section title A Commission for Environmental Cooperation

 The North American Mosaic

An Overview of Key Environmental Issues

June 2008
This report addresses the state of the environment in the territories of the Parties to the North 

American Agreement on Environmental Cooperation by providing an overview of key environmental 

issues. It provides an objective appraisal of environmental trends and conditions to inform the 

Council’s deliberations on strategic planning and future cooperative activities.

This publication was prepared by the Secretariat of the Commission for Environmental Cooperation. 

The design and implementation of this report benefited from the participation of the State of 

the Environment Advisory Group, which is composed of environmental reporting experts from the 

Parties. The views contained herein do not necessarily reflect the views of the governments of 

Canada, Mexico or the United States of America. 

In general, this report does not address the wide variety of responses to the environmental issues 

described herein. Likewise, an evaluation of the efficacy of these responses is beyond its scope.

More information, including detailed references for the findings in this report, is available on the 

CEC website: <http://www.cec.org/soe>.

Publication details

Type: Project report

Date: June 2008

Original language: English

Review and Quality Assurance Procedures

• Review by the Parties: February – April 2008; April – May 2008

• For more information please consult the Acknowledgements.

Published by the Communications Department of the CEC Secretariat.

© Commission for Environmental Cooperation, 2008

ISBN 2-923358-50-3 (Spanish edition: 2-923358-51-1; French edition: 2-923358-52-X)

Legal Deposit-Bibliothèque et archives nationale du Québec, 2008

Legal Deposit-Library and Archives Canada, 2008

For more information:

Commission for Environmental Cooperation

393, rue St-Jacques Ouest, bureau 200

Montreal (Quebec) Canada H2Y 1N9

T 514 350-4300 F 514 350-4314

info@cec.org www.cec.org
 The North American Mosaic

An Overview of Key Environmental Issues

June 2008
Acknowledgements

Project Management

Cody Rice, CEC, Program manager

Jessica Levine, Consultant, Project coordination

Marilou Nichols, CEC, Administrative support

Contributing Authors

Tundi Agardy, Consultant, Oceans and Coasts

Inés Arroyo-Quiroz, Consultant, Land Use; 

Species of Common Conservation Concern

Jane Barton, Consultant, Acid Deposition; Ground-level Ozone; Particulate Matter; 

Persistent Bioaccumulative Toxic Substances; Stratospheric Ozone

Ramón Pérez Gil, Consultant, Land Use; Species of Common Conservation Concern

Erica Phipps, Consultant, Industrial Pollution and Waste

Jamie Reaser, Consultant, Invasive Species

Elizabeth Shoch, Consultant, Invasive Species

Kent Thornton, Consultant, Shared Water Resources; Water Quality; Water Quantity and Use

Editing

Sabra Ledent, Consultant, Consulting Editor

Johanne David, CEC, French 

Jacqueline Fortson, CEC, Spanish

Douglas Kirk, CEC, English

Secretariat Review

Evan Lloyd, CEC, Director of programs

Orlando Cabrera-Rivera, CEC, Program manager

Hans Herrmann, CEC, Program manager

Luke Trip, CEC, Program manager

Coordination of Party Review

Wayne Bond, Environment Canada

Paula Brand, Environment Canada

Terence McRae, Environment Canada

Heather Case, US Environmental Protection Agency

John Dombrowski, US Environmental Protection Agency

Guy Tomassoni, US Environmental Protection Agency

Arturo Flores Martínez, Secretaría de Medio Ambiente y Recursos Naturales de México

César E. Rodríguez Ortega, Secretaría de Medio Ambiente y Recursos Naturales de México
 The North American Mosaic: An Overview of Key Environmental Issues 3

Preface 5

Air and Atmosphere

Climate Change 7 

Ground-level Ozone 11 

Particulate Matter 15 

Stratospheric Ozone 19

Biodiversity and Ecosystems

Land Use 23 

Oceans and Coasts 27 

Invasive Species 31 

Species of Common Conservation Concern 35

Pollutants

Acid Deposition 39 

Industrial Pollution and Waste 43 

Persistent Bioaccumulative Toxic Substances 47

Water

Water Quality 51 

Water Quantity and Use 55 

Shared Water Resources 59

Table of Contents

 The North American Mosaic: An Overview of Key Environmental Issues 5

Preface

The North American Agreement on Environmental Cooperation obliges the Secretariat of 

the Commission for Environmental Cooperation to “periodically address the state of the 

environment in the territories of the Parties.” 

To meet this obligation, the Secretariat has 

developed this report—The North American 

Mosaic: An Overview of Key Environmental 

Issues—with the support of environmental 

reporting experts from the governments of 

Canada, Mexico and the United States. 

This report describes a wide variety of 

environmental trends and conditions across 

North America. The breadth and diversity of 

the subject are astounding: from tiny invasive zebra mussels to global greenhouse gases 

measured by the teragram; from the last remaining vaquita porpoises to vast expanses of 

boreal forests and marine ecosystems; from 

invisible molecules of toxic chemicals to the 

all-too-visible smog and haze that blanket 

our cities from time to time. 

That said, as the title indicates, this report is an overview. It is not a comprehensive treatment of environmental issues, many 

of which have been described elsewhere in 

much greater detail. The assemblage of environmental indicators in this report is far 

from complete; many measurements are 

not available at the national level, let alone 

in comparable forms across North America. 

And yet there is value in this depiction of the 

North American environment. As a mosaic 

of existing information, this report prompts 

us to consider the following questions: 

n What are the central environmental 

challenges confronting North America?

n What are the greatest priorities for 

cooperative action among our three 

countries to address these environmental 

challenges? 

n How can we measure our progress and 

create effective feedback mechanisms?

n How can we enhance the relevance of 

trinational cooperation through the 

Commission for Environmental 

Cooperation?

Over the next year, we will use this report 

along with other important information to 

engage the public, subject matter experts 

and the Parties in a careful evaluation of 

our progress to date and future opportunities for cooperation. The information assembled in this report is a starting point 

for discussion.

In some cases, the news is good. As described here and in our annual Taking Stock

report, releases of many criteria air contaminants and toxic chemicals have declined over 

time. In other instances, North America 

still faces challenges. In the area of climate 

change, emissions of greenhouse gases continue to increase above 1990 levels here and 

in the rest of the world as energy use and 

population expand. Likewise, the cumulative 

impact of human activities continues to have 

important repercussions for biodiversity and 

ecosystem services.

The challenges may seem daunting, but 

we can meet them with innovation and effective international cooperation. Consider 

reductions in chemicals responsible for the 

depletion of the stratospheric ozone layer. In 

just 15 years, North America reduced the production and use of these harmful substances 

by almost 97 percent as part of a larger global 

agreement to shift to substitutes. Recovery 

of the ozone layer is now expected by the 

middle of this century based on international 

cooperation prompted by the 1987 Montreal 

Protocol and its amendments. 

As we begin to craft the Commission’s 

strategic plan for 2010–2015, our challenge 

is to identify those environmental issues 

where sustained environmental cooperation 

through the Commission can achieve real 

and important results. Such an effort will 

benefit the citizens of our three countries 

and demonstrate North America’s environmental leadership in the world. It is an effort that will repay us with rich dividends.

Felipe Adrián Vázquez-Gálvez 

Executive Director 

Secretariat of the Commission 

for Environmental Cooperation

 The North American Mosaic: An Overview of Key Environmental Issues 7

Key Findings

n During the last few decades, 

the earth’s climate has been 

disrupted by the rising temperature 

of the earth’s surface. This global 

warming very likely stems 

from increases in atmospheric 

greenhouse gas (GHG) 

concentrations produced by 

human activities.

n In North America, the largest 

source of GHG emissions is 

energy-related activities, including 

electric power generation, 

transportation and industrial fuel 

use. Some of these emissions are 

offset by factors such as forest and 

agricultural carbon sinks.

n North America is responsible 

for about a quarter of global 

GHG emissions.

n Since 1990, North American GHG 

emissions have increased by 

almost 18 percent, or at roughly the 

same rate as total energy use.

Air and Atmosphere

What Is the Environmental Issue?

Climate change is any change in climate properties that can be measured statistically (such 

as mean temperature, precipitation or wind) 

caused by natural internal processes, external forcings or human activities and lasting 

decades or longer. The global climate has 

experienced substantial variability over the 

history of the planet, but during the last few 

decades the climate has been disrupted in 

an unprecedented fashion. This disruption 

consists of an exceptionally rapid increase in 

the global average temperatures of the earth’s 

near-surface air and oceans. Unless current 

policies and practices change, this warming trend and a variety of associated climate 

impacts are projected to continue.

The Greenhouse Effect

Energy from the sun, which arrives mainly 

in the form of visible light, drives global climate and is the basis for life on earth. About 

30 percent of the sun’s energy arriving at earth 

is scattered back into space by the outer atmosphere, but the rest reaches the surface, where 

it is reflected back in the form of infrared 

radiation. The eventual escape of this infrared 

radiation into space is delayed by greenhouse 

gases such as water vapor, carbon dioxide, 

Climate Change

Climate change refers to a change in the state of the climate that can be identified by 

changes in the mean and/or the variability of its properties and that persists for an 

extended period, typically decades or longer. Climate change may be caused by natural 

internal processes or external forcings, or by persistent anthropogenic changes in the 

composition of the atmosphere or in land use.

The Greenhouse Effect

Some energy is reflected 

back out to space

Solar energy from the sun passes 

through the atmosphere

Earth’s surface is heated by 

the sun and radiates the heat 

back out toward space

Greenhouse gases 

in the atmosphere 

Sun trap some of the heat
8 Commission for Environmental Cooperation

ozone and methane. These gases make up only 

about 1 percent of the atmosphere, but they 

act like the glass roof of a greenhouse, trapping heat and keeping the planet warmer than 

it would be otherwise. Without the natural 

greenhouse effect, the average temperature 

at earth’s surface would be below the freezing 

point of water. The natural greenhouse effect 

is therefore a prerequisite for life on earth. 

Human activities are, however, very likely 

intensifying the natural greenhouse effect. 

Natural levels of greenhouse gases are being 

supplemented by emissions of carbon dioxide 

from the burning of fossil fuels, by the additional methane and nitrous oxide produced 

by farming activities and changes in land use, 

and by releases of long-lived industrial gases 

that do not occur naturally. As a result, global 

greenhouse gas emissions have grown since 

preindustrial times—70 percent between 1970 

and 2004 alone.

Because of these emissions, global atmospheric concentrations of greenhouse gases have 

increased markedly and now far exceed preindustrial values. The concentration of carbon 

dioxide in the atmosphere has reached a record 

high relative to the last half-million years, and it 

has done so at an exceptionally fast rate.

Global Climate Change 

The impact of additional greenhouse gases on 

global climate is evident in increases in the 

average global air and ocean temperatures 

(especially at the higher latitudes), the widespread melting of snow and ice, and the rising average sea level globally. Eleven of the 

last twelve years (1995–2006) are among the 

twelve warmest years since 1850. Over the 

last 30 years, the Arctic ice pack has shrunk 

on average each year by an area equivalent to 

Texas and Arizona combined, and the melting 

trend is accelerating. 

Most of the observed increase in the 

globally averaged temperatures of the last 50 

years is very likely attributable to the observed 

increase in concentrations of anthropogenic 

greenhouse gases. Indeed, the human impact 

on climate greatly exceeds that of known 

changes in natural processes, such as solar 

changes and volcanic eruptions. The current 

global temperatures are warmer than those of 

at least the past five centuries, perhaps those 

of more than a millennium. If warming continues unabated, the resulting climate change 

within this century would be extremely 

unusual in geological terms. 

Why Is This Issue Important to 

North America?

North America is already experiencing locally 

severe economic damage and substantial ecosystem, social and cultural disruption from 

weather-related events, including hurricanes, 

other severe storms, floods, droughts, heat 

waves and larger and more frequent wildfires. 

Although climate change does not account for 

all weather extremes, it exacerbates the risk 

from these events by affecting the frequency, 

intensity and duration of extreme climate events 

and associated natural disasters. The economic 

damage from the severe weather is growing dramatically, largely because of the rising 

value of the infrastructure at risk. The annual 

costs to North America have now reached tens 

of billions of dollars in property damage and 

lost economic productivity, as well as lives 

disrupted and lost. These patterns of climate 

change will continue unless the greenhouse gas 

emissions and the related greenhouse gas concentrations in the atmosphere that are causing 

global warming are substantially reduced.

Greenhouse Gas Emissions in North America

Home to some 7 percent of the world’s population, North America is responsible for 25 

percent of the total emissions of the most 

important greenhouse gas, carbon dioxide (see 

graph). Per person, North America emits twice 

as much carbon dioxide as Europe, over five 

times as much as Asia, and over 13 times as 

much as Africa. Per capita emissions are several 

times higher in Canada and the United States 

than in Mexico. These high rates are a result of 

higher per capita levels of economic activity, 

which drive greenhouse gas emissions, especially those related to energy consumption.

Sources of Emissions

Since 1990, North American greenhouse gas 

emissions have increased by almost 18 percent (see graph)—or at roughly the same rate 

as total energy use, but more slower than the 

Worldwide carbon dioxide emissions

 Asia & Oceania North America Europe 

 Eurasia Middle East Central & South America Africa

30,000

25,000

20,000

15,000

10,000

5,000

0

1990 1991 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

Worldwide carbon dioxide emissions, 1990–2005. Source: US Energy Information Administration. Million tonnes CO2

Trees ravaged by insect infestation.
Climate Change 9

overall gross domestic product. Without significant advances in energy efficiency and productivity over this period, this rate would have 

been even higher. 

Similar to the global picture, carbon 

dioxide constitutes over 80 percent of total 

greenhouse gas emissions in North America. 

The largest source of that gas, and of overall 

greenhouse gas emissions, is energy-related 

activities, including electric power generation, 

transportation and industrial fuel use. 

The conversion of fossil fuels to energy 

(primarily electricity) is the single largest contributor to North American emissions of carbon dioxide. More than half of the electricity 

produced in North America is consumed in 

buildings, making that single use one of the 

largest factors in North American emissions. 

As of 2003, the carbon dioxide emissions 

from US buildings alone were greater than the 

total carbon dioxide emissions of any country, 

except China.

The transportation sector is the secondlargest contributor to emissions of carbon 

dioxide in North America. This sector and 

its associated carbon dioxide emissions have 

grown steadily during the past 40 years. Growth 

has been the most rapid in Mexico, the country 

most dependent on road transport.

Important contributors to the remaining 

20 percent of greenhouse gas emissions are 

releases of methane from natural gas systems, 

landfills and agricultural sources; nitrous oxide 

from nitrogen fertilization and fuel combustion; and certain fluorinated industrial gases.

The fluorinated industrial gases—hydrofluorocarbons (HFCs), perfluorocarbons 

(PFCs) and sulfur hexafluoride (SF6

)—are 

potent greenhouse gases with long atmospheric 

lifetimes. Although they add only about 2 percent to the overall North American total greenhouse gas contribution, releases of these gases 

have increased sharply, up 72 percent between 

1990 and 2005. Some industrial releases have 

fallen, but this achievement has been more than 

offset by the rapid switch to HFCs and PFCs as 

substitutes for chlorofluorocarbons and other 

ozone-depleting substances, in particular the 

in-troduction of HFC-134a as a chlorofluorocarbon (CFC) substitute in refrigeration and 

air-conditioning applications. 

Recapture of Carbon

Land management activities can remove some 

portion of industrial greenhouse gas emissions. Forests and other vegetation act as a 

natural sink through carbon sequestration, but 

their net impact varies across North America. 

In 2005 land use, land use change and forestry 

activities captured more than 11 percent of US 

greenhouse gas emissions. Net carbon sequestration—primarily through a higher rate of net 

carbon accumulation in growing forests—was 

16 percent greater than in 1990. In Canada, 

the contribution of land management activities is highly variable: over 20 percent net carbon accumulation in 1990, but only 2 percent 

in 2005. This fluctuation is attributable to the 

large and variable impact of emissions from 

wildfires. In Mexico, land management activities added to total greenhouse gas emissions 

because of deforestation and land clearing. In 

2002 land use, land use change and forestry 

activities accounted for 14 percent of Mexico’s 

total greenhouse gas emissions.

What Are the Linkages to Other North 

American Environmental Issues?

As climate change continues, North America 

is expected to face additional challenges, 

some of which are described in the Fourth 

Assessment Report of the Intergovernmental 

Panel on Climate Change.

Human Health

By the end of the twenty-first century, climate change, with its higher temperatures 

and extreme weather in the form of longer 

and more intense heat waves, will have pronounced effects on human health, especially 

that of the elderly. Warming and climate 

extremes, including added exposure to pollen 

and ozone, are likely to lead to more respiratory illnesses. Periods of extreme weather and 

intense rainfall may spark as well an increase 

in water-borne diseases and degrade water 

quality. Climate change is also likely to hasten 

the spread of vector-borne infectious diseases, 

including Lyme disease and West Nile virus. 

In Mexico, the Instituto Nacional de Ecología 

has established a direct link between higher 

temperatures and an increase in the incidence 

of dengue fever since 1995.

Water Quantity and Quality

In making their projections, scientists are 

less certain about future precipitation patterns than about future temperatures. They 

do, however, project that warming in western 

mountains will reduce the snow pack, increase 

evaporation, produce more winter flooding and reduce summer flows, exacerbating 

the competition among agricultural, municipal, industrial and ecological uses of water in 

the west. In the Great Lakes and major river 

systems, lower water levels are likely to spur 

adaptation challenges related to water quality, 

navigation, recreation, hydropower generation, 

water transfers and binational relationships. 

Some studies project widespread increases in 

extreme precipitation with greater risks of not 

only flooding, but also drought. In Mexico, 

studies indicate that almost 97 percent of the Million tonnes CO2 equivalent

 Canada Mexico United States

9,000

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

North American 

greenhouse gas emissions 

Greenhouse gas emissions in North America, excluding land 

management, 1990 and 2005 (latest data from Mexico from 

2002, not 2005). Source: United Nations Framework Convention 

on Climate Change. 

More than half of the electricity produced in 

North America is consumed in buildings, 

making that single use one of the largest factors 

in North American emissions. 

1990 2005
10 Climate Change

California wildfire.

North American forests are indirectly influenced by climate through effects on natural 

disturbances such as wildfire, insects, and disease. 

Wildfires

The area consumed by wildfires in the United States and Canada has increased dramatically over the last three decades. The intensity of wildfires is closely related to the availability of the dry, dead biomass they use as fuel. A warming climate produces longer 

summer periods that dry fuels, thereby promoting the easier ignition and faster spread 

of wildfires. Since 1980, US wildfires have consumed an average of 22,000 square kilometers (km2

) a year, or almost twice the 1920–1980 average of 13,000 km2 a year. From 

1987 to 2003 in the western United States, the forested area burned was 6.7 times that 

burned from 1970 to 1986. In Canada, the burned area has exceeded 60,000 km2

 a year 

three times since 1990, or twice the long-term average. The wildfire-burned area in the 

North American boreal region increased from 6,500 km2

 a year in the 1960s to 29,700 

km2 a year in the 1990s. The human vulnerability to wildfires has also increased with rising population and housing development in forested areas.

Insects and disease

Insects and disease are a natural part of ecosystems. In forests, periodic insect epidemics kill trees over large areas. Recent epidemics have been related to the climatesensitive stages in insect life cycles. Many northern insects have a two-year life cycle, 

and warmer winter temperatures allow a larger fraction of overwintering larvae to survive. Recently, spruce budworm in Alaska completed its life cycle in one year rather than 

the normal two. The mountain pine beetle has expanded its range in British Columbia 

into areas previously too cold. The susceptibility of trees to insects is increased when 

multiyear droughts degrade the ability of trees to generate defensive chemicals. The 

recent dieback of aspen stands in Alberta was caused by light snowpacks and drought 

in the 1980s, which triggered defoliation by tent caterpillars, followed by wood-boring 

insects and fungal pathogens. Extensive areas of dead, standing dry trees exacerbate 

the risk of large wildfires. 

country is susceptible to a moderate or high 

degree of desertification and reduction in precipitation as a result of climate change. 

Oceans, Coasts and Fisheries

Coastal communities and habitats are especially vulnerable to climate change. Sea levels 

are rising along much of the coast, and the rate 

of change will accelerate in the future, worsening the impacts of progressive inundation, 

storm surge flooding and shoreline erosion. 

The destruction inflicted by storms is likely to 

grow, especially along the Gulf and Atlantic 

coasts. As for wildlife, coastal habitats and 

dependent species are threatened by rising sea 

levels, changes in vegetation and a built environment that blocks landward migration. 

Habitat Change and Disturbances

Climate change is a factor in the growing 

number of climate-related disturbances in 

North America such as wildfire and insect outbreaks, which are only likely to intensify with 

the drier soils and longer growing seasons that 

appear to lie ahead. Although in some areas 

vegetation growth may respond positively to 

recent climate trends, a rising incidence of 

disturbances is likely to limit carbon storage, 

enable the proliferation of invasive species 

and disrupt ecosystem services. As summers 

grow warmer, the window of high fire risk is 

expected to widen (see case study).

Over time, species will respond to climate pressures by moving north and to higher 

elevations in search of more suitable habitats, 

thereby rearranging North American ecosystems. The structure, function and services 

of ecosystems will change in response to the 

various capacities of species to shift ranges 

and from the constraints imposed by development, habitat fragmentation, invasive species 

and other pressures. This ecosystem alteration 

will be enhanced where high disturbance rates 

leave large areas open to recolonization by vegetation. In Mexico, half of the national vegetative cover may suffer alterations, including the 

disappearance of some zones and changes in 

others. In central and southern Mexico, climate 

change and land use patterns are projected to 

replace tropical forests with savannas, and the 

semiarid vegetation in most of central and 

northern Mexico with arid vegetation. Change 

in habitat distribution is expected to affect the 

species that inhabit these ecosystems. In tropical areas of Mexico, some species may become 

extinct altogether. 

Case Study – Accelerating Forest Ecosystem Disturbances
 The North American Mosaic: An Overview of Key Environmental Issues 11

Key Findings

n Ground-level ozone—unlike 

ozone in the stratosphere—

damages human health, vegetation 

and materials. Ozone and its 

precursor chemicals travel 

across both North American and 

continental boundaries.

n Humans contribute to the 

formation of ground-level ozone 

primarily through burning fossil 

fuels in the transportation, 

industrial and electricity generation 

sectors. Evaporation of liquid 

fuels and solvents also adds to 

ozone formation. 

n In certain areas of North America, 

levels of ground-level ozone 

exceed national standards for the 

protection of human health. 

n Since 1990, total emissions of 

ozone precursor chemicals have 

declined in North America, but the 

trend in human exposure across the 

three countries is mixed, reflecting 

differences in location conditions 

and reporting methods.

Ground-level Ozone

Ground-level ozone is a colorless, highly irritating gas created by photochemical 

reactions between nitrogen oxides and volatile organic compounds produced largely 

by fuel combustion, gasoline vapors and chemical solvents. 

Air and Atmosphere

)))))))))))))))))))))))

How ground-level ozone is formed

What Is the Environmental Issue?

Ozone (O3) is a gas found in different parts 

of the atmosphere. Ozone in the upper atmosphere, or stratosphere, is an essential gas that 

helps to protect the earth from the sun’s harmful ultraviolet rays. By contrast, the ozone found 

near the ground in the troposphere harms both 

human health and the environment. For this 

reason, ozone is often described as being “good 

up high and bad nearby.”

Ground-level ozone is produced when 

nitrogen oxides (NOx) and volatile organic compounds (VOCs) react through photochemical 

processes in sunlight (see figure). Power plants, 

motor vehicle exhaust, industrial facilities, gasoline vapors, and chemical solvents are the major 

sources of these emissions.

Ozone is also formed at ground level 

from natural emissions of VOCs, NOx

 and 

carbon monoxide, as well as stratospheric 

ozone that occasionally migrates down to 

the earth’s surface. Natural sources of ozone 

precursors include emissions from plants and 

soils, forest fires, and lightning. High ozone 

concentrations are observed at many remote 

mid-latitude sites in late winter and spring, 

especially at high elevations. However, longrange transport and the winter buildup of O3

precursors also contribute to these springtime 

levels, so it is not possible to attribute these 

high levels solely to natural sources.

VOCs + NOx + Sunlight = 

Ground-level ozone
12 Commission for Environmental Cooperation

Levels of ground-level ozone are often 

higher during hot summer days or downwind 

of the heavily populated areas that are emitting the necessary precursors. In the Northern 

Hemisphere, ozone levels are typically highest 

during the afternoon hours of the months in 

which temperatures are warm and the influence of direct sunlight is the greatest.

Why Is This Issue Important to 

North America? 

Ground-level ozone has deleterious effects on 

human and animal health and the environment. Despite reduction efforts by the three 

countries, it still exceeds national air quality 

standards in some areas of North America.

Effects of Ground-level Ozone

Ground-level ozone, a key component of 

smog, is considered a “nonthreshold” problem because even very small amounts in the 

air have deleterious effects on human health, 

especially the cardiovascular and respiratory 

systems. Exposure to ozone has been linked 

to premature mortality and a range of morbidity outcomes that include hospital admissions and asthma symptoms. After analyzing 

the air pollution and mortality data of eight 

major Canadian cities, Health Canada estimated that in these cities almost 6,000 deaths 

a year could be attributed to air pollution of 

which ground-level ozone is a major component. According to the Ontario Medical 

Association, air pollution costs Ontario citizens more than C$1 billion a year in hospital 

admissions, emergency room visits and absenteeism. In the United States, studies of 95 

major urban areas by researchers at Yale and 

Johns Hopkins revealed that an increase in 

daily ozone levels was associated with more 

than 3,700 deaths each year from cardiovascular and respiratory illnesses. 

Vegetation, crop productivity, flowers, 

shrubs and forests are also damaged by groundlevel ozone. Moreover, it can deteriorate cotton 

and synthetic materials, produce cracks in rubber and accelerate the fading of dyes, paints 

and coatings.

