time over the past several thousand years.

A warming trend has been recorded since the late 19th century,

with the most rapid warming occurring over the past two

decades. If emissions of greenhouse gases continue unabated,

scientists say we may change global temperature and our

planet’s climate at an unprecedented rate for our society.

This brochure addresses the most frequently asked

questions about the science of global warming and our

changing climate. Answers are based on the

assessments of the Intergovernmental Panel on Climate

Change and on the most recent peer-reviewed

scientific literature.
20th century was the warmest in the last 1,000 years.

The 1990s were the warmest decade and 1998 was the

single warmest year of the past millennium.

Trend

58.0°F

58.5°F

Source: NASA’s Goddard Institute for Space Studies 5-year surface annual mean

57.0°F

57.5°F

56.0°F

1880 1894 1908 1922 1936 1950 1964 1999 1978 1992

56.5°F

Global Average Temperature

How do we take 

Earth’s temperature?

Earth’s temperature is taken through a

network of thermometers on ships, buoys

and land-based weather stations. The data

are compiled by organizations like the World

Meteorological Organization, NASA and the

U.S. National Oceanic and Atmospheric

Administration. This global temperature

record dates back to about 1860. During

this period, measuring techniques have

changed, some weather stations relocated

and others became surrounded by cities.

Scientists have taken special care to

address these problems to ensure the

global temperature record is reliable and

consistent. To know what temperatures

were like before 1860, scientists must rely

on limited records or reconstruct Earth’s

temperature history by examining tree rings,

pollen records and air locked away in

ancient ice.
disappear, our planet would be 60°F colder and uninhabitable.

Q. Are human activities responsible for the warming?

A. Separating out the impact of human activity from natural climate

variation is extremely difficult. Nonetheless, the IPCC concluded there

is a “discernible human influence” on climate. This means the observed

global warming is unlikely to be the result of natural variability alone

and that human activities are at least partially responsible.

Q. What are the most important greenhouse gases?

Where are they coming from?

A. Many greenhouse gases occur naturally, but human activities are

adding gases to the natural mix at an unprecedented rate. Water vapor

ese geeouse gases coue o se

A. Because human emissions of CO2 and other greenhouse gases continue to climb, and because they remain in the atmosphere for decades

to centuries (depending on the gas), we’re committing ourselves to a

warmer climate in the future. The IPCC projects an average global tem

perature increase of 2–6°F by 2100, and greater warming thereafter.

Temperatures in some parts of the globe (e.g., the polar regions) are

expected to rise even faster. Even the low end of the IPCC’s projected

range represents a rate of climate change unprecedented in the past

10,000 years.

Q. What are the potential impacts of global warming and

a changing climate?

A. Our health, agriculture, water resources, forests, wildlife and coastal

areas are vulnerable to global warming and the climatic changes it will

bring. The IPCC concluded that “climate change is likely to have wideranging and mostly adverse impacts on human health, with significant

loss of life.” A few degrees of warming increases the chances of more

frequent and severe heat waves, which can cause more heat-related

death and illness. Greater heat can also mean worsened air pollution, a

well as damaged crops and depleted water resources. Warming is likely

to allow tropical diseases, such as malaria, to spread northward in some

areas of the world. It will also intensify the Earth’s hydrological cycle.

This means that both evaporation and precipitation will increase. Some

areas will receive more rain, while other areas will be drier. At the same

time, extreme events like floods and droughts are likely to become more

frequent. Warming will cause glaciers to melt and oceans to expand.

The IPCC projects that sea level will rise one half foot to three feet over

the next century. This threatens low-lying coastal areas. Scientists are

also concerned that warming could lead to more intense storms.

Source: NASA’s Goddard Institute for Space Studies

Most radiation is absorbed by

the Earth’s surface and warms it.

Infrared radiation is emitted

from the Earth’s surface.

The Greenhouse Effect

Solar radiation

passes through

the clear

atmosphere.

Some of the infrared

radiation passes through

the atmosphere and some

is absorbed and re-emitted

in all directions by greenhouse gas molecules. The

effect of this is to warm

the Earth’s surface and the

lower atmosphere.

Some solar radiation is

reflected by the Earth and

the atmosphere.
changes in weather and climate differ as well. Because the weather

changes from day to day, current weather forecasts are reliable for

roughly ten days. Climate, on the other hand, can be thought of as

average weather, including weather’s variability over much longer time

horizons (e.g., from year to year). Natural changes in our planet’s climate happen over the course of years, centuries and many millennia.

Long-term climate forecasts are possible because scientists understand

many of the factors that influence climate over such long periods, such

as changes in the sun’s energy and the level of greenhouse gases in

the atmosphere. Climate scientists do not claim to know how to predict

day-to-day fluctuations (weather) over the 21st century. Rather, they are

predicting how they think average temperature and precipitation (climate) will change due to human activities.

Q. How do we know CO2 in the atmosphere is increasing?

And that humans are responsible?

A. Careful measurements have confirmed that CO2 is increasing in the

atmosphere and that human activities are the primary cause. CO2

measurements have been taken directly from the atmosphere over the

past few decades. CO2 trends for earlier times have been derived from

measurements of CO2 trapped in air bubbles in glacial or polar ice. The

30% increase in atmospheric CO2 observed since pre-industrial times

(shown in the graph above) cannot be explained by natural causes.

CO2 concentrations have varied naturally throughout Earth’s history.

However, CO2 concentrations are now higher than any seen in at least

the past 450,000 years.

pyy 

fluorocarbons — has led to cooling of the upper atmosphere.

Scientists think these temporary cooling effects have been masking

some of the long-term warming being caused by human emissions of

greenhouse gases. Over the 21st century, the ongoing buildup of

greenhouse gases in the atmosphere is likely to be the most dominant

influence on our planet’s climate.