Reducing Emissions

The 1970s saw the beginning of attempts to 

mitigate ground-level ozone concentrations 

across North America through directed reductions in precursor emissions. In response, both 

NOx

 and VOC emissions in the United States 

fell substantially, despite significant economic 

growth. In Canada, VOC emissions have decreased, but the trend in NOx

 emissions has 

been almost flat since 1990. Mexico has experienced reductions in emissions from vehicles, 

but increases in those from fixed or stationary 

sources for both NOx

 and VOCs. Overall, air 

emissions of ground-level ozone precursors in 

North America have declined since 1990, with 

releases of both NOx

 and VOCs falling over 20 

percent (see graphs).

In all three countries, fuel combustion 

by mobile sources is a major source of both 

NOx

 and VOC emissions, with fossil fuel–fired 

power plants adding significantly to NOx

 emissions in the United States and Mexico. In Canada, upstream oil and gas production is the largest industrial contributor of NOx

. In addition 

to fuels in the transportation sector, solvents 

are a major source of emissions of VOCs in all 

three countries, but oil and gas production is 

also a large contributor in Canada. 

Monitoring Ozone Trends 

At present, considerable ozone data for North 

America are available from various networks. 

Characterization of North American trends 

and patterns is limited, however, by the lack 

of consistency in these data sets and by the 

inconsistent methods for preparing and reporting results. It is also difficult to derive 

meaningful North American trends because 

conditions vary greatly on a regional basis. 

Nevertheless, existing monitoring reveals that 

ambient levels of ozone exceed national standards in certain areas of all three countries.

In Canada, trends for ambient ozone 

based on the Canada-wide Standard (CWS) 

remained largely unchanged over the 15 years 

ending in 2005. However, the Canadian indicator for human exposure to ozone rose by 

an average of 0.8 percent a year, for a total increase of 12 percent between 1990 and 2005. 

The national ozone exposure indicator for 

Canada, which is weighted by population, is 

driven primarily by the ozone concentrations 

and populations in Ontario and southern 

Thousand tonnes

 Canada Mexico United States

30,000

25,000

20,000

15,000

10,000

5,000

0

Air emissions of NOx Air emissions of VOCs 

Sources: Environment Canada, Instituto Nacional de Ecología (latest data from Mexico from 2002, not 2005), US Environmental Protection Agency.

1990 2005 1990 2005
Ground-level Ozone 13

Quebec. In 2005, communities in these areas 

recorded the highest ground-level ozone concentrations for both the CWS and seasonal averages. Many stations in Alberta also reported 

high seasonal average concentrations. In 2005, 

at least 40 percent of Canadians lived in communities with ozone concentrations above the 

ambient CWS target.

In Mexico, the frequency of days on 

which ground-level ozone concentrations exceed the standard has remained constant over 

time in most monitored cities. However, in 

Mexico City and Guadalajara, ground-level 

ozone remains a serious air quality problem. 

In 2005 at least 27.7 percent of Mexicans lived 

in municipalities in which ozone concentrations were above the national standard at least 

one day a year.

In the United States, national ozone concentrations averaged over one hour and eight 

hours fell by 12 percent and 8 percent, respectively, in the period between 1990 and 2005. 

Despite the decrease, in 2005, more than 10 

percent of Americans lived in counties with air 

quality concentrations above the ozone onehour National Ambient Air Quality Standard, 

and at least 33 percent lived in counties with 

concentrations above the eight-hour standard. 

Transboundary Flows 

Both field studies and computer models 

confirm that the ozone problem in various regions of North America is a result of 

the complex interactions between meteorological processes on various scales and precursor emissions and their chemistry. At 

times, ozone levels are predominantly the 

result of local emissions, with only minor 

contributions from upwind sources. And at 

other times, local ozone levels are dominated 

by the transport of ozone and its precursors 

from upwind sources. 

Analyses of ozone levels within 500 kilometers of the Canada-US border found higher 

ozone levels in the lower Great Lakes–Ohio 

Valley region and along the US East Coast 

(see map). The lowest ozone values are largely 

found in the west and in Atlantic Canada. 

Levels are generally higher downwind of urban 

areas, such as in the western portions of lower Michigan. The locally higher levels in the 

complex terrain of the Georgia Basin–Puget 

Sound area of British Columbia–Washington 

State are not well resolved in the map shown, 

although they are lower than in the east. Between 1995 and 2004, there was a decrease in 

annual ozone levels within this border region, 

with trend lines on either side of the border 

tracking similarly.

Ozone concentrations in the US-Mexico 

border region remain a concern in some areas. 

Although in the Rio Grande Valley no days in 

2005 exceeded the binational eight-hour ozone 

standard, other monitoring locations in border 

sister-city pairs demonstrated exceedances, 

including Ambos Nogales (1 day), Ciudad Juárez/

El Paso (6 days), Tijuana/San Diego (11 days), 

and Mexicali/Imperial Valley (24 days). Although overall compliance with the ozone 

standard is generally improving, Mexicali/Imperial Valley and Tijuana/San Diego consistently remained above the applicable standard 

from 2001 to 2005.

Transport of ozone and precursor emissions extends beyond North America’s borders. North America is a source of groundlevel ozone for Europe just as Asia is for North 

America. More widely, ground-level ozone 

levels are rising across the planet and have 

created “background” ozone concentrations, 

even in remote areas that are not directly affected by human influence. Retrospective analysis of eighteenth-century data from Europe 

suggests that ozone concentrations in the 

Northern Hemisphere may have doubled over 

the past century in response to the massive 

industrialization that has taken place. Current 

“background” ozone concentrations in North 

America are about 30–40 parts per billion.

What Are the Linkages to Other North 

American Environmental Issues?

Ozone and its precursor pollutants are linked 

to particulate matter (PM), another component 

of smog, and to acidification, eutrophication 

and climate change. 

Source: Canada–United States Air Quality Committee.

Ozone concentrations along the Canada-US border (2002–2004) 

Analyses of ozone levels within 500 kilometers of 

the Canada-US border found higher ozone levels 

in the lower Great Lakes–Ohio Valley region and 

along the US East Coast. 

<55

55–60

60–65

65–70

70–75

75–80 

80–85

85-90

90–95

95–99 

Parts per billion

Source: Bilateral Air Quality Committee.
14 Ground-level Ozone

Particulate Matter

When nitrate, an oxidation product of nitrogen dioxide (NO2

), is combined with other 

compounds in the atmosphere, such as ammonia, it becomes an important contributor 

to the secondary formation of fine particulate 

matter (PM2.5). VOCs are also a precursor 

pollutant to the secondary formation of PM2.5. 

Ozone and PM have some common precursor gases, and reductions in any one of these 

precursors can have complex, and at times 

negative, results for concentrations of ozone 

or PM. Efforts to address and reduce concentrations of ozone and PM are often integrated 

in air quality management programs to avoid 

negative air quality results.

Acidification

Nitrogen oxides are formed primarily from the 

nitrogen liberated during combustion processes. Nitrogen oxide emitted during combustion 

quickly oxidizes to NO2

 in the atmosphere. The 

NO2

 then dissolves in water vapor in the air to 

form nitric acid (HNO3

), and interacts with 

other gases and particles in the air to form particles known as nitrates and other products that 

may be harmful to people and their environment. Both NO2

 in its untransformed state and 

the acid and transformation products of NO2

can have adverse effects on human health and 

the environment, harming vegetation, buildings and materials, and contributing to the 

acidification of aquatic and terrestrial ecosystems.

Eutrophication

Nitrogen releases not only contribute to the formation of acid depositions, but also can act as a 

nutrient in ecosystems, resulting in eutrophication or overenrichment of soils and waters. 

Climate Change 

When present in the upper troposphere, ozone 

is a very effective greenhouse gas. Strategies 

that reduce ozone concentrations on urban and 

regional scales probably help to limit the contribution of ground-level ozone to the greenhouse effect and global warming. 

Case Study – Transporting Asian Pollution to North America

Source: NASA

A recent study suggests that the transpacific transport of pollution from Asia influences 

North America’s air quality during the spring and summer. Even small quantities of 

Asian emissions over North America during the summer can have significant implications for air quality management.

In the summertime, emissions from Asia and Europe contribute 4–7 parts per billion by volume (ppbv) to afternoon ozone concentrations in the surface air over the 

United States, instigating violations of the air quality standard. If Asian anthropogenic 

emissions triple from 1985 to 2010 as expected, surface ozone in the United States 

could increase by 1–5 ppbv during the summer. 

The long-range transport of Asian pollution across the Pacific reaches a maximum 

in the spring because of the active cyclonic activity and strong westerly winds. The 

strongest Asian outflow occurs in the middle troposphere; it can be transported across 

the Pacific in 5–10 days. During the summer, the export of Asian pollution by convection competes with the export of mid-latitude cyclones. Transpacific transport occurs 

primarily in the middle and upper troposphere, with an average transpacific transport 

time of 6–10 days. 

According to the analysis, the Asian air masses contained elevated levels of carbon 

monoxide, ozone, particulate matter, and other chemicals consistent with the dominant 

influence of combustion emissions over East Asia. High levels of methanol and acetone 

indicated that natural emissions were combining with the polluted outflow.

South Korea

China

Japan

North America
 The North American Mosaic: An Overview of Key Environmental Issues 15

Key Findings

n Airborne particulate matter (PM) 

is an underlying cause of some 

serious human health problems, 

including cardiac and respiratory 

diseases. PM also adversely 

affects vegetation and building 

materials and contributes to 

regional haze and poor visibility. 

PM and its precursor chemicals 

are carried through the air across 

state, provincial, national and 

continental boundaries.

n PM has many natural and 

anthropogenic sources, among 

them direct releases to air from 

heavy equipment, fires, burning 

waste and dust from unpaved 

roads, stone crushing and 

construction sites. PM is also 

formed from precursor chemicals 

emitted by vehicles, power plants 

and industrial facilities. 

n In certain areas of North America, 

levels of PM exceed national 

standards for the protection of 

human health. 

n Since 1990, total emissions of PM 

and its precursor chemicals have 

declined in North America, but the 

trend in human exposure across the 

three countries is mixed, reflecting 

differences in local conditions and 

reporting methods.

What Is the Environmental Issue?

Particulate matter (PM), made up of solid particles and liquid droplets in the air, can be both 

large enough to appear as dirt and much smaller than the diameter of a human hair. Ambient 

PM mass is a complex mixture that is strongly 

dependent on source characteristics.

Particles are commonly tracked within 

two size ranges: PM2.5 or “fine” PM, which 

has aerodynamic diameters less than or equal 

to 2.5 micrometers (μm) and PM10, which 

includes fine PM as well as larger “coarse” 

particles up to 10 μm (about one-seventh the 

diameter of a human hair)—see figure. Particles of different sizes behave differently 

in the atmosphere. Smaller particles can remain airborne for long periods and travel 

hundreds of kilometers. Larger particles do 

not remain airborne as long because they tend 

to deposit closer to their point of origin. 

In general, the coarse fraction of PM10 

is composed largely of primary particles released directly into the atmosphere by both 

natural events (e.g., forest fires and volcanoes) and human activities (e.g., agriculture, 

construction activities, dust from unpaved 

roads, residential wood burning and industrial activities). Conversely, PM2.5 tends to 

be composed of more secondary particles. 

These secondary particles are formed in the 

atmosphere from chemical reactions involving 

Particulate Matter

Particulate matter is a mixture of solid particles and liquid droplets found in the air. 

Its components include sulfates, nitrates, ammonium, organic carbon, black carbon, 

metals and soil dust.

Air and Atmosphere

Source: NARSTO.

Representative composition of particulate matter

PM < 10 μm

Metals

Soil dust

Organic carbon

Sea salt

Nitrates

Pollen, spores

Precursor gases:

Sulfur dioxide

Nitrogen oxides

Volatile organic compounds

Ammonia

PM < 2.5 μm

Sulfates

Nitrates

Ammonium

Black carbon

Organic carbon

Metals
16 Commission for Environmental Cooperation

the precursor emissions of nitrogen oxides 

(NOx

), sulfur dioxide (SO2), volatile organic 

compounds (VOCs) and ammonia (NH3

). 

Why Is This Issue Important to 

North America?

Particulate matter has harmful effects on human health and the environment. Despite the 

efforts of all three North American countries to 

reduce PM, it still exceeds national air quality 

standards in some areas.

Effects of Particulate Matter

Research indicates that exposure to PM air pollution is linked to thousands of excess deaths 

and widespread health problems. Numerous 

studies have linked PM to aggravated cardiac 

and respiratory diseases such as asthma, bronchitis and emphysema and to various forms of 

heart disease. Fine PM has greater effects on human health than coarse PM because the smallest 

particles can travel deepest into the human lung, 

causing the greatest harm. Sensitive groups that 

appear to be at the greatest risk of such PM effects include older adults, individuals with cardiopulmonary disease, such as asthma or congestive heart disease, and children.

PM deposition also affects the environment by altering nutrient and chemical cycles 

in soils and surface water. For example, the deposition of particles containing nitrogen and 

sulfur may change the nutrient balance and 

acidity of aquatic environments, thereby altering species composition and buffering capacity. Some particles can corrode leaf surfaces or 

interfere with plant metabolism. PM also soils 

and erodes materials and buildings, including 

monuments, statues and other objects of cultural importance.

In addition to its effects on human health 

and the environment, fine PM is a main contributor to reduced visibility. This kind of haze 

is often noticeable in parks and wilderness areas, where periods of poor visibility result in 

lost tourist revenues.

Reducing Emissions

Between 1990 and 2005, direct fine PM air 

emissions in Canada and the United States declined by about a third (see graph). Important 

sources of direct fine particle emissions are 

diesel engines, burning activities and industrial sources. Only the Canadian and US PM2.5

emissions can be displayed over this time period because PM2.5 emission estimates for Mexico are available only for 1999. As of that year, 

Diesel Emissions in the US-Mexico Border Region

Diesel emissions are a source of particulate and hazardous air pollution. The trucking, shipping and rail industries are responsible for a significant share of diesel emissions within the US-Mexico border region. Other important sources of on-road diesel 

emissions are school buses, garbage trucks and municipal buses. Of non-road diesel engines, construction vehicles and agricultural equipment are the largest mobile 

sources of PM10 and PM2.5.

As the US-Mexico border opens more widely under terms of the North American 

Free Trade Agreement, cross-border truck transportation is expected to increase. Diesel 

emissions will become worse if the majority of Mexican diesel truck fleets continue to 

consist of vehicles built before 1993—the year engine manufacturers began to incorporate technology to reduce emissions and improve performance and fuel economy.

Because diesel emissions contribute so significantly to pollutants, retrofits and collaboration to reduce these emissions are under way. In January 2006, Mexico modified 

its fuel standard for gasoline and diesel fuels. An accelerated calendar was included 

for the border region, with the goal of providing the region with ultra-low sulfur diesel 

by January 2007.

Meanwhile, the San Diego Air Pollution Control District is retrofitting 60 heavy-duty 

diesel trucks from Tijuana with diesel oxidation catalysts. These devices reduce PM10

by 25 percent. In addition, the Laredo (Texas) Independent School District is modifying 

50 school buses to burn ultra-low sulfur diesel fuel, and the Rio Rico, Arizona, school 

district is implementing a similar project. These two projects will reduce the exposure 

of schoolchildren to fine PM and serve as a demonstration project for other school districts on both sides of the border. 
Mexico’s contribution to North America’s total 

PM2.5 emissions was about 7 percent.

Because PM is also formed in the atmosphere by precursor emissions, it is important 

to understand which human activities contribute to precursor emission inventories. For NOx

emissions, fossil fuel–fired power plants are important sources in the United States and Mexico, 

and all three countries count the transportation 

sector as a key contributor. For SO2

 emissions, 

coal-fired power plants in the United States and 

Mexico and smelters in Canada are large sources. Volatile organic compounds are produced 

by similar sources in all three countries—fuels, 

solvents and oil and gas development—but in 

Canada residential wood burning also makes a 

large contribution. For ammonia, agriculture is 

a common source throughout North America. 

Across the continent, air emissions of PM precursors have declined since 1990 (see graph).

Monitoring PM Trends 

At present, considerable PM data for North 

America are available from various networks using a number of different measurement techniques. Characterization of North 

American trends and patterns is limited, 

however, by the inconsistency of these data 

sets, lack of monitoring stations and suitable 

measurement technology in certain areas, 

as well as by differing methods for preparing and reporting results. It is also difficult 

to derive meaningful North American trends 

because conditions vary greatly on regional 

basis. In all three North American countries, 

however, the existing monitoring reveals that 

ambient levels of PM exceed national standards in certain areas.

In Canada, there was no statistically significant increasing or decreasing trend in PM2.5

exposure either nationally or regionally during 

the years 2000–2005. Between 2003 and 2005, 

at least 30 percent of Canadians lived in communities with PM2.5 levels above the Canadawide Standard target. The communities affected were located in southern Ontario, southern 

Quebec and British Columbia. 

Data on the levels of concentrations of 

PM2.5 are not available for most Mexican cities. However, PM10 measurements are available in various metropolitan areas. In 2005 the 

standard for PM10 was exceeded on 173 days in 

Toluca, 163 days in Monterrey, 51 days in Guadalajara, 34 days in Mexico City and 11 days in 

Puebla. Over the past decade, most monitored 

cities have experienced a tendency toward 

fewer days of exceeding the standard, with the 

exception of Monterrey and Toluca. In 2005 at 

least 27 percent of Mexicans lived in municipalities where PM10 concentrations were above 

the national standard at least 11 days a year.

Although concentrations of PM in the 

United States have generally fallen nationwide, 

they still exceed national standards in dozens 

of metropolitan areas. In 2006 some 14.7 million people were living in counties with PM10

levels above the national air quality standard, 

and 66.9 million people were living in counties 

that exceeded both the annual and daily standards for PM2.5. 

Transboundary Flows 

Efforts to reduce PM to meet air quality standards in North America are confounded by 

the fact that PM levels are affected by local 

pollution, as well as pollution transported 

across state, provincial and national borders. 

PM can remain in the atmosphere for days to 

a few weeks, depending on the size and rate 

at which it is removed from the atmosphere 

through, for example, precipitation. Therefore, particles in any given area may originate 

locally or from sources hundreds to thousands 

of kilometers away. Regional contributions 

from sources distant to eastern North American urban areas can account for 50–75 percent 

of the total observed PM2.5 mass concentration 

within a specific urban area.

Transboundary flows are important in the 

border region shared by Canada and United 

States. In 2005 the concentrations at stations in 

southern Ontario were influenced by significant 

contributions flowing from the United States, 

and in southern Quebec levels were affected by 

pollution from both the United States and Ontario. At the same time, PM2.5 and its precursor 

emissions from Canada led to elevated concentrations of PM2.5 in the eastern United States.

Along the US-Mexico border, the Rio 

Grande Valley remained consistently below 

the annual US standard for PM10 from 2001 

to 2005, but four other monitoring areas exceeded the standard (Ambos Nogales, Tijuana/San Diego, Ciudad Juarez/El Paso, and 

Mexicali/Imperial Valley). During this period, 

the Mexicali/Imperial Valley area consistently 

experienced annual PM10 concentrations more 

than four times the US standard.

Thousand tonnes

 Canada United States

9,000

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

Air emissions of PM2.5 

in Canada and United States

Sources: Environment Canada, US Environmental Protection Agency.

Particulate Matter 17

1990 2005

Thousand tonnes

 NOX VOCs SOX

30,000

25,000

20,000

15,000

10,000

5,000

0

Air emissions of PM precursor 

chemicals in North America 

Sources: Environment Canada, Instituto Nacional de Ecología 

(latest data for Mexico from 2002, not 2005), US Environmental 

Protection Agency.

1990 2005

Despite the efforts of all three North American 

countries to reduce PM, it still exceeds national 

air quality standards in some areas.
18 Particulate Matter

On a periodic basis, fires in one country can contribute to high PM concentrations 

in a neighboring country. For example, during April and May 2003 air quality in Texas, 

Oklahoma and other states suffered from large 

amounts of aerosol PM carried as smoke from 

fires in the Yucatán Peninsula and southern 

Mexico (see photo). The smoke plumes, which 

significantly degraded visibility and air quality in coastal regions along the Gulf of Mexico, 

were large enough to create circulation patterns 

in the atmosphere that trapped smoke aerosols 

and other PM in the lower atmosphere, further 

worsening air quality. 

Particle pollution can also cross into 

North America from outside the region. Intercontinental transport of PM in the form of 

dust and desert sand has been tracked from Africa and Asia to North America. Although this 

transport of dust from both Asia and Africa 

does not contribute significantly to annual average concentrations in North America, it may 

occasionally contribute significantly to daily 

concentrations. For example, in the summer of 

1997 a plume from North Africa contributed 

to PM10 concentrations at sites in the Houston, 

Texas, area by as much as 15–20 micrograms 

per cubic meter over two days. 

What Are the Linkages to Other North 

American Environmental Issues?

Particulate matter plays a role in various environmental issues, especially ground-level 

ozone, climate change and water quality. 

Ground-level Ozone

PM2.5 and ground-level ozone are closely related through common precursors, sources 

and meteorological processes. Because of this 

close relationship, changes in the emissions of 

one pollutant can lead to changes in the concentrations of PM or ground-level ozone. This 

finding is particularly important because certain regions, such as the eastern United States 

and southeastern Canada, experience high 

PM and ozone concentrations during the same 

season, whereas other regions, such as the San 

Joaquin Valley in California, have high PM 

and ozone levels in opposite seasons.

Climate Change

All particles affect climate change by scattering incoming and, to a lesser degree, outgoing 

radiation. Black carbon and other dark particles absorb radiative energy. Coarse particles 

and cloud droplets formed by the condensation 

of water vapor on particles also have radiative 

effects, which can have local and global impacts 

on climate change. 

Water Quality

Particles and their precursors—particularly 

sulfur dioxide, nitrogen oxides and ammonia—can be carried long distances by the wind 

and eventually be deposited on the ground 

or in water. Their deposition makes lakes and 

streams acidic, changes the nutrient balance 

in coastal waters and large river basins and 

encourages eutrophication, depletes the nutrients in soil, damages sensitive forests and farm 

crops, and affects the diversity of ecosystems. 

Particles also carry toxic components such as 

mercury, which can degrade water quality and 

aquatic ecosystems. 

Fires in the Yucatán Peninsula and southern Mexico, 2003. Photo: National Aeronautics and Space Administration.
 The North American Mosaic: An Overview of Key Environmental Issues 19

Key Findings

n Stratospheric ozone protects the 

earth’s surface from excessive 

solar radiation, but this protective 

layer has become thinner, allowing 

penetration of harmful levels of 

ultraviolet radiation. Excess levels of 

ultraviolet radiation are harmful to 

human health and the environment.

n In response, countries have sought 

to control production, consumption 

and trade of ozone-depleting 

substances (ODS) through an 

international agreement. By the end 

of 2005, the parties to the Montreal 

Protocol had together phased out 

production and consumption of over 

95 percent of ODS—which are used 

as refrigerants, aerosol propellants 

and for other purposes.

n Currently the earth’s protective 

ozone layer remains thinner than 

historical averages; the ozone hole 

over Antarctica was at its largest and 

deepest ever observed in 2006. 

n Canada, Mexico and the United 

States have substantially reduced 

emissions of ODS over the last 20 

years, but these substances are still 

released from various sources in 

North America and globally. Recovery 

of the ozone layer is expected by the 

mid-twenty-first century based on 

compliance with the international 

agreement now in place. Reduction 

in ODS has delivered substantial 

climate benefits, because some ODS 

also act as greenhouse gases.

What Is the Environmental Issue?

Stratospheric ozone protects the earth’s surface 

by absorbing ultraviolet (UV) radiation from the 

sun (see illustration). It is naturally formed by 

chemical reactions involving ultraviolet sunlight 

and oxygen. Approximately 90 percent of ozone 

is in the stratosphere, the layer of the atmosphere 

that begins 10–15 kilometers above the earth’s 

surface at the midlatitudes. The ozone in the 

stratosphere is referred to as the ozone layer. 

Thinning of the ozone layer

The stratospheric ozone layer is now thinner 

than it has been historically because of certain 

ozone-depleting substances, such as refrigerants and aerosol propellants. These substances 

were first produced commercially during the 

twentieth century and some continue to be 

produced and used. When released, these 

chemicals make their way to the upper atmosphere and gradually convert to more reactive 

gases that destroy ozone. Overall thinning of 

the ozone layer has been recognized since the 

1970s, and the total loss is currently estimated 

to average 3 percent over the globe, with more 

thinning occurring toward the polar regions 

and less near the equator. Above Antarctica, an 

ozone hole now forms each September. During 

September 2006, the average area of the ozone 

hole, at 27.5 million square kilometers, was 

the largest ever observed (see photo). A little 

over a week later, instruments recorded the 

lowest concentrations of ozone ever observed 

over Antarctica, revealing that the ozone hole 

was the deepest it had ever been.

Stratospheric Ozone

Ozone is a gas present throughout the earth’s atmosphere. Stratospheric ozone protects 

life on earth by absorbing harmful rays from the sun as they pass through the upper 

atmosphere (the stratosphere). Because the ozone present at the earth’s surface is 

detrimental, ozone is often described as being “good up high and bad nearby.” 

Air and Atmosphere

UV-B

Ozone layer

Source: United Nations Environment Programme.

Sun

UV-A

UV protection by the ozone layer

UV-C
20 Commission for Environmental Cooperation

Because the ozone layer absorbs some of the 

biologically harmful ultraviolet radiation from 

the sun, reductions in stratospheric ozone levels allow more UV radiation to reach the earth’s 

surface, where it affects human health, disrupts 

biological processes and damages materials.

Ozone-depleting Substances 

The substances most responsible for the destruction of the ozone layer are listed in the following 

table along with their uses in everyday life. Alternatives to these substances include hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons 

(HFCs) and perfluorocarbons (PFCs). HCFCs, a 

transitional CFC replacement, are used as refrigerants, solvents and fire extinguishers. HFCs 

and PFCs are used as refrigerants, aerosol propellants and solvents.

Why Is This Issue Important to 

North America?

Ozone depletion has significant health and 

economic consequences for North America. 

It is generally worse at latitudes approaching 

the poles, specifically the northern and Arctic 

regions in the North American context.

Effects of UV-B Radiation

Overexposure to UV-B radiation, the most 

damaging type of ultraviolet radiation, can 

cause a range of health effects, including 

skin cancers and premature aging, eye damage (such as cataracts) and suppression of the 

immune system. The physiological and developmental processes of plants are also affected 

by UV-B radiation, which can damage sensitive crops such as soybeans and rice and 

reduce crop yields.

Marine phytoplankton, which serves as 

the base for the ocean’s aquatic food chain, is 

under stress from UV-B radiation as well. Studies have also found that solar UV-B radiation 

damages fish, amphibians and other animals in 

their early developmental stages. 

More broadly, increases in solar UV radiation could affect terrestrial and aquatic biogeochemical cycles, thereby altering both sources 

and sinks of greenhouse and chemically important trace gases.

Finally, synthetic polymers, naturally 

occurring biopolymers and other commercially useful materials are adversely affected 

by solar UV radiation. Increases in solar 

UV-B levels accelerate their breakdown outdoors.

Reducing Emissions 

Canada, Mexico and the United States are 

addressing the destruction of the ozone layer 

by eliminating the production and consumption of ozone-depleting substances on a schedule determined by the Montreal Protocol on 

Substances that Deplete the Ozone Layer. This 

agreement has led to a phase-out of the production and consumption of CFCs and other 

ODS. Currently, 191 countries and the European Community are parties to the Protocol 

and are implementing its requirements. 

Source: National Aeronautics and Space Administration.

The Antarctic ozone hole (21–30 September 2006) 

Substance Uses

Chlorofluorocarbons (CFCs) Refrigerants, cleaning solvents, aerosol propellants and 

blowing agents for plastic foam manufacture

Hydrochlorofluorocarbons 

(HCFCs)

Refrigerants, cleaning solvents, aerosol propellants and 

blowing agents for plastic foam manufacture

Halons Fire extinguishers/fire suppression systems

Carbon tetrachloride Production of CFC (feedstock), solvents

Methyl chloroform Industrial solvents for cleaning, inks, etc.

Methyl bromide Fumigant used to control soil-borne pests and diseases in 

crops prior to planting

Common ODS and Their Uses
Stratospheric Ozone 21

By the end of 2005, the parties had 

together phased out over 95 percent of ODS, 

reducing production levels from a 1990 level 

of over 1 million Ozone Depletion Potential 

(ODP) tonnes a year to about 93,000 ODP 

tonnes a year in 2005. North American production and consumption declined from 

about one-third to under one-fifth of the 

global total (see graph).

Also through 2005, the production and 

consumption of ozone-depleting substances in 

North America fell by almost 97 percent (see 

graph). Because of the long time that it takes for 

ODS to move from ground level to the stratosphere, the impact of their elimination will not be 

felt for many years. It is estimated that the ozone 

layer could recover by about 2050, provided that 

all ozone-depleting substances of anthropogenic 

origin are eliminated. However, long-term predictions are uncertain because not all of the processes of ozone depletion are understood. The 

role of very short-lived ODS is still being studied, 

along with the impact of climate change on the 

stratosphere and ozone depletion.

Monitoring Stratospheric Ozone Trends

Over North America, total stratospheric 

ozone levels began falling in 1965, reaching 

their lowest levels in 1993. The ozone layer has 

since begun to recover, but as of 1998–2001 

average overall levels were still 3 percent lower 

than those observed 20 years earlier. Since 

1993, ozone levels over North America have 

been trending upward as result of reduced 

ODS emissions and reformation of stratospheric ozone. 

Illegal Trade in ODS

Somewhat complicating this picture of progress is the illegal trade in significant amounts 

of ODS on a global basis. Although all new 

CFCs are now banned in industrialized countries, millions of refrigerators, automobile air 

conditioners and other equipment that use 

CFCs are still in service. Servicing this equipment with CFC replacements is possible, but 

often more expensive. In addition, used CFCbased equipment is exported to developing 

countries by countries that have phased out 

CFCs. These factors create incentives for the 

illegal trade in ODS, which has been estimated 

at 10–20 percent of the legitimate global trade. 

Issues related to the legal trade of equipment 

and illegal trade of ODS may complicate progress toward the ultimate elimination of ODS 

on a worldwide basis.

What Are the Linkages to Other North 

American Environmental Issues?

Stratospheric ozone depletion has major linkages to other key North American environmental issues—principally climate change and the 

health of terrestrial and aquatic ecosystems.

Climate Change

The depletion of the ozone layer and climate 

change were originally understood to be separate threats. Recently, however, both the Environmental Assessment Panel for the Montreal 

Protocol and the Intergovernmental Panel on 

Climate Change stated that there is conclusive 

scientific evidence that ozone depletion and 

climate change are linked.

Some ozone-depleting chemicals (CFCs, 

HCFCs and Halon 1301) and their replacements (HFCs and PFCs) are powerful greenhouse gases. The accumulation of greenhouse 

gases, including ODS, increases warming of the 

lower atmosphere, which leads, in turn, to the 

cooling of the stratosphere. Stratospheric cooling hampers the formation of ozone and favors 

Thousands of ODP Tonnes

300

250

200

150

100

50

0

 Canada Mexico United States

North American reductions in ozone-depleting substances

ODP = Ozone Depletion Potential. Source: United Nations Environment Programme. 

1990 1995 2005

Production

1990 1995 2005

Consumption

Thousands of ODP Tonnes

1200

1000

800

600

400

200

0

1990 1995 2000 2005

Total reported production of ozone-depleting 

substances globally and in North America

 Global North America

ODP = Ozone Depletion Potential. Source: United Nations Environment Programme. 
22 Stratospheric Ozone

the development of polar ozone holes. Studies indicate that within two decades climate 

change may exceed CFCs as the principal cause 

of overall ozone loss.

Global efforts to phase out ozone-depleting 

substances have benefited the earth’s climate in 

two ways. First, the net global decline in emissions of ODS has resulted in a drop in greenhouse gas emissions equivalent to several billion 

tonnes of carbon dioxide. Second, the reductions needed to meet international obligations 

have frequently required equipment upgrades 

and more efficient energy practices that reduce 

greenhouse gas emissions. 

Health of Terrestrial and Aquatic Ecosystems

The linkages between terrestrial ecosystems 

and the higher levels of UV-B radiation resulting from ozone depletion are complex. The 

responses of plants and other organisms to 

increased UV-B radiation are influenced by a 

variety of environmental factors such as carbon dioxide, water availability, mineral nutrient availability, heavy metals and temperature. 

Many of these factors are also changing as the 

global climate is altered.

Higher levels of UV-B radiation damage terrestrial organisms, including plants and microbes. 

They change patterns of gene activity and affect 

life cycle timing, as well as change plant shape and 

the production of plant chemicals not directly 

involved in primary metabolism. Plant chemicals 

not only are important in protecting plants from 

pathogens and insect attacks, but also affect food 

quality for humans and grazing animals. 

The effects of ozone depletion and 

increased UV radiation on aquatic ecosystems are complex as well. Higher levels of 

solar radiation have negative impacts on the 

growth, photosynthesis, protein and pigment 

content and reproduction of phytoplankton 

and on the sea grasses that are important biomass producers in aquatic ecosystems. 

Zooplankton and other aquatic organisms, including sea urchins, corals and amphibians, are also sensitive to UV-B radiation. Polar 

marine ecosystems, where the increases in 

ozone-related UV-B radiation are the greatest, 

are likely to be the oceanic ecosystems most 

influenced by ozone depletion.

The linkage between UV-B radiation, 

aquatic ecosystems and global warming is 

also important. When these ecosystems are 

exposed to higher levels of to UV-B radiation, 

their ability to act as a sink for atmospheric 

carbon dioxide is reduced. 

Case Study – Phasing Out Methyl Bromide in North America

Methyl bromide (MeBr), a highly toxic, odorless, colorless gas, has been used as an 

agricultural soil and structural fumigant to control a wide variety of pests. However, 

because MeBr depletes the stratospheric ozone layer, Canada, Mexico and the United 

States agreed under the Montreal Protocol to phase out methyl bromide as a crop 

pesticide by 2005 in the United States and Canada and by 2015 in Mexico. The phaseout allows an exemption for quarantine applications and preshipment applications to 

eliminate quarantine pests as well as a Critical Use Exemption designed for agricultural users with no technically or economically feasible alternatives.

The United States manufactures MeBr and exports it to both Canada and Mexico, 

because neither country produces MeBr. In all three countries, MeBr is used in growing crops such as strawberry, tobacco, asparagus, flowers, potatoes, tomatoes, peppers and cucumbers. Competition among agricultural producers in the face of different 

phase-out schedules in the three countries has influenced the ways in which Canada, 

Mexico and the United States are phasing out MeBr (see graph for these countries’ 

consumption levels of MeBr over the period 1994–2006). 

Canada’s consumption of MeBr (25 ODP tonnes in 2006) accounts for less than 

1 percent of global methyl bromide consumption. The MeBr ban in effect is accompanied by the promotion of alternative technologies emphasizing integrated pest management. Canada has requested a small number of Critical Use Exemptions for the 

production of strawberry plantlets and fumigation of flour and pasta mills. 

With consumption of about 7 percent of the global total, Mexico has the right 

to a flexible approach as a developing country under the Montreal Protocol. Mexican 

consumption peaked in 1994 at 3,253 ODP tonnes, falling to 723 ODP tonnes by 2006. 

Mexico is reducing its MeBr consumption in a stepwise fashion.

The United States remains a significant producer and consumer of MeBr, requesting 

exemptions for MeBr beyond the original phase-out date in 2005 (see graph). In 2006 

the United States produced 6,502 ODP tones of MeBr—55 percent of the global total. That 

same year, the United States consumed 3,885 ODP tonnes of MeBr—almost 40 percent of 

the global total. Despite its exemption requests, the United States still reduced MeBr production by over 60 percent and consumption by 75 percent between 1991 and 2006.

North American consumption of methyl bromide

ODP = Ozone Depletion Potential. Source: United Nations Environment Programme. 

 Canada Mexico United States

Thousands of ODP Tonnes

18

16

14

12

10

8

6

4

2

0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
 The North American Mosaic: An Overview of Key Environmental Issues 23

Key Findings

n Human land use affects ecosystem 

function, biological diversity, 

water quality and quantity and 

climate. Humans have extensively 

altered natural land cover in ways 

that affect the provision of vital 

ecosystem services. 

n The most important human 

alterations of the natural land 

cover include widespread changes 

that have reduced and disturbed 

forested areas, native grasslands 

and wetlands to allow farming, 

ranching, resource extraction and 

human settlements.

n About 16 percent of North America 

is designated as “protected” by 

national governments to preserve 

valued species, natural spaces and 

environmental services. Some of 

these protected areas are affected 

by encroaching human activities, 

while other, more remote areas are 

less directly influenced.

n Compared with overall landscape 

modification since European 

colonization, current annual 

changes are small. However, 

rates of deforestation and 

urbanization in certain areas are 

affecting local ecological systems 

and global climate. 

What Is the Environmental Issue?

Land use is one of the most striking manifestations of humans’ presence and physical impact 

on the planet. More fundamentally, humans 

have altered the global patterns and prevalence 

of species and ecosystems. Several recent studies confirm that human-dominated ecosystems 

now cover more of earth’s land surface than do 

natural or “wild” ecosystems. According to one 

estimate, more than 75 percent of the earth’s 

ice-free land shows evidence of alteration from 

human residence and activity, with less than a 

quarter remaining as wildland. Together, croplands and pastures have become one of the largest land use categories; they occupy about 40 

percent of the earth’s land surface. 

Intact landscapes with little or no visible 

signs of influence from human activities such 

as agriculture, tree felling, mining, highways, 

pipelines or power lines are increasingly rare. 

One approach to measuring the extent of intact 

landscapes is the human influence index, which 

uses data on population density, settlement patterns, land use and infrastructure to measure 

the direct human impact on terrestrial ecosystems (see map). The direct human influence is 

highest in coastal regions and row crop farming 

areas, along transportation corridors and near 

population centers.

Although the amount of land in North 

America is constant, how land is used changes 

continually. The relationship between land use 

and land cover is complex because a particular 

kind of land cover may correspond with a variety of land uses. For example, forested land may 

be used for timber production, habitat, recreation or watershed protection. Likewise, some 

land uses such as agriculture may require maintaining several distinct land covers over time, 

such as cultivated crops, fallow land, woodlots 

or even burnt area. Despite this complexity, 

attempts to categorize land use and land cover 

can be useful for analyzing humans’ impact on 

natural ecosystems. Changes in land use can 

affect the distribution and type of land cover 

(such as forests, cropland and urbanized areas), 

the ability of ecosystems to provide valuable 

services that support life, and even elevation 

and terrain.

Why Is This Issue Important to 

North America?

Human activities have modified the original 

vegetation cover and landscape of North America in ways that have important implications 

for the environment. Land use and land cover 

affect many aspects of environmental quality 

and the services provided by ecosystems.

Forests 

Forests, both managed and unmanaged, cover 

about a third of North America’s land area. 

Within this forested area is a great diversity of 

forest types of which some 45 percent is classified as boreal, mostly in Canada and Alaska. 

Temperate and tropical forests make up the 

remainder of the forested area. North America 

has almost 20 percent of the world’s forests and 

over a third of its boreal forests. 

The extent of forested land is relatively 

stable in Canada, increasing slightly in the 

United States and declining in Mexico. Since 

1990, Canada has experienced a net increase 

in forested area of less than 1 percent, whereas 

Land Use

Land use refers to the purposes to which humans commit land cover such as forests and 

grasslands. Some land uses—particularly those that are less intensive or involve less 

alteration of natural systems—cause less disruption to ecosystem services such as water 

purification, recharging of groundwater, nutrient recycling, decomposition of wastes, 

regulation of the climate and maintenance of biodiversity.

Biodiversity and Ecosystems
24 Commission for Environmental Cooperation

forest cover in the United States has grown by 

about 1.5 percent. In Mexico, between 3.5 and 

5 million hectares of temperate and tropical 

forests have been lost over the last decade. The 

estimated annual deforestation rates in Mexico 

range from 0.5 percent to 1.14 percent from the 

early 1990s to 2000.

Changes in the structure of forest ecosystems introduced by human pressures can 

make forests more susceptible to damage by 

fires, drought, insect infestations and air pollution. For example, in Canada, clear-cutting 

has led to the proliferation of the balsam fir, 

which is vulnerable to the spruce budworm. 

In the conterminous United States, almost 

half of all forests are considered highly fragmented—that is, much of the forested area is 

in close proximity to a forest edge. Although 

the United States has many large regions of 

forest, fragmentation is so pervasive that edge 

effects disrupt ecological processes and suitability for wildlife habitat on most forested 

lands. In Mexico, the structure and composition of the remaining woodlands have been 

altered by the selective extraction of certain 

preferred tree species and by the extensive 

conversion of forests to pasture.

Agriculture 

In all, almost a third of North America’s surface is devoted to agricultural uses. Although it 

accounts for only 12 percent of the world’s agricultural area, North America produces almost 

20 percent of the world’s cereals and an equal 

percentage of the world’s meat. 

Since 1990, the overall amount of land 

dedicated to agricultural uses in North America 

has declined by about 1.5 percent. In Mexico, the 

most significant opening of land for farming and 

cattle occurred from 1940 to 1965, with annual 

growth rates of up to 10 percent a year. Although 

this trend has slowed, Mexico’s agricultural 

land uses continued to expand by 3.5 percent 

a year after 1990 and remain a major driver of 

land transformation. In Canada and the United 

States, the amount of land devoted to crops has 

declined since the 1950s. However, even with 

declines in overall agricultural area, the environmental effects of agricultural practices are still 

significant. Recent research has revealed that 

excessive nutrient loading from agriculture has 

created a considerable hypoxic zone of low dissolved oxygen in the northern Gulf of Mexico, 

which is causing ecological stress and the death 

of bottom-dwelling aquatic organisms.

The agricultural area devoted to permanent pasture in North America has remained 

relatively steady since 1990. However, based 

on historical land use changes for farming and 

ranching, temperate North American grasslands have undergone significant changes that 

have transformed the ecosystem and led to significant losses of biodiversity, especially species 

such as grassland birds, bison, prairie dogs and 

the black-footed ferret. Grasslands are one of 

the continent’s most endangered ecosystems 

(see case study). In 2001 about 55 grassland 

wildlife species in the United States were either 

threatened or endangered. In Mexico, overgrazing is reducing the productivity of grasslands and threatening biodiversity.

Wetlands 

Wetlands cover over 10 percent of North America. At 2.5 million square kilometers, this area 

represents some 40 percent of the global wetland area. Historically, wetlands, which include 

swamps, bogs and marshes, were undervalued 

as wasteland to be dredged for ports and marinas or drained for farms, housing and other 

development. In recent years, scientists have 

cataloged the many important ecological contributions of wetlands—as breeding grounds 

for waterfowl, fish and crustaceans; as areas to 

capture and filter sediments and organic matter; for water retention and flood mitigation; 

and as protective barriers against storms in 

coastal areas, among others. As of 2004, North 

America had almost 200,000 square kilometers 

of “Ramsar” wetlands, denoted as having international importance.

In the conterminous United States, almost 

half of all wetlands have been drained since 

European settlement. In Canada, only 14 

percent of wetlands have been lost over this 

period, primarily in southern Canada. In both 

countries, agricultural uses have accounted 

for about 85 percent of the historical loss. But 

agricultural conversion has slowed in recent 

years, and urban and suburban development 

has become a more important driver of wetland loss. Mexico’s wetland area is estimated 

at 36,000 square kilometers, and the historical 

loss is estimated to be 16,000 square kilometers. Much of Mexico’s wetlands are found in 

coastal areas, where they are pressured by oil 

infrastructure, urban and tourist development, 

livestock production and aquaculture.

Urban Areas 

Human settlements such as cities, towns and 

suburbs vary widely in density, form and disSource: Center for International Earth Science Information Network and Wildlife Conservation, Last of the Wild Data Version 2, 2005 (LWP-2): 

Global Human Influence Index.

Human influence index for North America (2000) 

Value

High: 64

Low: 0

Scale

0 500 1,000 km

Human influence index
Land Use 25

tribution. Urban settlements, as they have 

been defined by the census bureaus of Canada, 

Mexico and the United States, contain 75–80 

percent of the population of the continent. 

Determining the extent of human settlements 

across North America presents a challenge 

because definitions of such settlements vary 

greatly, particularly among nations. However, 

one estimate, based on satellite imagery of 

nighttime lights, puts North America’s human 

settlement area at almost 5 percent of the 

total continental land area. With settlement 

and urbanization, there has been an increase 

in the construction of impervious surfaces, 

which reduce the absorption of water on-site 

and groundwater recharge and increase storm 

water diversion, flows and impacts on surface 

water systems.

Because both the majority of North America’s population and its best agricultural land 

generally occupy the same regions, urbanization and sprawl have also led to the loss of agricultural land. Over the last 30 years, about half 

of the area transformed to urban uses in Canada was once agricultural land. In the United 

States, of the more than 36,400 square kilometers of land developed between 1997 and 2001, 

20 percent came from cropland, 46 percent 

from forestland and 16 percent from pastureland. In recent years, the extent of developed 

land (urban and industrial) in the United States 

has increased rapidly. More specifically, from 

1982 to 2002 the area of developed land grew 

at a rate of 47 percent, almost twice the rate of 

population growth. In Mexico, 995 square kilometers were converted to urban uses between 

1993 and 2000.

The expansion of low-density suburban 

and rural developments are associated not 

only with the loss of prime agricultural land, 

but also with the fragmentation and loss of 

forests, wetlands, grasslands and other wildlife 

habitats and the associated loss of biodiversity. 

Development in rural areas has also increased 

the risk of “interface” fires, which are associated with the intermingling of settlements with 

flammable forests and grasslands.

What Are the Linkages to Other North 

American Environmental Issues? 

Changes to land cover threaten biological 

diversity, contribute to climate change and 

alter the functioning and provision of ecosystem services.

Biodiversity

Habitat loss is the single greatest threat to biodiversity. When habitat is lost or fragmented, 

species that depend on this habitat experience 

a variety of pressures that ultimately lead to 

reduced species populations. During the last 

200 years, North America has experienced 

dramatic transformations of ecological systems 

and significant changes in the abundance of 

species. As nations have sought to find solutions to transportation, housing, energy and 

other material needs, the natural environment 

has been subjected to pressures arising from 

land cover conversion, habitat fragmentation 

and pollution. At the same time, protected areas 

have been established in an attempt to preserve 

valued species and natural spaces. 

Currently, 16 percent of North America 

is covered by nationally designated protected 

areas. In some areas, this legal protection status has reduced the extent of human impacts 

in populated regions. Elsewhere, remoteness, 

Source: Commission for Environmental Cooperation and Center for International Earth Science Information Network and Wildlife Conservation, Last of the Wild Data Version 2, 2005 (LWP-2): Global Last of the Wild.

Protected areas of North America (left) and areas of least human influence (right)

Decisions about land use can have significant 

effects on the contribution of human activities 

to the emissions of greenhouse gases associated 

with climate change. 

Protected area

Scale

0 500 1,000 km

Least human 

influence

Scale

0 500 1,000 km
26 Land Use

terrain and climate have provided large expanses 

of territory with de facto protection from direct 

human influence (see maps). However, as the 

climate changes the impacts of human activity 

will be felt directly and indirectly in even the 

most remote areas. 

Climate Change

Decisions about land use can have significant 

effects on the contribution of human activities 

to the emissions of greenhouse gases associated 

with climate change. For example, decisions 

about the extent and patterns of human settlement have had important and long-lasting 

implications for transportation and its associated greenhouse gas emissions. Likewise, 

decisions affecting the protection of North 

American forests affect, in turn, the service 

provided by those forests as a carbon sink—

they have drawn in some 269 million metric 

tonnes of carbon per year over the last decade 

or so. Indeed, North American forests contain 

more than 170 billion tonnes of carbon, of 

which 28 percent is in live biomass and 72 percent in dead organic matter. Most of the current 

net removal of carbon from the atmosphere 

and its storage in vegetation and soil is not a 

product of deliberate management practices, 

but instead can be attributed to a combination 

of past management and the response of terrestrial ecosystems to environmental changes. 

The substantial carbon removals by the forests 

of Canada and the United States result largely 

from the abandonment of agricultural land and 

subsequent regrowth of shrubs and trees. 

Water Quality and Quantity 

Land use activities often affect water quality and hydrology. For example, deforestation may lead to greater susceptibility 

to flash flooding and sediment loading in 

nearby streams. Urban development creates 

large volumes of excess storm water runoff, 

which can cause flooding, add pollution, create groundwater recharge deficits and alter 

stream ecology. Development of rural areas 

also has impacts on stream flows, altering 

aquatic ecosystems and their ability to maintain habitat and sediment balance. Some of 

the common impacts of changes in land use 

on water quality include increased organic 

matter and biological oxygen demand, 

changes in stream temperature and sediment 

load, salinization, changes in water flow and 

loadings of toxic chemicals, including pesticides and fertilizers. 

Case Study – North American Grasslands

Source: Commission for Environmental Cooperation.

The natural prairie of central North America is a transboundary ecological region shared 

by Canada, Mexico and the United States. The prairie grasslands are an immense, contiguous geographic region (see map) with a wide variety of species, land uses and cultural and social practices, as well as economic conditions and political-administrative 

regimes. The North American prairie is one of the planet’s largest biomes. 

The northern grasslands are North America’s most productive breeding grounds 

for aquatic birds, featuring species characteristic of both the eastern and western 

regions of the continent. The prairies maintain resident bird populations, in addition 

to providing nesting sites and stopover sites for migratory species. More than half 

the nesting ducks and many other grassland-dependent wildlife species in the United 

States depend on this crucial habitat. This region is also home to the largest known 

populations of certain species of hummingbirds, orioles, buntings, warblers, quail 

and thrashers. The southern grasslands are known for their varied mosaic of species, 

including 23 percent of the more than 1,500 cactus species found worldwide. 

Unfortunately, this ecosystem has suffered extensive deterioration over the last 

150 years. In the United States, less than 10 percent of the native Tallgrass prairie 

remains as grassland; 71 percent has been converted to cropland and 19 percent to 

urban areas. The main causes of the extensive loss of grassland habitats are changes 

in land use, such as the historical conversion to farmland or pasture, chemical pollution from farming, overuse of aquifers and unsustainable ranching practices. Extensive 

cattle raising in such a fragile region often has a negative impact on vegetation and 

soil properties and characteristics, and thus on the survival of multiple plant and animal species. The diminished grassland coverage also increases the area’s vulnerability to wind erosion, which reduces its suitability as a wildlife habitat. Soil compaction 

impedes natural recovery and leads to desertification. Other major threats include gas 

and oil drilling; urbanization with the associated highway networks, population density and groundwater overutilization; the growing presence of invasive species; and 

the increasing aridity arising from climate change.