Q. How will global warming affect the polar ice caps?

A. Polar ice caps are some of the largest surface features on our planet

and any changes to them, however small, could have far-reaching

effects. Melting due to global warming is expected to reduce the size

and extent of the polar ice caps, even after taking into account the

potential for more snow and ice accumulation atop the ice sheets due to

increased precipitation. Melting of polar ice and land-based glaciers is

expected to contribute to the one half foot to three-feet sea level rise

projected by the IPCC for the 21st century. Shrinking ice caps may also

cause changes in ocean circulation and even storm tracks. To be sure,

not all of the melting currently occurring is due to global warming, and

the melting of floating sea ice does not affect sea level. Further warming will likely accelerate the shrinkage of ice caps and glaciers, however

Of particular concern is the stability of the West Antarctic Ice Sheet. A

sudden collapse would raise sea levels 16–20 feet but the IPCC considers the likelihood of such a collapse before the year 2100 low.
complex interactions among increased CO2, rising temperatures, and

water and nutrient availability. Elevated levels of CO2 can essentially

fertilize plants and crops. However, plant growth is also affected by

other factors in addition to CO2 — factors that will be influenced by climate change. Modest temperature increases, for example, can

enhance growth, but if temperatures increase too much, growth actually

declines. Rising temperatures also increase the process by which

plants release CO2. Higher temperatures can increase the rate of evaporation, drying out soils. Insufficient water decreases plant growth.

Plants also cannot respond to more CO2 unless sufficient nutrients are

available. Furthermore, the growth-enhancing effects of CO2 may

diminish over time. Real-world crop yields would also be subject to the

hazards of droughts and floods under a changing climate.

Q. I’ve heard CO2 emissions from human activities are

small compared to what’s released by nature.

A. The Earth has a natural CO2 cycle that moves massive amounts of

CO2 into and out of the atmosphere. The oceans and land vegetation

release and absorb over 200 billion metric tons of carbon into and out of

the atmosphere each year. When the cycle is balanced, atmospheric

levels of CO2 remain relatively stable. Human activities are now adding

about 7 billion metric tons of carbon into the atmosphere every year,

which is only about 3–4% of the amount exchanged naturally. But that’s

enough to knock the system out of balance, surpassing nature’s ability to

take our CO2 emissions out of the atmosphere. The oceans and land

vegetation are absorbing about half of our emissions; the other half

remains airborne for 100 years or longer. This is what is causing the

rapid buildup of CO2, a buildup that dwarfs natural fluctuations.

scale effects of global warming, such as global temperature increase

and average sea level rise. The models are less reliable when it comes

to simulating changes in other weather variables such as changes in

rainfall. Also, current models are still ill-equipped to predict with any

confidence what will happen in local areas.

Q. Can you say the recent extreme weather is a

manifestation of climate change?

A. Given our knowledge of global warming and our changing climate,

we can expect more extreme weather, including more frequent hot days

and droughts, less frequent cold days, and more precipitation (including

more snowfall in cold areas). But attributing any particular extreme

weather event to global warming remains beyond the current limits of

scientific capability.

Q. Does El Niño have anything to do with global warming

and climate change?

A. El Niño is a natural phenomenon that has been occurring throughout

the centuries, though not always with the same regularity; it now occurs

about every two to seven years. El Niño is the strong warming of the

equatorial Pacific ocean. Its effects are felt worldwide, which demonstrates the interconnected nature of the Earth’s climate. Recent El Niño

events have been very strong and have contributed to the record-setting temperatures of the 1990s — evidence that El Niño events can

warm parts of the Earth. But now scientists are examining how human

induced global warming could affect El Niño. Scientists are concerned

that the accumulation of greenhouse gases in our atmosphere may

inject enough heat into the Pacific Ocean to make El Niño events more

frequent and fierce.
usage. If you live along the coast, your home may be threatened by sea

level rise and an increase in storm intensity. The cost of food may

change as farmers and the food industry adapt to new climate patterns

And the outdoor activities that you and your family enjoy could be

affected by increased beach erosion, decreased snow fall and retreating

glaciers, and loss of forests and wildlife, where species are unable to

adapt to the changing climate.

Q. How might global warming affect my business?

A. As a business owner, your costs, competition, and planning decisions

may be affected. Your health care costs could increase if the public

health sector is burdened by increases in heat and climate-related mortality and illness. Like homeowners, your business’s energy costs will

reflect the need for greater cooling in the summer and less heating in the

winter. Your property insurance premiums could go up due to more

droughts and floods and possibly more intense storms. If your business

is located along the coast, sea level rise may also affect property insurance, not to mention how rising seas may directly impact your business.

If your business depends on waterways for transportation, those shipping

costs could increase in some areas due to reduced river flow and lower

lake levels, though in northern areas shipping could be eased by a longer

ice-free season. If you’re in the agricultural or food industry, changing cli

matic and growing season conditions will require adaptations. Your competitors in this sector may experience either more or less favorable climatic changes than you. The same is true if you’re in a forestry-related

business. Some of global warming’s impacts may be most severe in

other nations less capable of adapting. This may create social and economic disruptions that ripple across the globe to affect your business.

For all of these reasons, long-term business planning will increasingly

have to consider the changing nature of our planet’s climate.

Here are some other websites 

we recommend

Intergovernmental Panel on Climate Change: 

www.ipcc.ch

U.S. Global Change Research Program: 

www.usgcrp.gov

NASA’s Goddard Institute for Space Studies: 

www.giss.nasa.gov

NOAA’s National Climatic Data Center: 

www.ncdc.noaa.gov

to learn what you can do... 

www.epa.gov/globalwarming