Scale

0 500 1,000 km

North American 

grasslands
 The North American Mosaic: An Overview of Key Environmental Issues 27

Key Findings

n Oceans and coastal regions are critical 

to the social and economic well-being 

of North America. The continent’s 

coastal and offshore marine ecosystems 

are home to a remarkable diversity of 

species, including marine mammals, 

fish, invertebrates and plants. Coastal 

regions also lay claim to some of the 

continent’s highest population densities 

and rates of population growth.

n North America’s oceans and coastal 

regions provide a wide range of goods 

and services, such as fisheries, trade 

routes, recreation and tourism, and oil 

and gas production. Direct human uses, 

coupled with the impacts of climate 

change, affect the condition of these 

ecosystems and their biodiversity. 

n The diversity of coastal ecosystems and 

the variability of human activity affecting 

them complicate efforts to summarize 

overall conditions. In general, coastal 

ecosystems are subject to substantial 

cumulative impacts from human 

activities, particularly from development 

and land use change. Offshore, the 

productivity of certain fisheries has been 

greatly reduced by harvesting pressures.

n Despite progress using ecosystem 

approaches to management and the 

precautionary approach, trends in North 

America show continuing overfishing 

in some areas and ongoing challenges 

related to land use change, habitat 

damage and bycatch. Climate change 

is expected to impose additional stress 

on marine and coastal ecosystems, with 

particularly profound effects in the Arctic 

Ocean from both environmental changes 

(such as a shrinking sea ice cover) and the 

associated increase in economic activity.

What Is the Environmental Issue? 

Coastal and marine ecosystems support some 

of the most productive and valuable habitats in 

the world, including estuaries, coastal wetlands, 

beaches, mangrove forests, seagrass meadows, 

coral reefs, sea mounts and upwelling areas. 

In the three North American countries, these 

ecosystems extend up to 100 kilometers inland 

and fully across their marine jurisdictions. The 

health of these habitats depends on the quality of 

the ecosystems’ physical and chemical processes 

and associated biological communities. Their 

degradation and loss affect the viability and productivity of invaluable natural resources.

Globally, coastal areas produce disproportionately more ecosystem services than 

most other geographic areas, even those 

with a larger total geographic extent. At the 

same time, these ecosystems are experiencing the most rapid environmental change. 

During the last few decades over a third of 

mangroves have been lost or converted in 

countries that monitor these areas. Similarly, 

approximately 20 percent of coral reefs have 

been destroyed and an additional 20 percent 

or more degraded globally. In some countries, 

the decline of coastal wetlands is reaching 20 

percent a year. In the Arctic Ocean, the effects 

of climate change on marine and coastal areas 

are already evident or are expected. These 

effects include loss of sea ice cover, shoreline 

erosion, flooding caused by rising sea levels 

and melting permafrost.

Oceans and Coasts

Oceans—the continuous saltwater bodies that cover more than 70 percent of the earth’s 

surface—shape its climate, provide a means of transport and are home to an important 

part of the planet’s biodiversity. Oceans and coasts—where land meets the sea—provide a 

wide range of valued goods and services, including fisheries, trade routes, recreation and 

tourism, oil and gas production and ecological diversity.

Biodiversity and Ecosystems

Miami, Florida, USA.
28 Commission for Environmental Cooperation

A recent global analysis of the cumulative 

effects of human activities on the oceans found 

that ecosystems with the highest predicted 

cumulative impact scores are the hard and soft 

continental shelves and rocky reefs. Almost half 

of all coral reefs were categorized as experiencing medium-high to very high impacts. Shallow 

soft-bottom and pelagic deepwater ecosystems 

experienced the lowest impact because of the

lower vulnerability of these ecosystems to most 

anthropogenicdrivers. Overall, the results highlight the greater cumulative impact of human 

activities on coastal ecosystems (see map).This 

analysis does not, however, fully account for the 

emerging pressures in Arctic coastal ecosystems 

from the effects of climate change. 

Why Is This Issue Important to 

North America?

Marine resources and coastal areas are important contributors to North America’s social and 

economic well-being. Indeed, the continent’s 

coastal areas are characterized by some of its 

highest population densities and rates of population growth. As of 2000, 36 percent of the 

total North American population lived within 

100 kilometers of the coast.

Fisheries

Fisheries play a critical role in North America’s 

coastal economies, but ongoing improvements 

are needed in the management of these resources 

to ensure their long-term sustainability.

Canada harvests more than 100 commercially valuable species of fish. In 2004 it ranked 

as the sixth-largest exporter of fish and fishery 

products in the world, generating revenues 

of over C$2 billion. The major marine finfish 

species in Canada are hake, redfish, cod, herring, salmon and capelin; shellfish products 

include shrimp, scallop, lobster and snow crab. 

However, fisheries in Canada are not without 

problems. Many of the groundfish fisheries off the Atlantic coast and Pacific salmon 

stocks have declined precipitously. To ensure 

sustainability and manage the impacts of fishing on sensitive areas, Canada is adopting a 

precautionary and ecosystem-based management approach to fisheries. In 2004 Canada 

announced a vision for a renewed fishery sector that aims to improve the economic and 

biological performance of Canadian fisheries 

based on these principles. 

Mexico is one of the top 20 seafood producers in the world, contributing 1.5 percent 

of total world fisheries production by weight. 

Mexico’s marine ecosystems also offer other 

economic benefits that are even more valuable—the coral reefs, clear tropical waters 

and white sand beaches that serve as a draw 

for Mexico’s lucrative tourism industry. And 

yet industrial and coastal development, agriculture and tourism have all strained Mexico’s 

ecosystems, and it has not been able to maintain the nearly exponential growth in fisheries that occurred in the final decades of the 

last century, in part because of the collapse 

of the anchovy fishery. Stagnant or declining 

catches, overexploitation of socially important 

species for artisanal fisheries, and a scarcity of 

rural development alternatives have created 

challenges, especially in places where fisheries 

have great local importance such as Sinaloa 

and Sonora.

The United States is the third-largest seafood producer country in world behind China 

and Peru, based on the value of marine and 

inland capture fisheries. As of 2004, the 

United States was the fourth-largest exporter 

and second-largest importer of fish and fishery products by value. Overall, the status of 

some US fishery stocks have improved, and 

others have declined. The number of stocks 

considered “overfished” increased from 43 in 

2005 to 47 in 2006. Stocks that are overfished 

have biomass levels below biological thresholds specified in their fishery management 

plans. The number of stocks “subject to overfishing” increased from 45 to 48. A stock that 

is subject to overfishing has a fishing harvest 

rate above the level that provides for the maximum sustainable yield. The majority of the 

530 assessed stocks in the United States are 

either not overfished (75 percent) or subject to 

overfishing (80 percent). In the United States, 

legislation signed in 2007 contains significant 

new provisions to end overfishing, promote 

market-based approaches to fisheries management, improve fisheries science, enhance 

international cooperation and address illegal, 

unreported and unregulated fishing, as well as 

bycatch of protected species.

Over time, the significant fishing industries in all three North American countries 

have experienced declining production. In 

2004 North American commercial landings 

were over 7.6 million tonnes, a reduction of 

almost 20 percent since 1990 (see graph).

In North America, fishing pressures 

have been particularly acute in the northThousands of tonnes, live weight

 Canada Mexico United States

10,000

9,000

8,000

7,000

6,000

5,000

4,000

3,000

2,000

1,000

0

North American catches

Source: Fisheries and Oceans Canada.

1990 2004

Atlantic salmon smolts. Photo: US Fish and Wildlife Service.
Oceans and Coasts 29

eastern regions, but these pressures have also 

been felt in the Gulf of Mexico and Caribbean, in the Gulf of California and on the 

West Coast. Affected species include Atlantic cod, Atlantic salmon, haddock, yellowfin tuna, flounder, grouper, red snapper and 

others. Overfishing affects not only target 

stocks, but also a wide array of species in the 

food web, and can cause cascading ecological 

effects that change the nature of marine ecosystems, sometimes permanently.

Habitat Damage and Bycatch 

Overfishing is not the only problem. The unintended harmful effects of human activities in 

the oceans, including fishing impacts on habitats and fishing incidental take are also a concern. Habitat damage includes that to living 

seafloor structures as well as alterations to the 

geologic structures that serve as nursery areas, 

refuges and homes for fish and organisms living 

in, on or near the seafloor. This damage reduces 

the ability of marine ecosystems to sustain fisheries. Bottom gears such as dredges and bottom 

trawls are associated with high levels of impact 

on certain types of habitat.

Bycatch refers to the incidental take 

of fish, other vertebrates and invertebrates 

not targeted by fishing gear and that may be 

retained or discarded alive, injured or dead. 

Currently, almost one-quarter of what is 

caught by global fisheries is discarded at sea 

each year. Although nontarget species generally have little or no commercial value, they 

can become entangled or hooked accidentally 

during the capture of targeted species such as 

shrimp, swordfish or tuna. Fisheries bycatch 

has been implicated as an important factor in 

the decline of many populations of protected 

species, including loggerhead and leatherback 

sea turtles, albatrosses and petrels, sharks and 

marine mammals such as the vaquita porpoise (see case study). Like those associated 

with habitat damage, these losses can have 

cascading effects through marine ecosystems. 

Although no management strategy has yet succeeded in eliminating bycatch, effective mitigation approaches have been proposed and 

adopted in some cases. Examples are accounting for bycatch in fishing quotas and installing 

equipment such as turtle excluders, streamer 

lines to reduce the catch of seabirds and finemesh net aprons to avoid entangling dolphins. 

Canada, Mexico and the United States participate in domestic and international initiatives 

to address bycatch.

What Are the Linkages to Other North 

American Environmental Issues?

Oceans and fisheries are not, as once thought, 

vast and inexhaustible resources immune to 

human activity. In fact, oceans and the coastal 

areas that border them are linked through 

important ecological processes. 

Land Use and Habitat

The ability of coastal systems to provide highly 

valued services is not limited to the marine area 

in question. Ocean health is intimately linked to 

that of adjacent marine, freshwater and terrestrial ecosystems, and vice versa. Port development, urbanization, resort development, urban 

sprawl, aquaculture and industrialization can 

destroy coastal forests, wetlands, coral reefs and 

other habitats. Dredging, reclamation and engineering works also account for widespread, usually irreversible destruction. Large segments of 

North America’s coastal areas are at risk of development-related habitat conversion and decline. 

Impacts of human activities on North America’s marine ecosystems

Source: Adapted from Benjamin S.Halpern, “A Global Map of Human Impact on Marine Ecosystems,” Science 15 February 2008, 948–952.

The continent’s coastal areas are characterized 

by some of its highest population densities 

and rates of population growth. As of 2000, 

36 percent of the total North American population 

lived within 100 kilometers of the coast.

Very low impact (<1.4) 

Low impact (1.4 – 4.95)

Medium impact (4.95 – 8.47) 

Medium high impact (8.47 – 12)

High impact (12 – 15.52)

Very high impact (>15.52)

Scale

0 500 1,000 km

Hawaii (US)
30 Oceans and Coasts

The loss of wetlands, coastal sands and mudflats 

also has implications for fisheries, because many 

of these areas provide critical nursery habitat 

for valuable marine species. In the Arctic, the 

effects of climate change on coastal habitats are 

expected to be particularly pronounced.

Water Quality

Marine and coastal ecosystems play an important role in maintaining water balance and 

providing freshwater for human consumption. 

Freshwater is also the main link between land 

use and the provision of coastal ecosystem 

services. Land-based sources of pollutants are 

delivered via rivers, from runoff and through 

atmospheric deposition. Logging of forested 

areas contributes to erosion and sedimentation, leading to estuarine decline in coastal 

and marine ecosystems. This pressure reduces 

the available feeding and nursery habitats for 

many marine species. Agriculture introduces 

harmful fertilizers, nutrients and toxics into 

coastal ecosystems. Polluted waters entering 

the marine environment cause degradation, 

lead to loss of ecosystem services and often 

pose human health issues. The removal of buffers such as riparian and estuarine wetlands 

compounds the problem by reducing the natural waste management that these ecosystems 

provide. Diversion of freshwater from estuaries 

results in losses of water and sediment delivery 

to nursery areas and fishing grounds.

Climate Change

The condition of the oceans and climate change 

are inseparable—not only because marine and 

coastal systems suffer the effects of climate 

change, but also because oceans drive both 

climate and weather. Global climate change 

imposes additional stress on coastal and marine 

systems that have been degraded by chronic 

multiple impacts and may impede the resilience 

of marine and coastal ecosystems. Coastal systems are simultaneously vulnerable to rises in 

sea level, erosion and acute storm events.

Although all oceans are susceptible to the 

impacts of climate change, the relatively pristine 

Arctic Ocean is particularly vulnerable. Rising 

temperatures are already rapidly and profoundly 

affecting sea ice cover, ocean processes and 

coastal habitat integrity. When those effects are 

coupled with the associated increase in natural 

resource development and shipping activities, 

North Americans may find that the Arctic will 

require considerably more effort to protect the 

integrity of its marine ecosystems and the communities that depend on them. 

Case Study – The Vaquita

The vaquita (Phocoena sinus), a small porpoise endemic to the northern Gulf of 

California, Mexico, is the most critically endangered marine small cetacean in the 

world—it is estimated that the population numbers only about 150. The vaquita is 

threatened primarily by the fishing gill nets used to catch fish and shrimp. Although 

other risk factors are the trawling that affects vaquita behavior and the uncertain 

effects of dam construction on the Colorado River and the resultant loss of freshwater 

input to the upper Gulf, entanglement is the clearest and most immediate concern. 

Captive breeding is not feasible for vaquitas because of the difficulty in capturing 

these small, solitary elusive animals in relatively deep water and the complete lack of 

experience with this species in captivity. 

In only a few years, the options for conserving the vaquita will be severely reduced. 

Although conservation groups, concerned scientists and government officials in Mexico 

have invested significant time and financial resources in vaquita conservation over the 

last 25 years, progress toward reducing entanglement has been slow, in spite of efforts 

to phase out gill nets in the vaquita’s core range and provide compensation schemes 

for fishermen. Indeed, the Biosphere Reserve in the northern Gulf has fallen far short 

of its potential for vaquita conservation. On 29 December 2005, Mexico declared the 

area in which about 80 percent of verified vaquita sightings had been made a Vaquita 

Refuge. In the same decree, the state governments of Sonora and Baja California were 

offered $1 million to compensate affected fishermen. The effectiveness of this major 

initiative remains to be seen. 

 Photo: Omar Vidal.
 The North American Mosaic: An Overview of Key Environmental Issues 31

Key Findings

n Biological invasion and the 

diversity and abundance of 

species spreading beyond their 

natural ranges are at the highest 

rates ever recorded with serious 

consequences for the environment, 

economy and human health. 

n A significant increase in the 

introduction of non-native species 

into and within North America has 

been an unintended consequence 

and cost of the growing scale of 

global trade, travel, and transport 

since the early 1900s. 

n Individual invasive species 

have already had profound and 

quantifiable negative impacts on 

the environment, economy, industry, 

infrastructure, human health and 

ecological function in North America. 

Climate change is making northern 

ecosystems more receptive to 

invasive species because of milder 

winters creating the potential 

for a significant increase in the 

introduction of these species.

n The issue of invasive species is 

recognized and the spread of certain 

individual species is monitored on 

a regular basis but comprehensive 

trend indicators are not available 

for major biomes (i.e., terrestrial, 

freshwater, marine/estuarine) or 

North America as a whole.

What Is the Environmental Issue?

Invasive species are a significant environmental 

challenge. At no time in history has the rate of 

biological invasion, and the diversity and volume of invaders, been so high and the consequences so great.

Today, goods, services and people are 

on the move worldwide. These international 

movements and transactions have brought 

social and economic benefits to many people 

in North America, but they also have brought 

new challenges. The growing rate and scale 

of global trade, travel and transport since 

the early 1900s have been accompanied by 

an exponential increase in the introduction 

of non-native species into and within North 

America. At times, non-native species are 

introduced intentionally for use in a broad 

range of industries such as agriculture, aquaculture, horticulture and the pet trade. But 

they also may arrive as inadvertent “hitchhikers” via imported plants and livestock, travelers and their baggage, manufactured goods, 

packaging materials and conveyances such as 

airplanes and ships in their ballast water or 

on their hulls (see table, which presents some 

common pathways of biological invasion).

North America’s intracontinental transportation systems are vast. They include 7.5 

million kilometers of roads, thousands of kilometers of navigable waterways and railroads, 

extensive coastal shipping routes and nearly 

half of the world’s airports. Once on the continent, invasive species can spread along roads 

and waterways and hitchhike on vehicles, in 

baggage and among cargo shipments, while 

marine/estuarine invaders can travel via intracoastal shipping or be transported by currents. Because invasive species do not respect 

political boundaries, species that invade one 

country have the potential to spread within a 

Invasive Species

An invasive species is a plant, animal or microscopic pathogen that, once transported out 

of its native range, has established itself, spread and caused harm to the environment, 

economy or human health in its new habitat.

Biodiversity and Ecosystems

Source: Adapted from National Invasive Species Council

Pathway categories

Living commodities

(biological species 

intentionally introduced)

Via living commodities 

that are intended for 

release into the environment

For example:

Horticultural plants

Crops

Stocked fish

Game animals

Living commodities

(biological species 

unintentionally introduced)

Via living commodities that 

are not intended for release 

into the environment

For example:

Fruits and vegetables 

for consumption 

Pets

Laboratory animals

Animals for zoos and public 

aquaria displays

Transport mechanisms

Via conveyances, 

packaging, equipment, 

and non-living commodites

For example:

Ship hulls

Ballast water

Solid wood packaging 

(pallets, dunnage)

Military equipment

Clay tiles
32 Commission for Environmental Cooperation

region. For example, both zebra mussels and 

quagga mussels were unintentionally introduced to North America through shipping. 

These invasive mussels were first discovered 

in the Canadian waters of Lake St. Clair 

near Detroit in 1988. They have since spread 

throughout watersheds across the continent 

with negative impacts on aquatic environments and local economies (see map, which 

depicts sightings of these invasive mussels in 

the United States and Canada). Recreational 

boats are major vectors in freshwaters to 

redistribute these mussels and other freshwater invasive species once they have invaded 

North America. Ballast water and hull fouling 

are important transport mechanisms for the 

introduction and spread of marine and estuarine species, especially in the Great Lakes and 

coastal estuaries. 

Once invasive species are introduced, 

ongoing changes in land use, climate and 

freshwater and marine ecosystems can facilitate biological invasion by making habitats 

more challenging for native species and more 

hospitable to invasive species. Because disturbed habitats often favor rapid colonizers, 

they are particularly vulnerable to the invasion of non-native species. From the perspective of the invasive species, it does not matter 

whether the environmental changes are natural or human-induced (see box). 

Why Is This Issue Important to 

North America?

With its many linkages to the global economy, North America is extremely vulnerable 

to the introduction of invasive species from 

abroad. Likewise, species native to Canada, 

Mexico and the United States can be spread 

via the international movements of people 

and trade goods to other countries (within 

North America and beyond) where they can 

become invasive. As trade, transport and 

travel expands, so do many of the risks associated with biological invasion. The environmental and economic consequences of 

invasive species can be significant. Climate 

change is increasing the risk of introduction 

of harmful non-native species.

Environmental Implications

Permanent elimination of native species unique 

to North America is one of the issues at stake. 

In the United States, invasive species rank second to habitat modification as a cause of species 

endangerment, and they are the primary driver 

of extinctions in island ecosystems, as well as 

many freshwater systems worldwide. Extinction of native species can result from a single or 

multiple impacts from invasive species, including competition for food, space, or reproductive sites; increased predation; and/or parasites 

and diseases for which native species have no 

defense. Invasive species can also degrade ecoSource: US Geological Survey.

Distribution of zebra and quagga mussel sightings in 

United States and Canada (March 2008)

Zebra mussel occurrences

Quagga mussel occurrences

Both species present

Zebra mussels trailered 

overland on boat hulls

Here are a few examples of invasive “hitchhikers” that have already had profound negative 

impacts on the environment, economy, industry, infrastructure and human and animal health 

in North America: 

Asian carp (Hypophthalmichthys nobilis, H. molitrix and others): environment

Asian longhorned beetle (Anolophora glabripennis): environment, industry

Asian tiger mosquito (Aedes albopictus): human and animal health

Brown tree snake (Boiga irregularis): environment, infrastructure, human health

Chytrid fungus (Batrachocytrium dendrobatidis): environment

Dutch elm disease (Ophiostoma ulmi): environment, industry

Emerald ash borer (Agrilus planipennis): environment, industry

European green crab (Carcinus maenas): environment, industry

Giant African snail (Achatina fulica): environment, industry, human health

Gypsy moth (Lymantria dispar): environment, industry

Norway rat (Rattus norvegicus): environment, infrastructure, human and animal health

Red imported fire ant (Solenopsis invicta): environment, human health

As of 2006, Mexico’s Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (Conabio) 

had identified at least 800 invasive species in Mexico, including 665 plants, 77 fish, 

2 amphibians, 8 reptiles, 30 birds and 6 mammals. In Canada, invasive alien species include 

at least 27 percent of all vascular plants, 181 insects, 24 birds, 26 mammals, 2 reptiles, 

4 amphibians, several fungi and mollusks, and 55 freshwater fish. Although extensive data on 

individual species are available, similar totals are not currently available for the United States.

When Prevention Fails . . .
Invasive Species 33

system functions and the production of ecosystem services, from food production to aesthetic 

value. Even the most well-protected natural 

areas are not immune to biological invasion. 

Predicting ecological impacts is made all the 

more difficult in that the effects of invasive species may be evident immediately or observable 

only after many years. 

Economic 

Invasive species can take a heavy financial toll 

on governments, industries and private citizens. Economic losses can be a direct cost such 

as lost or reduced crop production, or an indirect loss of tourist dollars from reduced quality reefs or sport fisheries. Globally, the economic losses from invasive species have been 

estimated at US$1.4 trillion a year. The cost 

to the United States is more than $100 billion 

a year. In Canada, the damage to agricultural 

crops and forestry from harmful invasive plant 

pests has been estimated at C$7.5 billion annually, and in the Canadian province of Manitoba 

alone, the economic losses from Dutch elm 

disease have been estimated at $30 million. 

Another type of direct economic impact is the 

cost to meet existing or proposed national and 

international regulations, such as the proposed 

requirement for ballast water treatment for all 

new ships under the International Maritime 

Organization ballast water treaty. The impact 

and management costs of a single species can 

carry a substantial price tag (see box for examples). If indirect costs such as loss of ecosystem 

services were also counted, these estimates 

would be substantially higher.

Human Health

The consequences of invasive species for human 

health can be direct from exposure to new diseases and parasites or indirect from higher and 

more frequent exposures to pesticides necessary to eradicate and control invasive species. Pathogens and parasites may themselves 

be invasive species or may be introduced by 

invasive vectors such as non-native mosquitoes. Cholera and some of the microorganisms that can cause harmful algal blooms are 

relocated and released in the ballast water carried by large ships. Other high-profile diseases 

Case Study – The Cactus Moth

The cactus moth (Cactoblastis cactorum) 

reproduces and feeds on cacti in the 

genus Opuntia, commonly known as 

prickly pears. Originally from South 

America, the cactus moth has been 

introduced around the world as a 

biocontrol agent for invasive cacti. 

In 1989 the moth was discovered in 

the Florida Keys, likely having arrived 

either by natural wind dispersal or 

on imported horticultural prickly 

pear cacti from the Caribbean. 

Since that time, the moth’s range 

has expanded northward along the 

Atlantic and Gulf coasts of Florida, 

despite active control efforts. This 

expansion has put North America’s 

native Opuntia at great risk.

Mexico is a hotspot of prickly pear diversity, with 38 endemic species covering 3 

million hectares. The United States also has 31 species of prickly pear, nine of which 

are endemic, including Florida’s extremely rare Opuntia corallicola. Numerous species 

of birds, bats, mammals and insects depend on prickly pears for food and habitat, and 

the cacti provide erosion control for fragile desert soils.

Aside from its enormous consequences for North American biodiversity, the cactus moth threatens the agriculture, landscaping and ranching industries. In 2000 the 

value of ornamental prickly pear cacti used for xeriscaping, or dryland gardening, in 

Arizona amounted to US$14 million a year. Prickly pear pads (nopales) and fruits (tuna) 

are the seventh-largest agricultural crop in Mexico, where they are frequently gathered 

from the wild to supplement dietary intake. A national symbol, the prickly pear is featured on the Mexican flag and currency.

In a display of international cooperation, the Mexican government has funded 

US Department of Agriculture efforts to halt the westward spread of the cactus moth. 

However, in 2006 the moth was discovered on the Mexican island of Isla Mujeres 

(9 kilometers from Cancún on the mainland), and the Mexican government is now 

attempting to eradicate it through an extensive trapping program.

Larvae of the invasive cactus moth. 

Photo: Peggy Greb, http://www.ars.usda.gov/is/graphics/photos/sep06/d588-2.htm.

The Costs of Control and Eradication

The Formosan termite (Coptotermes 

formosanus), introduced into the southeastern United States from East Asia, 

is an expensive visitor: an estimated 

US$1 billion a year is spent on property 

damage, repairs and control measures.

The European gypsy moth (Lymantria dispar), 

introduced into North Carolina in 1993 

and eradicated four years later, carried a 

$19 million price tag.

The Great Lakes Fishery Commission—

jointly administered by the Canadian 

and US federal governments—spends 

about $22 million a year to control the 

sea lamprey (Petromyzon marinus).

Researchers estimate the zebra mussel 

(Dreissena polymorpha) cost the power 

industry alone US$3.1 billion in the 1990s, 

with an impact on industries, businesses 

and communities of over $5 billion. 

In Canada, Ontario Hydro has reported that 

the zebra mussel has cost each generating 

station $376,000 a year.

The cost of eradicating one or more 

introduced mammals from 23 islands 

off the coast of northwest Mexico was 

about $750,000.
34 Invasive Species 

caused by invasive pathogens are malaria, dengue fever and the human immunodeficiency 

viruses that cause AIDS. Less well-known 

diseases can also be problematic. For example, 

the giant African snail, a potential food source 

as well as a pet, provides an intermediate host 

for rat lungworm, which can infect the human 

brain, causing headache, fever, paralysis, coma 

and even death.

What Are the Linkages to Other North 

American Environmental Issues? 

Invasive species are associated with a range of 

continent-wide environmental issues. Rapidly 

changing environmental conditions will tend 

to increase the diversity, spread and impacts of 

invasive species. 

Climate Change 

Climate change is likely to increase both the rate 

of new invasions into North America as well as 

promote the spread of invasive species already 

established. Stress on natural environments, 

such as that caused by climate change, may 

decrease their ability to resist biological invasion. Climate change is likely to increase the 

opportunities for invasive species to establish 

themselves after a storm or fire. Warmer temperatures or changes in rainfall patterns may 

enable certain species to expand their ranges 

and occupy new roles in ecological systems. 

Changes in the direction and strength of airflow could influence the spread and migration 

of airborne species, such as flying insects, while 

changes in near-shore currents could affect the 

distribution of marine/estuarine invaders. 

Land Use

Because many invasive species are fast-growing, highly opportunistic ecological generalists, land use change generally favors biological invasion. For example, road building, edge 

maintenance for roads and power transmission 

corridors, and logging can open new areas to 

invasive species and facilitate their spread via 

equipment and workers. Agricultural activities 

can introduce invasive species into new areas 

through seed contamination and crop “escapes.” 

And abandoned agricultural areas may be 

invaded by invasive species before natural succession can restore the local plant community. 

In urban and suburban environments, gardeners introduce non-native species for gardening 

that may spread into natural environments by 

means of “green space” corridors.

Water

Invasive species can place significant limits on 

the availability of potable water, as well as surface water for use by wildlife. Certain species 

such as pines and eucalyptus can draw down 

water tables and negatively influence regional 

water cycles. By affecting nutrient cycling, 

aquatic invasive species can promote eutrophication or the growth of undesirable algae. Invasive aquatic plants can choke waterways and 

trap sediment, causing the aquatic system to 

stagnate and eventually fill in. Stagnation can 

also increase the risk of disease such as West 

Nile virus by fostering mosquito populations.

Energy

Governments around the world are investing in biofuel energy production. Many of 

the characteristics that make plants good 

biofuel candidates—such as rapid growth 

rates and tolerance of disturbed environments—are the same characteristics that 

make a plant an effective invader. In fact, 

several species of invasive plants have been 

proposed for biofuel production in North 

America. The risk then becomes the potential escape of non-native species used for 

biofuel into the natural environment. 

Case Study – Tree Pests

In the United States and Canada, 

the recent appearance of several 

invasive species that are threatening forests and forest product 

industries turned a spotlight on 

a neglected pathway: solid wood 

packaging materials, including 

the crates, pallets and dunnage 

used to transport various commodities. These materials can 

harbor the eggs, larvae and adult 

forms of bark- and wood-boring insects. Recent examples of serious pests that may have 

been introduced through untreated packaging materials are the emerald ash borer (Agrilus 

planipennis) and the Asian longhorned beetle (Anoplophora glabripennis). 

The emerald ash borer was first discovered in 2002 on infested ash trees in Detroit, 

Michigan, and neighboring Windsor, Ontario, but apparently it arrived undetected and 

became established over a decade earlier. Native to China and eastern Russia, the 

beetle feeds on ash trees, killing them in the process. Ash trees are an important part 

of North American forests, providing food for numerous species of wildlife, and are a 

popular street tree in many midwestern US and Canadian cities. The emerald ash borer 

has spread into Ohio, Indiana, Illinois and farther into Michigan and Ontario, killing 

over 15 million ash trees in southeastern Michigan alone. Hitchhiking on nursery shipments, lumber and firewood, it has repeatedly escaped the quarantine areas set up by 

federal, state and provincial governments. Unfortunately, the prospects for successfully eradicating the emerald ash borer are not good. 

The Asian longhorned beetle was first discovered in New York in 1996, followed by 

detections in 1998 in Illinois, in 2002 in New Jersey and in 2003 in Ontario. The beetle 

attacks and kills many types of hardwood trees, including maples, and could drastically 

alter the region’s forests, as well as cost the forestry, landscaping, maple syrup and fall 

color tourism industries billions of dollars. This insect also could decimate 30 percent of 

urban street trees in the United States at a replacement cost of hundreds of billions of 

dollars. Since first detecting the insect, the US and Canadian governments have undertaken costly eradication efforts, requiring the removal of thousands of neighborhood, park 

and street trees. Although proceeding slowly, the eradication efforts have had promising 

results. Moreover, national, regional and international standards for fumigation and labeling are being developed to prevent invasive species from infesting packing materials. 

Asian longhorned beetle. Photo: US Fish and Wildlife Service 
 The North American Mosaic: An Overview of Key Environmental Issues 35

Key Findings

n North America’s species of 

common conservation concern are 

a group of migratory, transboundary 

and endemic species that Canada, 

Mexico, and the United States 

have identified from among the 

continent’s great wealth of wild 

flora and fauna as requiring 

cooperative attention for their 

effective conservation.

n North America is subject 

to pressures that affect the 

conservation of these species—

among them, climate change, 

land use and habitat conversion, 

invasive species and pollution.

n Across North America, almost 1,600 

species are critically endangered, 

endangered or vulnerable. The 

terrestrial and marine species of 

common conservation concern are a 

small but important sample of birds, 

mammals and reptiles selected for 

special conservation attention by 

the three countries.

n Some species have experienced 

population increases, while 

others are still declining in 

numbers. Although the status of 

individual species is assessed 

periodically, a North American 

trend indicator for this group as a 

whole is not available. 

What Is the Environmental Issue?

North America’s species of common conservation concern are a group of species selected from 

among North America’s great wealth of wild flora and fauna for special attention. Most of these 

species use or travel through a series of different 

habitats throughout North America, and thus 

can only be protected through the effective collaboration and action of multiple stakeholders.

Canada, Mexico and the United States 

share ecosystems that are home to species 

that move freely across their national borders. 

With that in mind, experts and representatives 

of the three countries’ federal wildlife services 

compiled a list of species whose conservation 

is of common concern and would require a 

regional approach. Priority was given to transboundary or migratory bird and mammal 

species that are endangered or threatened in 

one or more countries, extinct in at least one 

country, or warrant special concern, and to 

those likely to demonstrate the importance of 

trilateral or bilateral cooperation (see box for 

a list of the land species selected). 

In the selection process for marine species, 

priority was given to transboundary or migratory species that are at high risk of extinction 

Species of Common 

Conservation Concern

Species of common conservation concern are a group of North American migratory, 

transboundary and endemic species. As charismatic species, they were chosen for their 

ability to attract public attention and garner conservation resources. Conserving these 

species and their habitats requires regional cooperation. Successful conservation of these 

species will also have benefits for other species.

Biodiversity and Ecosystems

Golden-cheeked warbler, breeding adult male. Photo: Gene Nieminen, USFWS-NCTC.
36 Commission for Environmental Cooperation

because of current status or trends, inherent 

natural vulnerability or susceptibility to anthropogenic threats; are ecologically significant; 

are officially listed as being of conservation 

concern by one of the three North American 

countries, by the World Conservation Union 

or by the Convention on International Trade 

in Endangered Species; are capable of recovery 

or management; and have a high potential for 

public engagement. The list finally developed 

by the country teams focused on three taxonomic groups: marine mammals, seabirds and 

sea turtles (see box).

Migratory and transboundary species 

use or travel through a series of habitats in 

North America. Because of the large-scale migratory patterns and transboundary nature of 

these species, they depend on the continued 

availability of breeding and feeding habitats, 

as well as the important movement corridors 

and staging areas along the migratory routes 

linking the breeding and foraging grounds. 

The survival of many land and marine species 

of common conservation concern depends on 

the existence of ecosystems that are relatively 

intact. Changes to their status may point to 

deeper problems of biodiversity.

Why Is This Issue Important to 

North America?

Addressing the needs of these species requires 

paying attention to the root causes of biodiversity loss, especially landscape change and habitat loss on land and incidental take and habitat damage in the marine environment. The 

recovery of species that are migratory or have 

transboundary ranges is difficult or impossible 

without cooperation among the affected countries. Even endemic species may be affected by 

pressures originating outside the host country.

Categories of Species of Concern

The North American species of common conservation concern are a small group when compared with the almost 1,600 species that are 

critically endangered, endangered or vulnerable 

in North America (see graph), but they are important nevertheless. They include ecologically 

important species, flagship species, umbrella 

species, keystone species and indicator species, 

as well as species of taxonomic rarity and those 

having a high percentage of the global population located in North America.

Flagship species represent a wide range 

of taxa, different levels of risk and wide geographic spread. In essence, most are charismatic species—a trait that should help galvanize public attention and garner conservation 

resources. An example is the sea otter, one of 

the smallest marine mammals. Its captivating 

image appears on a variety of products from 

t-shirts to mouse pads and is well known to 

the general public. Another example is the 

vaquita, a small porpoise endemic to the 

northern Gulf of California, Mexico. The 

vaquita is threatened primarily by the fishing 

gill nets used to catch fish and shrimp that are 

consumed domestically and exported across 

North America.

American black bear. Photo: Steve Maslowski.

 Canada Mexico United States

Species in jeopardy in North America

Source: 2007 IUCN Red List of Endangered Species. Number of animal species

1,800

1,600

1,400

1,200

1,000

800

600

400

200

0

Extinct or extinct 

in the wild

Critically endangered, 

endangered, or vulnerable

Terrestrial Species of Common 

Conservation Concern

Birds

Burrowing owl (Athene cunicularia)

California condor (Gymnogyps californianus)

Ferruginous hawk (Buteo regalis)

Golden-cheeked warbler 

(Dendroica chrysoparia)

Loggerhead shrike (Lanius ludovicianus)

Mexican spotted owl 

(Strix occidentalis lucida)

Mountain plover (Charadrius montanus)

Northern spotted owl 

(Strix occidentalis caurina)

Peregrine falcon (Falco peregrinus)

Piping plover (Charadrius melodus)

Whooping crane (Grus americana)

Mammals

Black bear (Ursus americanus)

Black-tailed prairie dog

(Cynomys ludovicianus)

Gray wolf (Canis lupus)

Lesser long-nosed bat 

(Leptonycteris curasoae yerbabuenae)

Mexican long-nosed bat 

(Leptonycteris nivalis)

Sonoran pronghorn 

(Antilocapra americana sonoriensis)

Black-tailed prairie dog. 
Species of Common Conservation Concern 37

Umbrella species are those whose effective conservation will result in the protection of many other species that share the 

same habitat. For highly migratory animals 

such as the leatherback turtle, hawksbill 

turtle, loggerhead turtle, right whale, gray 

whale, pink-footed shearwater, short-tailed 

albatross and whooping crane, protection of 

umbrella species means protecting a whole 

suite of linked habitats—and the myriad organisms they support.

Keystone species play a pivotal ecological 

role in maintaining the biological diversity and 

structure of the food web. For example, removal of the sea otter would cause cascading 

effects that ultimately would result in the loss of 

kelp forests and associated communities. The 

hawksbill turtle also plays a keystone role—

preventing the domination of the reefs by fastspreading sponges. For keystone species, the 

risk of extinction implies broader communitylevel consequences. 

Species of common conservation concern 

may also act as indicators or biological barometers of how well or badly their host ecosystems 

are faring. Such is the case for the grasslands, a 

highly modified ecosystem under extreme duress, where a majority of the terrestrial species 

of concern make their home. 

What Are the Linkages to Other North 

American Environmental Issues? 

North America is subject to natural and anthropogenic pressures that affect the conservation of these and other species. 

Human Use of Terrestrial and Marine Ecosystems

The detrimental effects of changes in land 

use and habitat fragmentation on animal 

populations are well known. The destruction 

of land habitat may stem from factors such 

as conversion of natural habitat to agricultural or urban development, physical modification of rivers or water withdrawal from 

rivers. Habitat loss also occurs in coastal and 

marine systems. For example, trawling of the 

seabed can significantly reduce the diversity 

of marine habitats, and destructive fishing 

and coastal development can lead to losses of 

coral reefs. When habitat is lost, plant species 

and the associated community of animals 

whose habitat is largely determined by the 

composition of the native plant communities 

become extinct. Even more widespread than 

total habitat loss is habitat fragmentation. 

The smaller pieces of the original habitat are 

not large enough to maintain viable populations of some species.

Invasive Species

After habitat destruction and fragmentation, 

the introduction of invasive species is regarded 

as the greatest threat to the continuity of biodiversity. Invasive species compete with native species primarily for space and food, and 

Two Kemp’s ridley hatchlings, Padre Island, Texas, USA. 

Photo: NPS, Padre Island National Seashore.

Marine Species of Common 

Conservation Concern

Marine mammals

Blue whale (Balaenoptera musculus)

Gray whale (Eschrichtius robustus)

Guadalupe fur seal 

(Arctocephalus townsendi)

Humpback whale 

(Megaptera novaeangliae)

Killer whale (Orcinus orca)

Right whale 

(Eubalaena glacialis and Eubalaena japonica)

Sea otter (Enhydra lutris)

Vaquita (Phocoena sinus)

Seabirds

Pink-footed shearwater 

(Puffinus creatopus)

Short-tailed albatross (Phoebastria albatrus)

Xantus’ murrelet 

(Synthlibiramphus hypoleucus)

Sea turtles

East Pacific green turtle 

(Chelonia mydas agassizii)

Hawksbill turtle (Eretmochelys imbricata)

Kemp’s Ridley turtle (Lepidochelys kempii)

Leatherback turtle (Dermochelys coriacea)

Loggerhead turtle (Caretta caretta)

Pink-footed shearwater. Photo: Hadoram Shiriha.

The survival of many land and marine species of 

common conservation concern depends on the 

existence of ecosystems that are relatively intact.
38 Species of Common Conservation Concern

the vulnerability of natural ecosystems to fire, 

flood and other natural phenomena is altered if 

the composition of native species is disturbed. 

Invasive species also propagate disease and disturb natural ecosystem processes. The diverse 

geography of North America allows invasive 

species from almost anywhere to find a hospitable place in some part of the region, with 

ecosystems ranging from Arctic tundra, tropical coral reefs and deserts to rain forests and 

freshwater rivers and lakes.

Climate Change

Climate change is expected to intensify habitat disturbance in North America. A greater 

number of disturbances are likely to enable 

the proliferation of invasive species and disrupt ecosystem services. Over time, species 

will respond to the climatic pressures by moving north and to higher elevations in search 

of more acceptable habitats, thereby rearranging North American ecosystems. From 

the tropical jungles of Mexico to the Arctic 

regions of Canada and the United States, the 

structure, function and services of ecosystems 

will change in response to the various capacities of species to undertake such range shifts 

and from the constraints imposed by development, habitat fragmentation, invasive species 

and broken ecological connections.

Pollution

Certain threats to biodiversity, especially those 

that undermine ecosystem integrity in ways 

not easily seen, are difficult to quantify. For 

example, it is known that pollution affects the 

hawksbill turtle. Pesticides, heavy metals and 

PCBs have been detected in turtles and eggs, 

and oil spills harm the animal’s respiration, 

skin, blood chemistry and salt gland functions. Like other marine turtles, hawksbills 

eat a wide variety of debris, including plastic 

bags, packing peanuts, tar balls, balloons and 

plastic pellets. Even at low levels of ingestion, 

this debris can interfere with metabolism and 

block the digestive system. Toxic byproducts 

can also be absorbed. The exact impact of pollution on this and other species is difficult to 

measure because the effects of specific pollutants at varying levels on the health of exposed 

species are unknown. 

Case Study – Burrowing Owl

The burrowing owl (Athene cunicularia) is a bird with resident and migrant populations 

alike in Canada, Mexico and the United States (see photo). Northern populations of 

the burrowing owl migrate south during winter, spending the season in Mexican territory and the southern United States (see map). The species prefers grasslands, desert 

zones and open areas. One distinctive characteristic of this bird is that it nests in burrows it digs itself, or in burrows dug by mammals such as prairie dogs, gophers and 

ground squirrels. These holes not only provide a place to nest, but also protect against 

wind, rain, sun and predators. 

Burrowing owl populations have fallen throughout Canada and the United States; 

Mexico does not have sufficient data to determine that country’s trends. In Canada, 

the situation of the burrowing owl is critical—it faces possible extinction. 

Intensive land use—particularly conversion of grasslands to farming—is thought 

to be an important factor in the drop in the burrowing owl population. Prairie dog and 

rabbit eradication programs may be 

another. Intensified land use has led 

to the loss and overall fragmentation 

of nesting grounds. Fragmentation 

hinders the owl’s ability to find a 

mate, and it appears to interfere with 

juvenile dispersal as well. Other factors underlying population decline 

include urban development, pesticide use and invasive grass species 

that change grassland physiognomy. 

In Canada, over 75 percent of the prairies have been cultivated, and much 

of the remaining grasslands have 

been altered by human activities. 

Problems along the burrowing owl’s 

migration routes and in the wintering 

grounds may be contributing to higher 

species mortality as well.

Burrowing owl. 

 

Regions populated by the burrowing owl

Source: NatureServe.
Key Findings

n Acid deposition (commonly called 

acid rain) degrades the quality 

of forests, coastal ecosystems, 

lakes and soils; harms wildlife; 

and corrodes building materials. 

Acidifying emissions can cross 

national and provincial or state 

boundaries, affecting ecosystems 

hundreds of kilometers away.

n Sulfur dioxide (SO2

) and nitrogen 

oxides (NOx

) are acidifying 

emissions that contribute to acid 

deposition. These chemicals are 

emitted mainly by human activities 

such as metal smelting and fossil 

fuel combustion in electricity 

generation and transportation. 

n Since 1990, emissions of SO2

in North America are down by 

one-third and those of NOx

 have 

declined by just over one-fifth. 

Sulfate deposition in the eastern 

United States and Canada has 

decreased substantially over 

the last 15 years, whereas the 

reduction in nitrate deposition has 

been less dramatic. 

n Despite the considerable progress 

made toward reducing emissions 

and the deposition of acidifying 

pollution, many sensitive 

ecosystems are still receiving 

levels of acid deposition above the 

threshold levels that cause longterm damage. Furthermore, some 

regions previously affected by high 

levels of acid deposition are not 

recovering as expected.

The acid deposition process

 The North American Mosaic: An Overview of Key Environmental Issues 39

What Is the Environmental Issue?

Acid deposition has already damaged North 

American forests, lakes, soils, buildings and 

historic monuments—in some cases, irretrievably. The air pollutants giving rise to acid 

deposition affect human health and air quality as well. But the problem is not just a North 

American one. Because acidic pollutants can 

travel great distances through the atmosphere 

to be deposited in ecosystems hundreds and 

even thousands of kilometers away, acid deposition is a global problem. Emissions from 

North America travel as far as Europe, and 

pollution from Asia affects human health and 

the environment in North America.

Air pollutants, particularly emissions of 

oxides of sulfur and nitrogen, are the precursors of acid deposition. In North America, sulfur dioxide (SO2

) and nitrogen oxides (NOx) are 

emitted by anthropogenic sources such as metal 

smelting and fossil fuel combustion in electricity 

generation and transportation, as well as by natural sources such as volcanoes, forest fires and 

lightning. However, the vast majority of SO2

 and 

NOx

 emissions that contribute to acid deposition 

are a product of human activities (see illustration 

of acid deposition process).

Acid Deposition

Acid deposition primarily results from the transformation of air pollutants such as sulfur 

dioxide and nitrogen oxides into secondary pollutants such as sulfuric acid, ammonium 

nitrate and nitric acid. Acidic particles and vapors can be deposited on the earth’s surface 

as acid precipitation (wet deposition) or via particles such as fly ash, sulfates, nitrates, and 

gases (dry deposition). 

Pollutants

Adapted from: Michael Pidwirny, Physical Geography.net—Fundamentals of Physical Geography, 

<http://www.physicalgeography.net/fundamentals/8h.html>.

SO2

NOx

O3

Dry deposition

Emission 

source

Dry deposition 

of gases, particles 

and aerosols

Wet deposition 

of dissolved acids

Wet deposition

Oxidation

Reaction with moisture

in atmosphere

Natural

Ammonia
40 Commission for Environmental Cooperation

Acidification of ecosystems occurs when 

the deposition of acidic compounds exceeds 

the neutralizing capacity of the receiving 

environment. Clean rainwater is slightly 

acidic, with a pH of about 5.6, because it 

contains dissolved carbon dioxide from the 

air. Acidic pollution has a pH lower than 

this, normally ranging between 4 and 5. A 

decrease in a single pH unit represents a tenfold increase in acidity. Thus rainwater with 

a pH of 4.2 is about 25 times more acidic 

than clean rain. By making soil and water 

more acidic, acid deposition harms plants, 

animals and ecosystem integrity in affected 

areas. It also damages buildings, monuments 

and painted surfaces. 

In a lake, acid deposition creates a cascade of effects that reduce fish populations and 

may even completely eliminate a fish species 

from a water body. As acid rain flows through 

soils in a watershed, metals such as aluminum are released into the lakes and streams 

in that watershed. Both low pH and increased 

aluminum levels are directly toxic to fish. 

In addition, they cause chronic stress that, 

although it may not kill individual fish, does 

lead to lower body weight and smaller size and 

makes fish less able to compete for food and 

habitat.Acidification of lakes and streams can 

also increase the amount of methyl mercury 

available in aquatic systems. In certain lakes 

in Canada and the United States that have a 

low pH, the common loon, a duck-like waterbird, has been found to have elevated blood 

mercury levels. 

In forest soils, excess acid deposition 

increases the susceptibility of forests to stresses 

from pests, pathogens and climate change, 

resulting in poorer forest health, lower timber 

yields and eventual changes in the composition of forest species. Acid rain weakens trees 

by damaging their leaves, limiting the nutrients available to them or exposing them to 

toxic substances slowly released from the soil. 

Quite often, these effects of acid rain, in combination with one or more additional threats, 

injure or kill trees.

Finally, the pollutants that cause acid 

rain are harmful to human health. In the 

air, they join with other chemicals to produce smog, which can irritate the lungs 

and make breathing difficult, especially for 

people suffering from asthma, bronchitis or 

other respiratory conditions. Fine particulate matter, containing sulfate derived from 

SO2

, is thought to be especially damaging to 

the lungs.

Thousand tonnes

 Canada Mexico United States

30,000

25,000

20,000

15,000

10,000

5,000

0

Air emissions of SO2 Air emissions of NOx

5-ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

5-ye ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

5-year (1990–1994) 

mean sulfate wet deposition

5-year (2000–2004) 

5-ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 mean sulfate wet deposition 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

5-ye ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-year (1990–1994)

mean nitrate wet deposition

5-year (2000–2004) 

mean nitrate wet deposition

5-ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

5-ye ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

5-ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

W et Dep5-yeositi o n (k g/ha /yr) ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

t De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

 < 5 

hate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

e W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

 5 – 10 

n Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

n Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

 10 – 15 

94) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

94) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

 15 – 20 

9 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep ositi o n 

(kg /ha /yr) 

9 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k g/ha/yr) 

 20 – 25 

ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et Dep osit

(kg /ha /yr) 

ye ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po siti o n (k 

 25 – 30 

5-ye ar (1 99 0 - 1994) Me an Su lphate W et Dep ositi o n (k g/ha /yr) 5-ye ar (2 00 0 - 2004) Me an Su lphate W et 

(kg /ha /yr) 

5-ye ar (1 99 0 - 1994) Me an Nitrate W et De po siti o n (k g/ha/yr) 5-ye ar (2 00 0 - 2004) Me an Nitrate W et De po si

 > 30

Sources: Environment Canada, Instituto Nacional de Ecología (latest data from Mexico from 2002, not 2005), US Environmental Protection Agency.

Source: NAtChem. Maps courtesy of Chul-Un Ro, Environment Canada.

Wet sulfate and wet nitrate deposition in the United States and Canada

1990 2005 1990 2005

(kg/ha/yr)
Acid Deposition 41

Why Is This Issue Important to 

North America?

The effects of acid deposition across North 

America can be addressed only in cooperation 

with the neighboring jurisdictions that contribute to acidifying emissions. The issue of acid rain 

first caught the public’s attention in the late 1970s 

and early 1980s when its devastating impacts on 

ecosystems in eastern North America were publicized. In 1980 Canada and the United States 

began working together to address this transboundary issue. The two countries signed the 

Canada-US Air Quality Agreement in 1991 to 

promote scientific understanding and pollution 

reduction in both countries. 

Efforts to Reduce Emissions

Canada and the United States gave priority to 

SO2

 emissions because lowering these emissions was understood to be most important 

in lessening damage to sensitive ecosystems. 

Since 1990, SO2

 air emissions in North America 

have declined by almost one-third (see graph). 

At present, electric power generation accounts 

for the largest emissions of SO2

 in the United 

States, while in Canada the dominant emitting 

sector is base metal smelting. 

Over the same period, air emissions of 

NOx

 have declined by just over one-fifth (see 

graph). Mobile sources such as cars and trucks 

are the most significant sources of NOx

 emissions in North America, with the remainder 

coming from power plants and other sources.

Results 

In response to emission reductions, levels of 

sulfate deposition in the eastern United States 

and Canada decreased substantially over the 

period 1990–2004, whereas changes in levels 

of nitrate deposition have been less dramatic 

(see maps). 

Affected Areas 

Many of the water and soil systems in eastern 

North America cannot neutralize acid naturally. As a result, these areas are sensitive to 

acid deposition. To understand the capacity 

of ecosystems to absorb acid deposition, scientists have developed the concept of “critical load”—that is, an estimate of the amount 

of deposition that a particular ecosystem can 

receive below which no harmful effects occur. 

The critical load depends on the quantity of 

acid-neutralizing bases, such as calcium and 

magnesium salts, in a region’s water and in 

the surrounding rocks and soils. 

Case Study – Acidification at El Tajín, Mexico 

Pyramid of the Niches, El Tajín, Veracruz, Mexico. Source: Luiz Castro.

Located in the present-day municipality of Papantla de Olarte in Veracruz, Mexico, El Tajín 

was one of the most important cities in the Mesoamerican Gulf zone. Its archaeological 

zone contains constructions dating back to 100 A.D. From 600 to 1150 A.D., the city reached 

its maximum size and influence. 

Humberto Bravo Álvarez and the section on environmental contamination at the 

Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, are 

studying the effects of acid rain on archaeological and historical sites at El Tajín. From 

18 August 2002 to 9 April 2003, they collected 40 rain samples at the El Tajín archaeological site and applied atmospheric trajectory analysis to each precipitation sample to 

determine air transport pathways corresponding to the precipitation events. Trajectory 

models are useful in identifying upwind regions likely to contribute to the pollutant 

burden at downwind receptors.

The analyses indicated that 85 percent of the precipitation events sampled at El 

Tajín were acidic (pH < 5.62). The back trajectory analysis of these acidic events showed 

a great variation, indicating there was no apparent directional preference for transport during these events and suggesting the importance of local sources. The El Tajín 

archaeological zone is surrounded by possible sources of acid rain precursors in the 

form of industries burning fuel oil with a high sulfur content (such as electric power 

plants and refineries). Thus both these sources and more distant ones may be important contributors to rainfall acidity at El Tajín.
42 Acid Deposition

Despite the progress in reducing acidifying emissions, some ecosystems are making a 

slower-than-expected recovery. In the United 

States, acidic surface waters are still found in 

the upper Midwest, Adirondack Mountains 

and northern Appalachian regions. In Canada, 

the areas receiving depositions higher than 

their critical loads are in provinces that are 

part of the Canadian Precambrian Shield. In 

Ontario, Quebec, New Brunswick and Nova 

Scotia, the susceptible hard rock (granite) 

areas lack the natural capacity to neutralize or 

buffer acid rain effectively. Historically, lower 

levels of industrialization combined with natural factors such as eastwardly moving weather 

patterns and soils with buffering capacity have 

largely protected western prairie ecosystems in 

Canada and the United States from the impacts 

of acid rain.

Although similar maps are not available 

for Mexico, the effects of acid deposition are 

evident in national parks near Mexico City 

where acid rain has damaged forests and soils, 

as well as in the damage to monuments and 

historic buildings in Mexico City and elsewhere (see case study, previous page).

What Are the Linkages to Other North 

American Environmental Issues?

Scientists’ initial concerns about the effects of 

acid deposition on forests and building materials have now expanded to include the relationship of acidifying pollution to biodiversity and 

sensitive coastal ecosystems. 

Biodiversity 

The various species that inhabit lakes, rivers 

and wetlands differ in their abilities to tolerate acidity. Acidification primarily reduces 

the variety of life inhabiting a lake and alters 

the balance among surviving populations. 

Changes in the mix of species inhabiting 

water bodies also affect birds and other species farther up the food chain, as some kinds 

of food resources become scarcer and others 

become more abundant. Scientists cannot 

say whether species that have disappeared 

from an acidified lake will ever return—even 

if pH levels return to normal.

Coastal Ecosystems

Linked to acid deposition is the effect of nitrogen deposition on coastal ecosystems, where 

nitrogen is often the limiting nutrient. Higher 

levels of nitrogen in coastal waters can cause 

significant changes to those ecosystems. Some 

Although sulfur emissions in the United States and Canada have fallen in response 

to control programs, the continued emissions of acidifying sulfur and nitrogen compounds present a serious long-term threat to forest health and productivity in parts of 

northeastern North America. This conclusion was reached by a study of the northeastern United States and eastern Canada conducted by the Conference of New England 

Governors and Eastern Canadian Premiers Forest Mapping Group.

According to the Forest Mapping Group’s study, the atmospheric deposition of 

sulfur and nitrogen from 1999 to 2003 exceeded the critical load in more than one-third 

of the study area (see map). In the eastern Canadian provinces, the most sensitive 

forest areas occur in southern Quebec, especially in the Lower-Laurentides north of 

the St. Lawrence River, in southeastern Nova Scotia, and in southern Newfoundland. 

In New England, the most sensitive forest areas occur in the mountain ranges and 

coastal areas where soils are poor and weathering rates low, and where there is greater 

demand for nutrients due to more intensive harvesting.

Based on forest monitoring in Quebec and the known effects of acid deposition, 

the Forest Mapping Group’s study concludes that high exceedances of critical loads 

lead directly or indirectly to reduced forest growth and health.

Case Study – Forest Sensitivity to Sulfur and Nitrogen Deposition 

60 percent of estuaries in the United States suffer from overenrichment of nitrogen, a condition known as eutrophication. Symptoms of 

eutrophication include changes in the dominant species of plankton (the primary food 

source for many kinds of marine life), which 

can cause algal blooms, low levels of oxygen 

in the water column, fish and shellfish kills, 

and cascading population changes up the food 

chain. In addition, the higher levels of turbidity 

in the water because of the large amounts of 

algae can kill off submerged aquatic vegetation, 

which is an important habitat for many estuarine fish and shellfish species. Although a large 

number of the most highly eutrophic estuaries 

are along the US Gulf and Mid-Atlantic coasts, 

overlapping many of the areas with the highest 

nitrogen deposition, eutrophic estuaries can 

be found in every region of the conterminous 

US coastline. 

Source: Conference of New England Governors and Eastern Canadian Premiers Forest Mapping Group.

Forest areas sensitive to acid deposition in the New England states 

and eastern Canadian provinces

Scale

0 150 300 km

 sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

< -600 

 sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

-599 – -400 

s sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

of data variability on page 7) 

-399 – -200

 sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

-199 – 0 

s sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

+1 – +200

 sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

+201 – +400 

+401 – +600

s sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

+601 – +800

s sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

f data variability on page 7) 

+801 – +1,000

s sensitive to acid deposition in the New England states and Eastern Canadian provinces. 

of data variability on page 7) 

> +1,000

Steady-State Exceedance 

(eq/ha/yr)
 The North American Mosaic: An Overview of Key Environmental Issues 43

Key Findings

n Industrial pollution and waste 

encompass the full range of 

materials generated by industrial 

activities that are unwanted by the 

producer. At times, they represent 

an unrealized opportunity to 

improve production efficiency and 

reduce disposal costs.

n Certain components of industrial 

pollution and waste are 

hazardous to human health 

and the environment. Related 

concerns include sensitive human 

populations such as children, the 

implications of low-level exposures 

to multiple pollutants, and 

contamination of ecosystems. 

n Every year, industrial activity 

in North America generates 

substantial quantities of toxic 

chemicals, air contaminants, 

hazardous and nonhazardous waste, 

and radioactive materials that must 

be managed to protect human 

health and the environment.

n Some trends in waste management 

are encouraging, such as industries’ 

adoption of pollution prevention 

methods and a sustained decline in 

releases of carcinogens and other 

toxins of concern, but progress has 

not been uniform. 

What Is the Environmental Issue? 

Industrial production contributes goods, services and jobs, but it is also a major source of 

pollution and waste. This pollution and waste 

can be classified into six categories: toxic chemicals, criteria air contaminants, greenhouse 

gases, hazardous wastes, nonhazardous wastes 

and radioactive wastes.

Toxic Chemicals 

These substances are hazardous to human 

health and the environment. In 2004 North 

American industrial facilities generated over 

5 million tonnes of toxic chemicals as production-related pollutants and waste (see box). 

Despite this large amount, data for comparable 

industries and chemicals in Canada and the 

United States reveal encouraging trends. Over 

the period 1998–2004, total releases of carcinogens and developmental/reproductive toxicants 

declined by 26 percent in the United States and 

Canada (see graph), compared with a 15 percent reduction in all tracked chemicals. Mexican data are not available for this time period. 

Although releases to most media for these 

Industrial Pollution and Waste

Industrial pollution and waste encompass the full range of unwanted substances and 

losses generated by industrial activities, including emissions to air or surface waters 

and the substances sent to sewage treatment plants, deposited in landfills, released or 

applied to the land, treated, injected underground, controlled through storage, recycled 

or burned for energy recovery. 

Pollutants
44 Commission for Environmental Cooperation

chemicals have declined over time, releases to 

underground injection have increased. This 

disposal method, in which fluids are released 

into subsurface wells, has increased by over 40 

percent since 1998 for carcinogens and developmental/reproductive toxicants. Furthermore, 

even though facilities with the largest reported 

amounts have made progress in reducing toxic 

releases and transfers, the more numerous 

facilities reporting smaller pollution amounts 

are tending to move in the opposite direction. 

Criteria Air Contaminants 

These substances, which include nitrogen 

oxides, sulfur oxides, carbon monoxide, 

particulate matter and volatile organic compounds, are associated with environmental 

effects such as smog, acid rain and regional 

haze, and health effects such as respiratory 

illness. These pollutants are emitted from a 

variety of sources, including residential fuel 

combustion, motor vehicles and agricultural 

activities. Industrial sources are also major 

contributors—among them, electric utilities, 

primary metal smelters and cement kilns. 

Although emissions of criteria air contaminants are trending downward, reductions 

from sources such as motor vehicles have 

been partially offset by increases from certain 

oil and gas industry subsectors attributed to 

expanded production.

Greenhouse Gases 

These gases, which include carbon dioxide 

(CO2

), methane and nitrous oxide, are linked 

to global climate change. Industrial energy 

use is a major source of CO2

 emissions in 

North America, roughly on a par with the 

CO2

 emissions arising from energy use in the 

agricultural, commercial and residential sectors combined. Although CO2

 emissions from 

industrial energy use dropped by more than 

30 percent from 1980 to 2005, emissions from 

transportation increased by about 50 percent 

and those from electricity generation and refineries by nearly 60 percent during the same time 

period. Total emissions of greenhouse gases in 

North America amounted to more than 8.5 billion tonnes of CO2

 equivalent in 2005.

Hazardous Wastes 

Hazardous wastes are industrial waste streams 

that may contain more than a single chemical or 

substance. They are typically defined by characteristics such as ignitability, reactivity, corrosivity and toxicity. Comparable North American 

data for hazardous waste generation and management are currently lacking, making it difficult to discern trends. Although the United 

States issues a biennial hazardous waste report, 

periodic nationwide data are lacking in Canada 

(except for cross-border shipments) and are at 

an early stage of development in Mexico. 

The amounts of hazardous wastes being 

generated are significant. In the United States, 

nearly 34.8 million tonnes of hazardous waste 

were generated in 2005, mostly in the form 

of liquid waste. Government estimates put 

Canada’s annual generation at about 6 million tonnes. In Mexico, data from over 35,000 

facilities put the annual total at 6.17 million 

tonnes in 2004. Mexico’s total generation of 

hazardous wastes is not known, but 8 million 

tonnes a year is frequently cited.

Nonhazardous Wastes 

Nonhazardous industrial wastes include coal 

ash, foundry sands, cement kiln dust, mining and mineral processing wastes, oil and 

gas production wastes, and other wastes that 

lack the characteristics of hazardous waste. 

Although these waste streams are not classified as hazardous, their management is not 

without risk and generally legal requirements 

are in place for their proper treatment and 

disposal. In Canada, disposal of wastes from 

nonresidential sources (industrial, commercial and institutional) increased from 14.6 to 

15.5 million tonnes between 2002 and 2004. 

In the United States and Mexico, overall estimates of nonhazardous industrial waste are 

not readily available, although estimates for 

various individual sources may exist.

Thousands of tonnes

 Air Surface Water Underground Injection 

 Land Off-site Releases

300

250

200

150

100

50

0

Releases of carcinogens 

and developmental/ 

reproductive toxicants

(Canadian and US data)

Source: Comission for Environmental Cooperation, Taking Stock 2004.

1998 1999 2000 2001 2002 2003 2004

In North America, all three countries track certain industrial pollutants using Pollutant Release 

and Transfer Registers (PRTRs). PRTRs compile facility-reported annual data on releases of 

specific substances to air, water and land, as well as disposal and transfers off-site for treatment 

or recycling. In 2004 over 5 million tonnes of releases and transfers were reported. 

Canada’s National Pollutant Release Inventory (NPRI), established in 1992 based on 

recommendations of stakeholders from industry and environmental organizations, tracks more 

than 300 chemicals as well as criteria air contaminants.

Mexico’s Registro de Emisiones y Transferencia de Contaminantes (RETC), which recently 

became mandatory, covers some 100 chemicals and forms part of the Cédula de Operación 

Anual (annual certificate of operations), which is also used to collect data on hazardous waste 

generation, energy use and other indices of environmental management. 

The US Toxics Release Inventory (TRI), begun in 1987, now tracks data from facilities on 

more than 600 chemicals. 

Enhancing the comparability of their PRTRs is a shared priority for the three countries. In June 

2002, the CEC Council signed Council Resolution 02-05: Action Plan to Enhance Comparability 

Among Pollutant Release and Transfer Registers in North America.

North America’s Pollutant Release and Transfer Registers
Industrial Pollution and Waste 45

Radioactive Wastes 

Radioactive wastes are by-products of certain 

industrial activities, in particular electricity 

generation. In 2005 nuclear power generation 

produced 1,697 tonnes of spent fuel (expressed 

as amounts of heavy metal) in Canada, 21 

tonnes in Mexico and 2,396 tonnes in the 

United States. 

Why Is This Issue Important to 

North America? 

Industrial pollution and waste pose potential 

threats to human and ecological health if not 

properly managed. The concerns range from 

toxic effects on fetuses and children to the 

health implications of low-level exposures to 

multiple pollutants and the degradation of 

habitats and ecosystems. These concerns do 

not stop at the borders, because some pollutants can travel long distances and waste is 

shipped to recycling and disposal sites across 

political boundaries.

Health and Environment 

The pollution and waste tracked through 

PRTRs and regulated by environmental laws 

in North America are those the national governments have identified as raising concerns 

about human health or the environment. 

The effects of certain toxic chemicals on the 

health and development of children and other 

vulnerable groups are a special concern. 

Researchers describe “windows of vulnerability” during fetal and child development in 

which toxic exposures can have particularly 

devastating effects. Although the traditional 

focus has been on overt health effects such 

as cancer, scientists are increasingly worried 

about the more subtle effects of low-level toxic 

exposures, such as impairments in endocrine 

and neurological functions.

Long-range Transport 

Industrial pollution and waste are important 

in the North American context because pollutants travel through the air and water to 

cross national borders and because wastes 

are also shipped across borders for recycling, 

treatment and disposal. The deposition of 

persistent contaminants in the distant north, 

in locations far from industrial sources, 

attests to the ability of pollutants to travel far 

from their points of origin. The industrial 

pollution and waste released into rivers or 

water bodies that span political boundaries, 

such as the Great Lakes and the New River, 

which runs from Baja California into California, is also a shared concern, especially 

the effects of persistent bioaccumulative 

toxic substances (PBTs).

Waste Management 

Decisions on how to manage wastes have 

environmental implications. Municipal waste 

incineration, medical waste incineration, 

burning of hazardous wastes in cement kilns 

and backyard waste burning were among the 

top sources of dioxins, according to US and 

Canadian inventories. Dioxins, like some 

other PBTs, can be dispersed long distances 

by air currents and other environmental pathways and tend to settle in colder regions. 

The regulatory requirements governing the management of hazardous wastes can 

influence the waste management decisions 

of industrial facilities. For example, such 

regulations may inhibit recycling by facilities 

because of concerns about higher compliance 

costs. Jurisdictional differences in regulatory 

requirements, in addition to differentials in 

waste management pricing, can also influence decisions about where and how wastes 

are managed. Whatever the differences, it is 

true that North American companies ship 

hundreds of thousands of tonnes of hazardous waste each year between Canada, Mexico 

and the United States. When wastes are sent to 

other jurisdictions for recycling, treatment or 

disposal, the waste shipments must be transported along roads and railways and through 

populated areas before reaching their final 

destinations.

Economic Costs

Apart from their potential effects on humans 

and the environment, wastes represent inefficiency in industrial production. Wastes 

impose costs on facilities; they must pay for 

waste management, regulatory compliance 

and underutilized material inputs. From 

a societal perspective, the economic costs 

include paying for cleaning up contaminated 

sites, regulating waste-generating industries and ensuring medical treatment for the 

adverse effects of environmental exposures. 

The nonmonetary costs include the depletion 

of nonrenewable resources, consumptive land 

use and degradation of ecosystems. 

Industrial pollution and waste are important in the 

North American context because pollutants travel 

through the air and water to cross national borders 

and because wastes are also shipped across 

borders for recycling, treatment and disposal.
46 Industrial Pollution and Waste

Companies and jurisdictions are increasingly striving to decouple waste generation from 

economic productivity. PRTR data demonstrate 

that facilities undertaking pollution prevention 

activities are able to reduce their wastes faster 

than those who do not (see graph). The graph 

also reveals that over three-quarters of waste 

is generated at facilities that have yet to pursue 

pollution prevention. Reducing waste does not 

require reducing economic activity. California 

has the largest subnational economy in North 

America, but ranks thirtieth among states and 

provinces in total releases of toxic chemicals. 

What Are the Linkages to Other North 

American Environmental Issues? 

Land use planning and climate change are just 

two of the other important environmental issues 

linked to industrial pollution and waste.

Land Use

Waste poses challenges for local land use planning, ranging from the siting of new treatment, 

storage and disposal facilities to the question of how to manage “brownfield” sites. In 

the United States, as of 2008, 1,581 sites (final 

and deleted) were on the Superfund program’s 

National Priorities List and 3,746 facilities are 

expected to need cleanup under the Federal 

Resource Conservation and Recovery Act. 

Numerous other sites are under local or state 

jurisdiction, and so the full extent of contaminated land is unknown. In Canada, about onequarter of the 17,866 contaminated sites under 

federal responsibility are on native reserves, 

placing an additional burden on populations 

already vulnerable to environmental threats 

because of socioeconomic factors or geography. In Mexico, the federal government has 

identified 300 contaminated sites covering 

200,000 hectares. The location of polluting 

industries, landfills and other waste management sites also raises questions of environmental justice.

Natural Resource Depletion

Inefficient use of materials and energy affects 

the use of natural resources. Depletion of natural resources is mediated by the renewability 

of the inputs used and the degree of recycling 

undertaken within or among industrial sectors. 

Recycling and energy recovery of industrial 

wastes enable the wastes from one process to 

become the material inputs or energy source for 

another. More than a million tonnes of materials, mostly metals, were sent for recycling by 

PRTR-reporting facilities in 2004, and nearly 

300,000 tonnes were sent for energy recovery. However, recycling and energy recovery 

can have their drawbacks. Recycling activities 

themselves can be sources of environmental 

contamination, and the air releases and residuals from energy recovery are a concern.

Climate Change

Industrial pollution and waste contribute to climate change as well. The anaerobic decomposition of wastes in landfills produces methane, a 

potent greenhouse gas, and waste incineration 

releases carbon dioxide. The transportation of 

wastes to recycling, treatment and disposal sites 

produces transportation-related carbon emissions. Finally, the materials disposed of as waste 

must be replaced by more raw materials, which 

implies further consumption of fossil fuels and 

additional carbon releases. 

Millions of kg

 2002 2004 

2,500

2,000

1,500

1,000

500

0

Total releases and transfers of PRTR 

substances for facilities with 

and without pollution prevention 

activities (Canadian and US data)

Source: Comission for Environmental Cooperation, Taking Stock 2004.

Facilities Without Pollution 

Prevention Activity

Facilities With Pollution 

Prevention Activity
 The North American Mosaic: An Overview of Key Environmental Issues 47

Key Findings

n Persistent bioaccumulative toxic 

substances (PBTs) are chemicals 

that break down slowly in the 

environment, accumulate in 

humans and other species and 

are toxic. Although only a limited 

number of PBTs are currently 

receiving attention, more may 

be identified through ongoing 

screening activities.

n PBTs may be released 

intentionally (such as pesticides) 

or unintentionally (such as 

combustion or manufacturing 

byproducts). Some PBTs are 

dispersed globally by air currents 

and other environmental pathways, 

resulting in contamination even in 

regions far from their points of origin.

n Comprehensive biomonitoring 

data are not available for all of 

North America, but local and 

country-specific studies have 

measured PBT levels in humans and 

wildlife. The implications of these 

findings are under investigation in 

all three countries.

n The levels of certain PBTs in the 

environment have been reduced 

by eliminating or reducing releases 

and adopting alternatives, but 

recovery times are slow because 

these chemicals do not break 

down to harmless byproducts easily 

or quickly. 

Persistent Bioaccumulative 

Toxic Substances

What Is the Environmental Issue?

There is substantial evidence that persistent 

bioaccumulative toxic substances (PBTs) 

cause long-term harm to human health and 

the environment. This evidence has provoked 

an international response to the problem 

(see box).

PBTs and Human Health

In North America, humans are exposed to many 

different environmental contaminants, including certain PBTs. Studies have linked various 

PBTs to a range of adverse effects in humans, 

including nervous system disorders, reproductive and developmental problems, cancer 

and genetic impacts. Certain PBTs mimic hormones, possibly altering sexual characteristics 

and other hormonal functions. 

PBTs and Animal and Plant Health

Like humans, animals and plants are exposed to 

PBTs in the environment through air, water and 

food. The animals most likely to be exposed to 

toxic levels of PBTs are those higher up in the 

food chain, such as marine mammals, birds of 

prey and certain fish species. The fish consumption advisories issued by governments around 

the Great Lakes and elsewhere are designed to 

protect people from the risks of eating contaminated fish. Mercury, polychlorinated biphenyls 

(PCBs), chlordane, dioxins and DDT—the PBTs 

that commonly contaminate fish—accumulate 

in fish tissue at concentrations thousands of 

times higher than in the water. PBTs can stay in 

sediments for years, a source of contamination 

for bottom-dwelling creatures that are then eaten 

by predators (see illustration of bioaccumulation 

and biomagnification). 

Persistent bioaccumulative toxic substances (PBTs) are chemicals that do not degrade 

easily in the environment. PBTs typically accumulate in fatty tissues and are slowly 

metabolized, often increasing in concentration within the food chain. Certain PBTs have 

been linked to adverse health effects in both humans and animals. 

Pollutants

An International Response to PBTs

With 151 other nations, Canada, Mexico and the United States are all signatories to the 

Stockholm Convention on Persistent Organic Pollutants (May 2001). Canada and Mexico 

have ratified this treaty, but the United States has not.

The Convention identifies 12 organic PBTs for control. These chemicals fall into three categories: 

Pesticides—aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene (HCB), 

mirex and toxaphene

Industrial chemicals and unintended byproducts—HCBs and PCBs

Unintended byproducts—dioxins and furans.

Many of the 12 chemicals covered by the Stockholm Convention are no longer produced, 

and yet they persist in the environment. Even though only a limited number of PBTs are 

currently receiving attention, more may be identified through ongoing screening activities.
48 Commission for Environmental Cooperation

The persistence of these substances in the 

environment is considerable. Chlordane was 

banned in the United States in 1988, but 105 

fish consumption advisories were still issued 

for this substance in 2006. Likewise, DDT has 

been banned since 1975, but 84 DDT-related 

fish consumption advisories were still issued in 

2006 in the United States.

Why Is This Issue Important to 

North America?

PBTs are intentionally and unintentionally 

released into the environment. Once in the 

environment, some PBTs can readily disperse 

throughout specific regions and across international boundaries, both within North America 

and globally. PBTs are of particular concern to 

North America because they are found in environmentally sensitive areas, such as the Arctic 

(see illustration of pathways into the Arctic), 

the Great Lakes and the Gulf of Mexico.

Sources of PBTs 

Worldwide, all industrial sectors use chemicals, but certain sectors are more likely to emit 

PBTs. These emissions may originate from 

either intentional releases—such as pesticides 

with PBTs as impurities—or unintentional 

releases—such as combustion byproducts (e.g., 

dioxins and furans). 

PBTs continue to be released as the byproducts of industrial activities. Mercury releases 

through coal-fired electricity generation, for 

example, have increased since the beginning 

of the industrial age in the mid-1800s. Rates of 

mercury deposition from the atmosphere have 

increased globally 200–400 percent since the 

industrial revolution, increasing the potential 

human health and ecosystem effects of mercury worldwide.

Transport

A major concern with some PBTs is the ease 

with which they can move through the environment. PBTs make their way into remote 

regions by traveling long distances in a series of 

“hops” involving a complex cycle of long-range 

atmospheric transport, deposition and revolatilization, collectively called the “grasshopper 

effect.” Eventually, they accumulate in cold 

regions such as the Arctic by a process called 

“global distillation” (see illustration). 

Because PBTs are often relatively volatile, 

they may enter the atmosphere where they 

can be carried with the winds, sometimes 

for long distances. Through atmospheric 

processes, either because the molecules are 

carried down with precipitation or because 

particulate matter settles, they are deposited 

onto land or into water ecosystems where 

they accumulate and may cause damage. 

From these ecosystems, they may evaporate, 

again entering the atmosphere, ultimately 

traveling from warmer temperatures toward 

cooler regions. Whenever the temperature 

drops, they condense out of the atmosphere, 

frequently reaching higher concentrations 

in circumpolar regions and in high altitudes 

because there is insufficient thermal energy 

to go through another evaporation cycle. 

Through these processes, some PBTs can 

move thousands of kilometers from their 

sources of emission and accumulate in polar 

latitudes. In addition to releases within the 

region, North America is also affected by the 

long-range atmospheric dispersal of PBTs 

from global sources.

Biomonitoring 

The human populations exposed to PBTs include 

groups of special concern such as children and 

developing fetuses. Children are especially 

vulnerable to toxic chemicals because of their 

unique physiology and developmental and 

behavioral characteristics. The biomonitoring 

data needed to measure the occurrence of PBTs 

in humans are not readily available for North 

America as a whole, but some insights can be 

drawn from more localized studies:

n In the Canadian Arctic, the Arctic 

Monitoring and Assessment 

Programme has determined that the 

high exposure levels found in some 

Arctic communities may have a 

negative influence on human health. 

Although there is still no direct 

evidence of adverse effects on health 

status (mortality and morbidity) at 

Less volatile

More volatile

Hotter equatorial regions

Temperate regions

Colder 

polar regions

The grasshopper effect 

and global distillation 

Source: Environment Canada.

Changes in the concentration of PBTs as they move up the food chain

Phytoplankton 0.025 ppm

Zooplankton 0.123 ppm

Smelt 

1.04 ppm

Lake trout 

4.83 ppm

Herring gull eggs 124 ppm

Adapted from: US Environmental Protection Agency.
Persistent Bioaccumulative Toxic Substance 49

the population level, there is reason 

for concern and a need to continue to 

reduce human exposure based on the 

weight of all available evidence.

n In Mexico, organochlorine (OC) 

pesticides, which are also PBTs, 

were measured in the ambient 

air of Chiapas during 2000–2001. 

Concentrations of some OC pesticides 

(DDTs, chlordane, toxaphene) in this 

area were elevated compared with levels 

in the Great Lakes region. This finding 

suggests that southern Mexico may 

be a source region for this group of 

chemicals, but comparably high levels 

have also been reported in parts of the 

southern United States, where their 

suspected sources are emissions from 

historically contaminated soil (DDTs, 

toxaphene) and past termiticide usage 

(chlordane). Agricultural workers 

may be at risk because of exposures 

to these PBT chemicals.

n In the United States, about 6 percent of 

women of child-bearing age had 5.8 parts 

per billion or more of mercury in their 

blood from 1999 to 2002. Concentrations 

below 5.8 parts per billion are unlikely 

to cause appreciable harm. Based on 

these survey results and the number of 

births each year, it is estimated that more 

than 300,000 newborns each year in the 

United States may have an increased risk 

of learning disabilities associated with in 

utero exposure to methylmercury.

What Are the Linkages to Other North 

American Environmental Issues?

Persistent bioaccumulative toxic substances are 

linked to biodiversity, international trade and 

climate change. 

Biodiversity

The effect of PBTs on biodiversity in North 

America was first evident when peregrine falcons, eagles and other top predators began 

to disappear in the 1970s because of DDT in 

the food chain. Although these highly visible 

species have recovered through interventions 

and through the banning of some PBTs, other 

species may still be affected by the presence of 

PBTs in the continent’s ecosystems. 

Trade

International trade can introduce PBTs into 

North America despite rigorous efforts to 

prevent their release here. Consumer goods 

and products sold in North America are 

increasingly manufactured, grown or otherwise handled in nations whose requirements related to PBTs may differ from those 

in North America. An example is food 

imported from other countries that have 

been treated with pesticides still in use there 

such as DDT, aldrin and chlordane.

Climate Change

Normal atmospheric conditions carry mercury and some other PBTs emitted by fossil 

fuel combustion and other industrial activities 

northward, where these substances eventually 

settle on land or water surfaces. For example, 

the boreal forest region in northern Canada 

and Alaska is a resting place for years of past 

emissions. Because climate change affects 

northern forests and wetlands, mercury previously deposited into cold, wet soils may be 

released again through wildfires. In response 

to the drier conditions in northern regions, 

soil will relinquish its hold on hundreds of 

years of mercury accumulation, sending it 

back into the air. The projected increases in 

boreal wildfire activity stemming from climate 

change are expected to increase atmospheric 

mercury emissions, exacerbating exposure in 

northern food chains. 

Selected Industries and Processes Associated with PBTs

Manufacturing Thermal processes Certain products with PBT impurities Recycling processes

n Production of chlorinated 

organic chemicals

n Pulp and paper production

n Oil refining and 

catalyst regeneration

n Chlorine production using 

graphite electrodes

n Iron ore sintering for blast furnaces

n Primary copper smelting

n Secondary scrap metal processing

n Cement kilns

n Mineral processing: lime, ceramic, 

glass, brick

n Waste incineration: municipal, 

hazardous, medical/clinical

n Coal and oil combustion 

vehicles and stationary motors

n Pesticide/herbicide application

n Preservatives for wood/leather/textile

n Solvent use and application

n Industrial bleaching processes

n Textile/wool/leather dying and finishing

n Metal, paper and 

plastics recycling

n Sewage and paper 

sludge and effluent 

application on land 

n Solvent and waste 

oil recovery

n Pentachlorophenoltreated wood

Source: Arctic Monitoring and Assessment Programme. 

Precipitation

Contaminant emissions transported by air

Ice fluxes transport 

particles in the ice

Ocean currents

River discharge

Deposition in ice

Pathways of contaminants to the Arctic

Source: United Nations Environment Programme.
50 Persistent Bioaccumulative Toxic Substance

Case Study – Toxics in Osprey Eggs: An Indicator of Contaminants in the Fraser and Columbia River Basins

Ospreys, which are fish-eating migratory birds, are exposed to pollutants that accumulate in aquatic food chains and, as a result, are 

a good indicator species of aquatic ecosystem health. 

A long-standing monitoring study of the levels of chemicals 

found in migrating ospreys has provided information about the 

PBTs that have accumulated in the birds and the origins of the 

substances. The results also suggest important questions about 

exposure and bioaccumulation in humans. 

Ospreys migrate between Latin America and the Fraser and 

Columbia River Basins in the Pacific Northwest of North America. 

The PBTs found in ospreys include industrial OCs (dioxins, furans 

and PCBs), OC pesticides (DDT metabolites, dieldrin, chlordane 

and toxaphene) and mercury. In particular, DDT, PCBs, and dioxins 

and furans are historically associated with reproductive failure and 

population declines in osprey.

Researchers have found that some of the toxic contaminants 

in the ospreys originate from industrial sites still operating in the 

Fraser and Columbia River Basins and the sites of closed industries 

where residues remain. The remaining toxics originate in Asia and 

potentially in food sources at the osprey’s wintering grounds in 

Latin America.

The Canadian Wildlife Service, along with Mexican and US 

agencies, has collected data on the migratory habits of ospreys. 

They were tracked from their nesting grounds in the Fraser River 

Basin to areas of intensive agriculture in Mexico (see map), and 

other Central American countries. 

From 1997 to 2004, the osprey population along the lower 

portion of the Columbia River increased from 94 to 225 occupied 

nests, almost a 14 percent annual rate of increase. The rate of population increase was associated with higher reproductive rates 

than in previous years and significantly lower egg concentrations 

of most OC pesticides, PCBs, dioxins and furans. Indeed, the levels of observed egg residue concentrations in 2004 indicated that 

reproduction at few, if any, nests was adversely affected by the 

presence of such pesticides. As recently as 1997–1998, the DDT 

metabolite DDE was still causing reproductive failure for a portion 

of this population. Only mercury showed a significant increase in 

eggs over time, but the concentrations in 2004 remained below 

those established to have effects on birds. 

Because ospreys feed on a variety of sport fish species, the 

ongoing monitoring of contaminant levels in this bird serves as 

an early warning about the toxic substances potentially consumed by humans. 

Adapted from: J. Elliott, D. P. Shaw and D. Muir, “Factors Influencing Domestic and International 

Sources of Chlorinated Hydrocarbons to Fish and Ospreys in British Columbia,” Toxic Substance 

Research Initiative, Final Report, TSRI #224. Vancouver, (unpublished).

Satellite telemetry of osprey migration routes

Osprey.

Osprey migration route

Nesting Wintering 

sites (BC) sites (MEX)

Golden* Sinaloa 

Golden* Sinaloa

Pitt River Durango

Atlin Lake Oaxaca

* Same bird, different years
 The North American Mosaic: An Overview of Key Environmental Issues 51

Key Findings

n The biological, chemical and physical 

characteristics of water affect its ability 

to sustain life and its suitability for 

human consumption and use. Several 

water quality issues have persisted 

for decades, including sedimentation, 

nutrient overenrichment and bacterial 

and toxic contamination.

n Many human activities yield waste 

products such as sewage, runoff, urban 

industrial releases and air pollution 

that affect the quality of water. 

Likewise, landscape modifications can 

undermine the natural processes of 

water purification through wetlands 

and infiltration of water through soil 

into groundwater.

n Although freshwater quality in 

many parts of North America is 

good, a significant portion of 

North American surface freshwater 

is degraded. A similar assessment 

of groundwater is not possible, but in 

certain areas it is known to be degraded 

by nitrates, pesticides and salinity.

n Conventional pollutant discharges 

from industrial point sources have 

largely fallen over the last 30 years 

in North America, but nonpoint and 

diffuse pollutant sources such as 

agricultural runoff, stormwater runoff 

and atmospheric deposition have 

become relatively larger contributors to 

impairment of water quality.

What Is the Environmental Issue?

The health of humans, wildlife and ecosystems 

depends on adequate supplies of clean water. 

But as populations grow and expand into previously undeveloped areas, governments are 

finding it more and more difficult to ensure 

water quality. The byproducts of this growth—

increased and accelerated runoff, sewage, inadequate infrastructure, land clearing, industrial 

point sources, air pollution—also pose risks 

for water quality. Meanwhile, development 

can undermine the self-maintenance of water 

resources—wetlands and infiltration of water 

through soil are the natural ways in which water 

is purified. Draining wetlands and impervious 

paving reduce these natural purification processes in terrestrial and aquatic ecosystems.

The three North American countries have 

different definitions and procedures for measuring surface water quality. A comparable 

assessment of North American water quality is 

therefore challenging. But it is clear that, based 

on national reporting, the overall percentage of 

North American surface freshwater in degraded 

condition is significant.

Canada’s water quality index, which is based 

on various parameters such as nutrients, assesses 

surface freshwater quality for its ability to protect aquatic life, including fish, invertebrates and 

plants; it does not assess the quality of water for 

human consumption or use. According to the 

most recent information available, freshwater 

quality in southern Canada was rated “excellent” 

or “good” at 44 percent of monitored sites, “fair” 

at 33 percent of sites, and “marginal” or “poor” 

Water Quality

Water quality refers to the physical, chemical and biological characteristics of surface 

and groundwater. These characteristics affect the ability of water to sustain human 

communities, as well as plant and animal life.

Water
52 Commission for Environmental Cooperation

at 23 percent of sites. Phosphorus, a nutrient 

derived mainly from human activities and a key 

driver of the water quality index, is a major concern for surface freshwater quality in Canada. 

Phosphorus levels exceeded limits set under the 

water quality guidelines for aquatic life over half 

the time at monitored sites.

In the United States, over 40 percent by 

length of small wadeable streams sampled in 

2004–2005 showed substantial disturbances to 

sensitive communities of small water-dwelling 

creatures, indicating significant pollution and 

habitat modification. The most widespread 

stressors were nitrogen, phosphorus, streambed sediments and riparian disturbance. About 

a third of sampled stream length contained 

high nitrogen or phosphorus concentrations, 

and a quarter revealed streambed sediments or 

riparian disturbance. As of 2002, almost half of 

assessed stream length and lake area and onethird of assessed bay and estuarine areas were 

not clean enough to support human uses such 

as fishing and swimming. The leading causes 

of impairment were excess levels of nutrients, 

metals (primarily mercury), sediment and 

organic enrichment from agricultural activities; 

hydrologic modifications; atmospheric deposition; and discharges from industrial, unknown 

or unspecified sources.

Mexico monitors surface water for biochemical oxygen demand (BOD), fecal coliform, 

nitrogen, phosphorus and other substances. BOD 

measures the amount of oxygen consumed by 

microorganisms in decomposing organic matter 

in water. The greater the BOD, the more rapidly 

oxygen is depleted in the stream and the more 

stress is placed on higher forms of aquatic life. 

In 2006, 16 percent of monitored sites had an 

average annual BOD of more than 30 milligrams 

per liter, indicating unacceptable contamination 

under Mexican standards. Fecal coliform are bacteria fed by human or animal waste that serve as 

indicators of contamination. In 2006, 58 percent 

of monitored sites in Mexico had average annual 

concentrations above acceptable levels for drinking water. And, as in the rest of North America, 

levels of nitrogen and phosphorus in surface 

water are also a problem for Mexico. Elevated levels of pollutants containing these elements were 

detected at a majority of monitored sites.

Pollution and contaminants associated 

with surface water also affect groundwater: 

point source contamination (bacteria, organics), nonpoint source pollution from agriculture 

(nitrates and pesticides), industrial contamination (heavy metals, organic compounds), and 

naturally occurring contaminants such as arsenic. Groundwater depletion can create cracks, 

fissures and fractures through land subsidence 

that permits contaminants to enter deeper 

groundwater aquifers. Salt water intrusion into 

coastal aquifers is a problem throughout the 

Gulf of Mexico and Gulf of California regions 

of Mexico and the United States as salt water 

replaces the freshwater being removed from the 

aquifer. Because there are no comprehensive 

surveys or sources of information on groundwater, regional patterns or trends in groundwater quality for North America are unknown.

Why Is This Issue Important to 

North America?

Sustainable access to clean water is vital to the 

human and ecological life of North America. As 

North Americans have experienced the vulnerability and finite nature of clean water supplies, 

they have realized that they must protect and 

conserve this essential resource. Water quality 

concerns that have persisted in North America 

over the last 30 years include sedimentation, 

Sites in which algal blooms have led to animal and plant deaths in North American coastal waters (1993–2002)

Source: Institut français de recherche pour l’exploitation de la mer.

Mercury most often enters North American water 

resources via the deposition of mercury emitted to the 

air from mining, industrial processes, and combustion 

of fossil fuels, municipal and medical waste.

Caged salmon

Seabirds Marine mammals

Fish kills

Sea grass

Farmed fish

Manatees

Seagrass/shellfish

Farmed shimp

Fish kills

Caged salmon

Hard clam larvae

Sampled, but no toxins detected

One time (one year)

2 – 5 times

6 – 10 times

during the 10 year period
Water Quality 53

eutrophication, pathogenic diseases and persistent toxics (mercury and organic chemicals).

Sedimentation

Soil erosion and sedimentation (deposition of 

eroded soil) in lakes, waterways and coastal areas 

are major water quality problems throughout 

North America. Although erosion, sediment 

transport and sedimentation are natural processes, human activity may exacerbate these 

processes in certain parts of the continent, and in 

local or regional situations may be the primary 

cause. Sediment affects water quality by reducing 

water clarity, smothering aquatic habitats and 

acting as a transport mechanism for pollutants 

such as pesticides and fertilizer. In the United 

States, sedimentation is associated with over 

60 percent of degraded stream miles. Similarly, 

Environment Canada has identified sediment as 

a Canadian water quality issue, whereas in Mexico, soil erosion is a major environmental issue. 

Erosion and sedimentation stem primarily from 

human modification of the landscape. As populations continue to grow and land use changes, 

sedimentation will continue to be an issue.

Eutrophication and Nutrient Overenrichment

Eutrophication and high nutrient loadings 

affect both freshwater and coastal systems. In 

eutrophication, excessive plant growth (bloom) 

occurs in water bodies receiving excessive nutrient loads. Eutrophic conditions can occur naturally in lakes as they age and in estuaries, but 

in North America human activities have led to 

widespread nutrient levels and eutrophication 

that far exceed natural levels. Eutrophication 

encourages the growth of toxic algae, which in 

some instances in the marine environment is 

also known as “red tide” (see map). Decomposition of these excess algal blooms reduces the 

level of oxygen in the water to the point that 

other organisms die (hypoxia). 

Canada, Mexico and the United States all 

struggle with nutrient overenrichment of water 

resources caused by poor sewage treatment, 

use of fertilizers and deposition of combustion 

byproducts (nitrogen oxides). In the United 

States, about 55 percent of freshwater impairments and some 20 percent of coastal system 

(estuaries, bays) impairments stem from nutrient loads or eutrophication. The Gulf of Mexico 

dead zone, a product of nutrient inputs (primarily nitrogen) from the Mississippi River basin, 

is the largest expanse of human-caused hypoxia 

in the Western Hemisphere. In Canada, similar 

concerns have arisen about St. Lawrence estuary 

hypoxia caused by factors such as nitrogen.

Pathogens

Contamination of water resources with pathogenic organisms (as indicated by fecal coliform 

bacteria) is still a concern in many areas of 

North America. The source of pathogen contamination of most concern is poorly treated 

and untreated sewage. In some areas, however, 

agricultural operations and wildlife are also a 

factor. Although 71 percent of the US population is served by sewage treatment plants, 

beaches were closed or health advisories were 

issued because of bacterial contamination for 

18,000 instances in 2003, up from 3,000 days in 

the mid-1990s. A similar portion (72 percent) 

of the Canadian population is served by sewage 

treatment plants, but municipal wastewater discharges still represent one of the largest sources 

of pollutant releases by volume to Canadian 

waters. In Mexico, where only 35 percent of the 

people are served by sewage treatment plants, 

bacterial contamination of freshwater and 

coastal systems is a significant issue.

Mercury

Mercury is a metal that accumulates in human, 

fish and animal tissue, sometimes reaching toxic 

levels (see case study). In aquatic ecosystems, 

mercury can enter the food chain through the 

action of bacteria and benthic organisms. Consumers of mercury-contaminated organisms 

can then accumulate mercury to toxic levels—

even where concentrations of mercury in water 

are barely detectable. 

Mercury most often enters North American water resources via the deposition of 

mercury emitted to the air from mining, industrial processes and combustion of fossil fuels, 

municipal and medical waste. In recent years, 

Canada and the United States have reduced 

their mercury emissions: Canada by 80 percent 

over 1990–2003 and the United States by 45 percent over 1990–1999. However, high mercury 

levels in fish still account for over 90 percent 

of fish consumption health advisories issued 

in Canada and 80 percent issued in the United 

States for both freshwater and coastal fisheries. In 2000–2003, mercury was found in 100 

percent of fish sampled as part of the national 

fish tissue study in the United States. Even after 

inputs to contaminated systems cease, mercury 

can continue to accumulate in the food chain 

for decades. And because mercury is easily 

transported long distances in the atmosphere, 

mercury emissions from other continents contribute to the mercury contamination of North 

American fisheries.

Persistent Organic Pollutants

Persistent organic pollutants are organic chemicals that accumulate in human and animal 

fatty tissue, possibly reaching toxic levels. The 

North American countries have long worked to 

reduce the use and release of persistent organic 

pollutants such as DDT, polychlorinated biphenyls (PCBs), dioxins and chlordane, and yet 

these compounds still persist in soils, sediment 

and fish tissue. In the United States, for example, PCBs, dioxins and furans, and DDT were 

widely detected in fish sampled in 2000–2003. 

Although long banned in the United States, 

PCBs were still found in 100 percent of both 

predator and bottom-dweller composite samples. Long-term monitoring of fish populations 

in the Great Lakes has documented a decline in 

PCBs, DDT and other persistent contaminants, 

but the concentrations of some of these constituents still exceed the human and wildlife health 

criteria in various regions of the Great Lakes. 

What Are the Linkages to Other North 

American Environmental Issues?

Water quality is affected by activities within 

a watershed or groundwater recharge area, as 

well as by the global climate and atmospheric 

transport from areas farther away.

Climate Change

As climate patterns change, precipitation and 

runoff patterns in North America will also 

likely change, bringing more drought in some 

areas and more flooding in others. Under 

drought conditions, pollutants can become 

concentrated in water resources to harmful 

levels. With greater runoff and flooding, even 

more pollutants (by quantity and in greater 

varieties) are washed into surface waters.

Walleye fishing.
54 Water Quality

Land Use 

Several studies have identified links between 

quality of water resources and land use in a 

watershed. Land clearing can increase the transport of sediment into surface waters. Pesticides 

and fertilizers applied to the land can be washed 

into surface waters or percolate into groundwater 

aquifers, as can any material spilled on land, such 

as toxic chemicals, oil from cars or gasoline. 

Energy

Levels of demand for energy are linked to water 

pollution. Water used in oil and gas exploration 

and production may become laden with toxics 

that must be removed before it is safe for use 

by humans or wildlife. Combustion byproducts 

from power plants, such as nitrogen oxides, 

sulfur dioxide and mercury, can travel long 

distances in the atmosphere and affect water 

resources far from the facility site, changing the 

pH, contributing nitrogen to the nutrient load 

and contaminating fisheries.

Biodiversity and Ecosystems

Plants and animals living in surface waters are 

accustomed to particular water quality conditions. If the water quality of a lake or stream 

changes, some plants and animals can no longer survive there. Because poor water quality 

is known to reduce biodiversity, the United 

States and Canada use the biodiversity of 

aquatic communities as an indicator of surface 

water quality. Changes in aquatic communities 

because of poor water quality can change the 

way in which the aquatic ecosystem functions, 

as well as the associated terrestrial plant and 

animal communities.

Pollutants

Greater runoff is associated with greater loading 

of sediments, nutrients, toxic contaminants and 

other pollutants—all of which have an effect on 

the quality of drinking water supplies and aquatic 

ecosystems. Meanwhile, as land is converted to 

urban or suburban uses, point source pollution 

also increases—a byproduct of the additional 

wastewater treatment facilities constructed to 

meet the needs of the expanding population 

and new industries. The cumulative influence of 

increased point and nonpoint sources can affect 

the suitability of water to support aquatic ecosystems and other desired water uses. The presence 

of new contaminants such as flame retardants, 

personal care products and pharmaceuticals 

is also beginning to be detected, although the 

overall and cumulative levels of risk for humans 

and ecosystems are still unknown. 

Case Study – Mercury in North American Waters

Raccoon

n n n n

Rainbow Trout

n n

Loon

n n n

Leopard frog

n n n

Minnow

n n

Panther

n n n n n

Human

n n n n

River Otter

n n n n

Bald Eagle

n n n n

Accumulation of mercury through the food chain

n Indicates the concentration of methylmercury as it moves up the food chain

Mercury is a naturally occurring metal that can damage the liver, brain, heart, kidneys, 

lungs and immune system of humans, fish and wildlife. Industrialization has increased 

the proliferation of mercury globally. In most of its chemical forms, mercury is easily 

transported through the atmosphere. Atmospheric mercury is the primary source of 

mercury in North American freshwaters and marine waters. Indeed, virtually no place 

on earth is untouched by the deposition of atmospheric mercury. As a result, mercury 

contamination is pervasive throughout North America, occurring even in areas far from 

cities and industry.

Mercury concentrations in most North American waters are too low to have toxic 

effects on those who come in contact with or ingest the water. However, under the 

right conditions mercury in the water can move into the food chain. In most organisms, mercury binds to proteins and accumulates in the tissues as methylmercury. 

When predators eat mercury-contaminated prey organisms, the mercury from the 

prey’s tissues is transferred to the predator’s tissues (see illustration). Thus the 

higher an organism is in the food chain, the more it can accumulate mercury in its 

tissues and the greater is the potential for toxic effects.

In the freshwater and marine systems of North America, methylmercury in fish is a 

concern. When they ingest mercury-contaminated fish, North American birds, animals 

and humans face the possibility of accumulating toxic levels of mercury in their tissues. Those whose diets consist primarily of fish are at greater risk of experiencing the 

health effects of mercury accumulation.

Source: National Wildlife Federation, <http://www.nwf.org/mercury/bioaccumulation.cfm>. 

Walleye

n n n
Key Findings

n Fresh surface water and 

groundwater are a common 

denominator for life. Continued 

access to freshwater is a concern in 

several areas across North America.

n The distribution of freshwater 

varies widely across North America. 

Industrial and agricultural uses 

account for the majority of North 

American water withdrawals, and in 

some areas human use competes 

with ecological requirements.

n Total water withdrawals in the 

United States grew from 1970 until 

1990, but since then they have 

been relatively constant despite 

continued population growth. In 

Canada and Mexico, total water 

withdrawals have continued to rise.

n Climate change, land use and 

population growth affect the 

availability of freshwater throughout 

North America. As climate 

changes, patterns of precipitation 

and runoff are also likely to 

change. In response to expected 

population growth and patterns 

of development, heightened 

competition among water users is 

anticipated.

 The North American Mosaic: An Overview of Key Environmental Issues 55

What Is the Environmental Issue?

Water—a finite but renewable resource—is 

essential to sustain life, development and the 

environment. Although over 70 percent of the 

earth’s surface is covered by water, 97 percent 

is salty ocean water and less than 3 percent is 

freshwater (see figure). Of the freshwater, 69 

percent is frozen in glaciers and permanent 

snow, and an additional 30 percent is “hidden” 

Water Quantity and Use

Water quantity and use are directly related to a variety of human and ecological needs: 

agricultural, industrial, domestic and environmental. Human development and the 

environment depend on adequate supplies of clean water.

Water

118

Freshwater

Total water

Oceans 97.5%

Freshwater 2.5%

Glaciers 68.7%

Groundwater 30.1%

Permafrost 0.8%

Freshwater lakes 67.4%

Soil moisture 12.2%

Atmosphere 9.5%

Other wetlands 8.5%

Rivers 1.6%

Plants and animals 0.8%

Surface and 

atmospheric water 

0.4%

Surface and 

atmospheric water 

Global distribution of the world’s water

Source: United Nations Environment Programme.
56 Commission for Environmental Cooperation

Average annual precipitation in North America

Average annual 

precipitation (mm)

0 – 99 

100 – 199

200 – 399 

400 – 599

600 – 999 

1,000 – 1,399

1,400 – 1,999

2,000 – 2,799

2,800 – 5,599

Source: ESRI.

groundwater. Thus less than 1 percent of the 

earth’s water is present as fresh surface water 

and atmospheric vapor.

Fresh surface water and groundwater are a 

common denominator for life in all countries. 

Not only does water sustain all life, but there are 

no substitutes for it in many commercial and 

industrial processes, and especially in growing agricultural crops. The use of freshwater 

for public water supplies, irrigation, industrial 

processes and cooling of electric power plants 

exerts pressure on water resources. Meanwhile, 

the patterns of freshwater use affect public 

water supplies, the salinization of freshwater 

bodies in coastal areas, food production and 

competition among various end uses. Another 

factor is that aquatic ecosystems typically need 

both minimum and maximum flows at specific 

times of the year to sustain their various communities of aquatic organisms. 

Why Is This Issue Important to 

North America?

Freshwater resources are of major environmental and economic importance to North America, but the distribution of these resources 

varies widely. Canada has some 20 percent of 

the world’s total freshwater resources. Much of 

it, however, is in remote locations or retained 

in lakes, underground aquifers and glaciers. By 

contrast, Mexico is primarily an arid country 

in which freshwater is abundant in some local 

areas only. Not just the amount of freshwater 

is important, but also how rapidly this water 

is replenished through rainfall and runoff. In 

many parts of North America, humans’ needs 

and uses for water compete with the need for 

water to sustain aquatic life.

Distribution of Water Resources

North America’s internal renewable water 

resources include the average annual flow of 

rivers and the recharge of groundwater (aquifers) by precipitation within a country’s borders 

(see table). Surface water produced internally 

represents the average annual flow of rivers 

generated from internal precipitation and the 

base flow generated by aquifers. Groundwater 

recharge is estimated by measuring rainfall in 

arid areas where rainfall is assumed to infiltrate 

aquifers, although the uncertainties about these 

estimates are considerable. The total of the two 

accounts for any overlap occurring when surface waters recharge aquifers, or when aquifers 

release to surface flow. Natural incoming flows 

originating outside a country’s borders are not 

included in the total in the table.

Freshwater resources are not distributed 

uniformly across North America (see map). In 

Freshwater withdrawals for domestic, 

industrial and agricultural uses in 

North America (2000)

Source: Food and Agriculture Organization.

Use Canada Mexico United 

States

Total withdrawal 

(km3/yr)

46 78 479

Domestic 19% 17% 13%

Industrial 69% 6% 46%

Agricultural 12% 77% 41%

Canada Mexico United States North America

Surface water produced internally 2,840 361 2,662 5,863

Groundwater recharge 370 139 1,300 1,809

Total (adjusted for overlap) 2,850 409 2,800 6,059

North America’s internal renewable water resources (km3/yr) 

Source: Food and Agriculture Organization.
general, the eastern regions of North America 

are considered water-rich or precipitationdominated, although droughts can significantly affect water availability on a periodic 

basis. Annual normal precipitation values 

of 800 millimeters or greater are generally 

associated with water-rich areas in Canada, 

southeastern Mexico and the eastern United 

States. Annual normal precipitation values 

of less than 600 millimeters are associated 

with arid and semiarid regions, including the 

Great Plains regions of Canada and the United 

States and much of Mexico. This distribution 

plays a role in the withdrawal of surface versus 

groundwater for use.

Uses of Water Resources

In North America, about 85 percent of water 

withdrawals are by industry and agriculture 

combined, but the distribution among uses varies by country (see table). In Canada, 69 percent of withdrawals are used by industry and 12 

percent by agriculture, whereas in Mexico agriculture uses about 77 percent and industry only 

6 percent. In the United States, agricultural and 

industrial withdrawals are roughly similar. 

Not all freshwater withdrawals have the 

same implications for the water supply. Agriculture is a highly consumptive use, returning 

only a small portion of water withdrawn back 

to the source. The rest is lost to evaporation 

or used for irrigation and livestock watering. 

Industrial uses are often much less consumptive because water is recycled internally and 

eventually returned in part downstream. An 

example is electric thermal power generation, which accounts for a large proportion of 

industrial withdrawals. Some of the water is 

converted to steam to drive the generator producing the electricity, but most of the water is 

used for condenser cooling and later released. 

In domestic water use, returns are largely via 

sewage, which is treated in most areas before 

being returned to surface waters. Other 

human uses of freshwater—such as those for 

hydroelectric power generation, shipping and 

recreation—are in-stream uses that are not 

counted as withdrawals. 

Withdrawals can also be expressed on 

a per capita basis (see graph). The United 

States and Canada are the highest per capita 

water users in the world when withdrawals 

for all uses are considered. Per person usage is 

more than two and a half times that in Asia or 

Europe and over six times that in Africa. One 

reason is the low cost of water relative to that 

in other industrialized countries. Per capita 

water use in Mexico is more comparable to 

that in other areas of the world, although it is 

still slightly higher. 

Total water withdrawals in the United 

States grew from 1970 until 1990, but since 

then they have been relatively constant, even 

though the population has grown by about 16 

percent. In Canada and Mexico, total water 

withdrawals have continued to rise. Between 

1972 and 1996, Canada’s rate of water withdrawals increased by almost 90 percent, 

although its population rose by only 34 percent over the same period. Mexico has also 

experienced higher water withdrawals over 

the last 30 years.

Irrigation is partly responsible. Land 

under irrigation has increased since 1960, 

doubling in Canada and Mexico and increasing by more than 50 percent in the United 

States. Much of this increase has occurred in 

arid or semiarid regions, where groundwater is the primary water source. Slightly over 

a tenth of North America’s cultivated area is 

irrigated, of which over 75 percent is in the 

United States and slightly more than 20 percent in Mexico. In the United States, areas 

equipped for irrigation account for about 12 

percent of cultivated land. In Mexico, almost a 

quarter of cultivated land is irrigated. By contrast, less than 2 percent of Canada’s cultivated 

area is under irrigation. 

The majority of withdrawals in North 

America are from surface water, but groundm3 / person / year

 Canada Mexico United States

1,800

1,600

1,400

1,200

1,000

800

600

400

200

0

Per capita water use in North America 

based on all withdrawals (2000)

Source: Food and Agriculture Organization. 

Case Study – The Pitfalls of Groundwater Mining

Land subsidence, or even collapse, can 

occur when groundwater withdrawal 

exceeds recharge and a void is created 

in the underground aquifer. One example is the San Joaquin Valley, one of 

the most productive agricultural areas 

in the United States. The California 

Central Valley, which includes the San 

Joaquin Valley, produces about 25 

percent of US table food on only 1 percent of US farmland. From 1900 until 

the 1970s, groundwater was mined to 

irrigate and grow this food. But eventually groundwater withdrawals significantly exceeded recharge, and the over 

75 years of pumping groundwater for 

irrigation caused the land to drop (subside) by over 8 meters. In this photo of 

a site near Mendota in the San Joaquin 

Valley, the top sign on the pole indicates 

the level of the land surface in 1925 compared with the level when the photo was 

taken, about 1977.

Water Quantity and Use 57

Subsidence in San Joaquin Valley, California 

Photo: US Geological Survey.
58 Water Quantity and Use

water withdrawals also serve many uses, from 

domestic supply to irrigation. Although globally the amount of groundwater exceeds that 

of surface water (i.e., about 30 percent groundwater to less than 1 percent surface water), 

surface water is much more rapidly replenished (through precipitation) than groundwater. Some groundwater is called “fossil water,” 

because its rate of recharge or renewal is measured in geologic time (millions of years), 

whereas the renewal of surface waters is measured in days or weeks.

In many areas of North America, the 

groundwater table is declining; withdrawals are 

simply outweighing recharge. In some areas of 

Mexico and the United States, the ground overlying these declining aquifers has collapsed 

or subsided. Mexico City has experienced up 

to 18 meters of subsidence over the past 100 

years because of groundwater withdrawal. 

Since the late 1980s, authorities in Mexico City 

have managed groundwater use in an effort to 

reduce subsidence, and yet the observed rate of 

subsidence during the last 20 years has been on 

the order of 20–30 centimeters per year. In the 

United States, more than 43,500 square kilometers in 45 states have been directly affected by 

subsidence (see case study).

What Are the Linkages to Other North 

American Environmental Issues?

Water is a pervasive factor in all aspects of the 

environment, but linkages with certain issues 

merit further consideration.

Climate Change

Water vapor is not only the most important 

greenhouse gas, but also a major influence on 

the earth’s climate. Water patterns are both 

affected by and influence climate change. Patterns of increased rainfall and drought have 

been linked with El Niño and La Niña events in 

North America, based on changes in the sea surface temperature. As climate patterns change, 

precipitation and runoff patterns are also likely 

to change, with more drought in some areas 

and more flooding in others. Scientists may not 

be able to predict the precise patterns of change 

with certainty, but they do understand that climate change will lead to changes in water availability in North America.

Land Use

Like climate change, changes in land use, 

particularly those stemming from population growth, are linked to water quantity and 

use. In general, population growth and land 

use changes are expected to play a greater role 

than climate change in the scarcity of water 

resources over the next 25 years. Certain areas 

of North America are likely to have greater 

water shortages than others, however, because 

of the spatial differences in the distribution of 

renewable and available water within countries and across geographic areas. In some 

areas, the trends in climate change, population 

change and land use are occurring simultaneously. Land use can also affect water supply by 

expanding the impervious surfaces that limit 

the extent of groundwater recharge.

Biodiversity and Ecosystems 

Precipitation and runoff sustain both terrestrial and aquatic ecosystems, as well as provide 

the freshwater required to sustain estuarine 

ecosystems. Competition for water between 

humans and aquatic ecosystems can significantly alter flow regimes—that is, as water is 

withdrawn for other uses, the lowered flow no 

longer supports aquatic communities. Aquatic 

ecosystems require more than just a minimum 

amount of water for maintenance; they also 

require flooding with the right frequency and 

magnitude to sustain the system. Dams and 

other flow impediments can alter both the 

timing of flows and the amount of water that 

flows in streams, significantly affecting the 

downstream biodiversity (see case study).

Pollutants 

Runoff is usually accompanied by increased 

loading of sediments, nutrients, toxic contaminants and other pollutants—all of which 

affect the quality of drinking water supplies 

and aquatic ecosystems. As land is converted to 

urban or suburban uses, point source pollution 

increases as additional wastewater treatment 

facilities are constructed to treat the waste from 

an expanding population. Impervious surfaces 

in developed areas also promote additional 

runoff that may not have an opportunity to be 

filtered through natural processes. The cumulative influence of increased point and nonpoint 

sources can affect the suitability of water to 

support aquatic ecosystems and to meet other 

desired water uses. 

The Colorado River Delta is located in the region below the California-Mexico border, where 

the Colorado River has historically flowed into the Gulf of California (Sea of Cortez). This oasis 

of 7,800 square kilometers used to be one of the largest desert estuaries in the world. In 

the 1920s, naturalist Aldo Leopold commented on the rich diversity of waterfowl, freshwater 

and brackish aquatic life, jaguar, deer, beaver and other wildlife found in the Colorado River 

Delta. Today, the Delta is only about 5 percent of its original size, and it no longer supports 

this rich biodiversity because its historical inflow no longer consistently reaches this estuary. Upstream dams and diversion canals have significantly reduced and altered flow in the 

Colorado River. Even though estuaries such as the Delta are naturally brackish, these ecosystems must have freshwater inflows to sustain their biodiversity and productivity.

Case Study – Colorado River Delta

An image of the Colorado River Delta taken September 2000 by the Spaceborne Thermal Emission and Reflection 

Radiometer (ASTER). Photo: National Aeronautics and Space Administration.
 The North American Mosaic: An Overview of Key Environmental Issues 59

Key Findings

n Shared water resources are 

water bodies that span political 

boundaries. Along the borders of 

the United States with Canada and 

Mexico, the effective management 

of the quality and use of these water 

bodies is a shared responsibility.

n Climate change, population 

growth, pollution, invasive species 

and land use change affect the 

quantity and quality of shared 

water resources and their ability to 

support aquatic ecosystems.

n In 2005 the US Environmental 

Protection Agency rated several 

important shared water bodies. 

The Gulf of Mexico was rated as 

in “fair” condition and the Great 

Lakes as in “fair to poor” condition. 

The Gulf of Maine was in generally 

better condition than the rest of the 

northeastern coastal region, which 

was rated as in “poor” condition. 

n It is difficult to characterize overall 

trends across all the important 

features of shared water resources. 

In the Great Lakes, for example, 

trends in ecosystem conditions 

show improvement in some areas 

and worsening in others. 

What Is the Environmental Issue?

Water is a shared global resource. The hydrologic cycle transports water around the globe 

through atmospheric vapor and ocean currents. 

On land, water (streams and rivers) forms the 

political borders between many nations. Countries also share the lakes that span their political 

boundaries and the groundwater aquifers that 

lie beneath those boundaries.

Because water is essential for supporting 

all life processes, many nations view its adequate 

availability as a fundamental human right. Conflicts over water rights were recorded as early 

as 2500 B.C., and such conflicts are expected 

to arise more frequently in the future as human 

populations and economic development continue to grow and climate patterns change.

Why Is This Issue Important to 

North America?

North America has extensive shared water 

resources, but there are vast differences in the 

Shared Water Resources

Shared water resources are the rivers and estuarine regions that form borders 

or flow across borders, the lakes that span political boundaries, marine areas 

with multiple jurisdictions, and the groundwater aquifers that lie beneath 

political boundaries.

Water

A view of the Rio Grande river bordering the United States (left) and Mexico as seen from an international bridge in 

the border city of Ciudad Juárez, Mexico. Photo: REUTERS/Tomas Bravo.
60 Commission for Environmental Cooperation

quality and quantity of those resources across 

the continent. Along the northern border 

between Canada and the United States and 

the southern border between Mexico and the 

United States, the management of shared water 

resources—both for quantity and quality—is 

an important issue. 

Management of Shared Water Resources

Canada and the United States share water along 

their almost 9,000-kilometer border from the 

Atlantic to the Pacific, and across a gradient 

from the relatively water-rich areas in the east 

to the more arid regions in the west. Likewise, 

Mexico and the United States share water along 

their 3,000-kilometer border, which runs amid 

the arid regions from Texas to California. But 

even with these gradients, many of the water 

quantity and quality issues among North 

American countries are similar. Potential conflicts over shared water resources in North 

America are addressed through bilateral water 

treaties, agreements and protocols.

The largest shared water resource between 

Canada and the United States is the Great 

Lakes–St. Lawrence River system, which contains one-fifth of the freshwater in the world. 

Other shared Canada-US resources range from 

the Gulf of Maine in the east to the Red River 

of the North in the central region to the Pacific 

Ocean in the west. Over the years, the United 

States and Canada have negotiated agreements 

to resolve water issues. As early as 1909, the 

Boundary Waters Treaty established the International Joint Commission to prevent and 

resolve disputes between the two countries. 

In 1972 Canada and the United States signed 

the first Great Lakes Water Quality Agreement 

(later revised in 1978 and 1987) to control pollution in these waters and to clean up wastes 

from industries and communities. The 1987 

revisions introduced the concepts of Areas of 

Concern, Lakewide Management Plans and 

other elements generally recognizing an ecosystem approach toward restoration and maintenance of the Great Lakes.

The water quantity and elevations of the 

Great Lakes are a concern for both Canada and 

the United States. Recently, the water levels of 

some of the Great Lakes have declined (notably Lakes Michigan and Huron). Some of the 

environmental consequences of lower water 

levels are smaller wetland areas, nursery areas, 

fish habitat and wildlife habitat, including resting and nesting areas for migratory waterfowl. 

As lake levels fall, dredging will be required to 

maintain shipping lanes. But dredging can disturb and resuspend contaminated sediments 

throughout the Great Lakes. Associated economic consequences are lower shipping tonnage because of shallower drafts in harbors, 

loss of recreation and less-efficient thermoelectric power generation.

Major shared resources between Mexico 

and the United States are the Colorado River 

and Rio Grande/Rio Bravo and the Gulf of 

Mexico. Many other surface water and groundwater resources important to local communities, states and provinces also lie along these 

borders. Ensuring that both countries have 

sufficient shared water resources has been one 

of the driving factors in treaties, agreements 

and protocols along the Mexico-US border. 

The Convention of 1906 between Mexico and 

the United States addressed water distribution 

issues for the Rio Grande. Several decades 

later, the US-Mexico Water Treaty of 1944 distributed waters in the lower Rio Grande, the 

Colorado River and the Tijuana River and also 

created the US-Mexico International Boundary Water Commission. In 1983 Mexico and 

the United States enacted the Border XXI 

Agreement to prevent, reduce and eliminate 

sources of pollution. 

Water Quality

Since 1994, Environment Canada and the US 

Environmental Protection Agency have jointly 

evaluated the Great Lakes, publishing the results 

Source: US National Oceanic and Atmospheric Administration.

Case Study – Resolving Water Quantity Issues in the San Pedro River

The San Pedro, which has its headwaters in Sonora, Mexico, and flows north into 

southern Arizona, is the largest undammed river in the southwestern United States. 

Water quantity is an issue for this river because the San Pedro is in an evaporationdominated region, with low rainfall. During dry months, groundwater maintains flow 

in portions of the San Pedro, and so it is an oasis in the arid Chihuahua and Sonoran 

Deserts. However, groundwater has been depleted because of withdrawals for mining 

and ranching in Mexico and domestic withdrawals by Sierra Vista and Ft. Huachuca 

in Arizona. In response, citizens formed the San Pedro National Conservation Area in 

1988, and those on both sides of the border implemented collaborative measures, 

management measures—such as creating land preserves, implementing county codes 

for water use and promoting water conservation practices (e.g., low-flow water fixtures, 

toilets, washers)—and stakeholder outreach and education programs. Monitoring programs are in place to track progress toward achieving goals for sustainable groundwater levels in order to restore and protect the San Pedro River.

The San Pedro River. Photo: Adriel Heisey.
Shared Water Resources 61

in the State of the Lakes Ecosystem (SOLEC) 

report. SOLEC assesses the Great Lakes basin 

ecosystem components using a suite of ecosystem 

health indicators. For 2007, the overall status of 

the Great Lakes ecosystem was assessed as mixed 

because some conditions or areas were good or 

improving, and others were poor or worsening. 

Some of the improving conditions were declining levels of most contaminants in herring gull 

eggs and predator fish, the achievement of phosphorus targets, improving lake trout stocks, 

and the partial recovery of mayfly (Hexagenia) 

populations. Some of the negative trends were 

the increasing concentrations of the flame-retardant polybrominated diphenyl ethers (PBDEs) 

in herring gull eggs, the nuisance growth of 

the green alga Cladophora, the persistence of 

pervasive non-native species, unsustainable 

groundwater withdrawals, the growing number 

of impervious surfaces in urban areas, the longrange atmospheric transport of polychlorinated 

biphenyls (PCBs) and other contaminants, the 

ongoing shoreline development, and the declining populations of some species of amphibians 

and wetland-dependent birds.

In 2005 the US Environmental Protection Agency scored several large shared water 

bodies based on a large amount of monitoring data collected between 1997 and 2000. In 

this assessment, the Gulf of Mexico was rated 

as in “fair” condition and the Great Lakes as 

in “fair to poor” condition. The Gulf of Maine 

was in generally better condition than the 

rest of the northeastern coastal region, which 

was rated as in “poor” condition, but signs of 

degraded water quality condition were still 

noted throughout the area north of Cape Cod 

and along the coastline of Maine.

Many of the water quality issues along 

country borders are similar. DDT and other 

chlorinated hydrocarbon pesticides, as well as 

PCBs and associated arochlors (aromatic chlorinated hydrocarbons), have contaminated fish 

tissue from the Gulf of Maine to the Great Lakes 

to the Gulf of Mexico. PCB and DDT concentrations are also a concern in the Rio Grande 

separating Mexico and the United States. Mercury contamination of fish tissue is widespread 

as well, not only in North America, but also 

globally. The mercury concentrations in top 

predator fish, such as walleye and largemouth 

bass, have been so high that fish consumption 

advisories have been issued for tributaries to the 

Great Lakes and along the Canada-US border. 

King mackerel have mercury concentrations 

high enough to trigger consumption advisories 

throughout the Gulf of Mexico. 

Source: US National Oceanic and Atmospheric Administration.

Case Study – Addressing Water Quality Issues in Lake Erie

During the late 1960s and early 1970s, water quality in Lake Erie deteriorated to the 

point that the lake was declared “dead.” Because Lake Erie is the shallowest of the 

Great Lakes, high phosphorus concentrations were contributing to serious eutrophication problems such as beaches covered with algal scum, loss of oxygen to support fish 

and other aquatic life in the bottom waters, and replacement of fish such as walleye 

with pollution-tolerant species such as carp.

In 1972 Canada and the United States signed the Great Lakes Water Quality 

Agreement and began to work on reducing phosphorus loading to the Great Lakes. 

Great Lakes provinces and states worked to reduce phosphorus in municipal and 

industrial effluents and to eliminate phosphorus from detergents. As a result, total 

phosphorus and chlorophyll concentrations decreased dramatically. Fish species richness increased with the return of pollution-intolerant species such as burbot, lake 

whitefish, smallmouth bass and white sucker and a decline in pollution-tolerant species such as brown bullhead, common carp and white crappie.

Over the last decade, however, Lake Erie’s concentrations of phosphorus have 

been on the rise again. Tributary loadings of dissolved phosphorus are increasing. 

Hypoxia and anoxia in the central basin are more extensive and occur over a longer 

period of time. Blooms of the hazardous cyanobacteria Microcystis and the extensive 

growth of Cladophora, a clinging filamentous green alga, are beginning to rival those 

of the 1970s. As a result, nutrient management, particularly for phosphorus, remains 

the top priority for improving the lake, and the United States and Canada are developing a new binational nutrient management strategy for the lake. Although yellow perch 

stocks are now recovering throughout the lake, the top predator species populations 

of walleye, lake trout and lake whitefish continue to struggle.

Lake Erie shore.
62 Shared Water Resources

Salinity is another problem—it rises as 

irrigation water seeps through mineral-rich 

soils and then returns to surface water, thereby 

transporting these dissolved minerals. In the 

Colorado River, salinity rose over the first half 

of the twentieth century as irrigated acreage 

increased in the Colorado River Basin. In 1973 

the International Boundary and Water Commission adopted Minute 242 to address the 

salinity issues in the Colorado River.

Rivers and streams throughout North 

America also exhibit degraded water quality 

because of the loading of oxygen-consuming 

organic matter, sedimentation that decreases 

water clarity and water depth and volume 

and nutrients that contribute to nuisance and 

harmful algal blooms.

What Are the Linkages to Other North 

American Environmental Issues?

Shared water resources are vitally linked to other 

important environmental topics such as climate 

change, land use, biodiversity and pollutants.

Climate Change

As climate patterns change, precipitation and 

runoff patterns are likely to change, with more 

drought in some areas and greater flooding in 

others. The warming temperatures of the Great 

Lakes have increased their evaporation during winter when they used to freeze, which, 

in turn, has contributed to lower lake levels. 

Warmer future temperatures are also expected 

to further reduce the Colorado River’s stream 

flow and water supplies. Meanwhile, increased 

runoff will result in greater loading of sediment, 

organic matter, nutrients and toxic contaminants to aquatic ecosystems throughout North 

America. Overall, reduced water quantity and 

degraded water quality will make it more difficult for all three countries to satisfy international treaty requirements.

Land Use

Along with climate change, population growth 

and changes in land use will play a greater role 

in the scarcity of water resources over the next 

25 years because of the growing urbanization 

and competition among water users, both 

within countries and along international borders. In view of the spatial differences in the 

distribution of renewable and available water 

within countries and across geographic areas, 

certain areas of North America are likely to 

have greater water shortages than others. 

Biodiversity and Ecosystems

Biodiversity in aquatic ecosystems is affected 

by both water quantity and quality, as illustrated by the examples of the San Pedro River 

and Lake Erie (see case studies). Biodiversity 

is also affected by the introduction of invasive 

species, which have already affected biodiversity in shared resources. Examples are zebra 

mussels in the Great Lakes and water hyacinth in the Rio Grande. Zebra mussels not 

only outcompete native mussels for habitat, 

but also affect food web dynamics by filtering 

the food needed by other organisms out of the 

water column. Water hyacinth are clogging the 

Rio Grande and its tributaries with their dense 

growth, blocking sunlight for native plants and 

depleting the water of oxygen needed to support fish and other aquatic organisms.

Pollutants

Pollutants often cross political boundaries. 

Although DDT has been banned and PCB use 

has been restricted for decades, the atmospheric 

transport and deposition of these compounds 

are continuing to maintain concentrations in 

fish tissue from the Great Lakes to the Gulf of 

Mexico. Canada and the United States have identified Areas of Concern and developed Remedial Action Plans for the Great Lakes (see map). 

Nutrient overenrichment has contributed to the 

hypoxia problem in the Gulf of Mexico and to 

eutrophication of lakes, reservoirs, streams and 

rivers throughout North America. Atmospheric 

nitrogen contributes as much as 30 percent of 

the nitrogen loading to Chesapeake Bay. Atmospheric sources also contribute nitrogen to the 

Great Lakes and Gulf of Mexico. 

Areas of concern identified for water quality improvement 

in the Great Lakes by Canada and the United States

Source: International Joint Commission.

1 Nipigon Bay

2 Thunder Bay

3 St. Louis Bay/River

4 Torch Lake

5 Deer Lake – Carp Creek/River

6 Manistique River

7 Menominee River

8 Fox River/Lower Green Bay

9 Sheboygan River

10 Milwaukee Estuary

11 Waukegan Harbor

12 Grand Calumet River/ 

 Indiana Harbor Canal

13 Kalamazoo River

14 Muskegon River

15 White Lake

16 Saginaw Bay

17 St. Clair River

18 Clinton River

19 Detroit River

20 Rouge River

21 River Raisin

22 Maumee River

23 Wheatley Harbour

24 Black River

25 Cuyahoga River

26 Ashtabula River

27 Presque Isle Bay

28 Buffalo River

29 Niagara River

30 Eighteen Mile Creek

31 Rochester Embayment

32 Oswego River (delisted)

33 St. Lawence River/ 

 (Cornwall/Massena)

34 Bay of Quinte

35 Port Hope

36 Toronto and Region

37 Presque Isle Bay

38 Severn Sound (delisted)

39 Sapnish Harbour

40 St. Marys River

41 Penisula Harbor

42 Jackfish Bay

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Commission for Environmental Cooperation

393, rue St-Jacques Ouest, Bureau 200

Montréal (Québec) Canada H2Y 1N9

t (514) 350-4300 f (514) 350-4314

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