EXECUTIVE SUMMARY

WMO/UNEP 

“Scientific Assessment of Ozone Depletion: 2010”

PREPARED BY THE 

SCIENTIFIC ASSESSMENT PANEL

OF THE

MONTREAL PROTOCOL ON SUBSTANCES 

THAT DEPLETE THE OZONE LAYER

************************** 

Contents 

Overview ............................................................................................. 1 

Changes in Gases that Affect Stratospheric Ozone and Climate .......................... 1 

 Ozone-Depleting Substances and Substitutes: Tropospheric Abundances 

and Emissions 

CFCs, HCFCs, HFCs, and Climate Change 

 Total Chlorine and Bromine and Implications for Ozone Depletion 

Figure ES-1: Emissions of ODSs and Their Substitutes 

Ozone and Climate: Antarctic .................................................................... 5 

Figure ES-2: Schematic of the Influence of Ozone-Depleting 

 Substances and Climate Change on the Changes in the 

 Stratospheric Ozone Layer, and the Influence of Ozone 

 Changes on Surface Ultraviolet Radiation 

Ozone and Climate: Global and Arctic ........................................................ 7 

Information for Policymakers and Options for Policy Formulation ...................... 9 

 Information for Policymakers 

 Options for Policy Formulation 

 Table ES-1: Hypothetical Cases 

Appendix: Scientific Summaries of the Chapters .......................................... . 12

 Chapter 1: Ozone-Depleting Substances and Related Chemicals 

 Chapter 2: Stratospheric Ozone and Surface Ultraviolet Radiation 

 Chapter 3: Future Ozone and Its Impact on Surface UV 

 Chapter 4: Stratospheric Changes and Climate

 Chapter 5: A Focus on Information and Options for Policymakers 

************************** 
1

Executive Summary

Scientific Assessment of Ozone Depletion: 2010 

OVERVIEW 

 It has been recognized since the 1970s that a number of compounds emitted by human 

activities deplete stratospheric ozone. The Montreal Protocol on Substances that Deplete the 

Ozone Layer was adopted in 1987 to protect global ozone and, consequently, protect life from 

increased ultraviolet (UV) radiation at Earth’s surface. Chlorine- and bromine-containing 

substances that are controlled by the Montreal Protocol are known as ozone-depleting substances 

(ODSs). ODSs are responsible for the depletion of stratospheric ozone observed in polar regions 

(for example, the “ozone hole” above Antarctica) and in middle latitudes. The severe depletion 

of stratospheric ozone observed in the Antarctic has increased UV at the surface and affected 

climate at southern high latitudes. 

 The Montreal Protocol and its Amendments and Adjustments have successfully 

controlled the global production and consumption of ODSs over the last two decades, and the 

atmospheric abundances of nearly all major ODSs that were initially controlled are declining. 

Nevertheless, ozone depletion will continue for many more decades because several key ODSs 

last a long time in the atmosphere after emissions end. 

 In contrast to the diminishing role of ODSs, changes in climate are expected to have an 

increasing influence on stratospheric ozone abundances in the coming decades. These changes 

derive principally from the emissions of long-lived greenhouse gases, mainly carbon dioxide 

(CO2), associated with human activities. An important remaining scientific challenge is to 

project future ozone abundances based on an understanding of the complex linkages between 

ozone and climate change. 

 Most ODSs are potent greenhouse gases. The buildup of ODS abundances over the last 

decades contributes to global warming. The actions taken under the Montreal Protocol have 

reduced the substantial contributions these gases would have made to global warming. 

 There is now new and stronger evidence of the effect of stratospheric ozone changes on 

Earth’s surface climate, and of the effects of climate change on stratospheric ozone. These 

results are an important part of the new assessment of the depletion of the ozone layer presented 

here. 

CHANGES IN GASES THAT AFFECT STRATOSPHERIC OZONE AND CLIMATE 

 Changes in the global atmospheric abundance of a substance are determined by the 

balance between its emissions and removals from the atmosphere. Declines observed for ozonedepleting substances controlled under the Montreal Protocol are due to global emission 

reductions that have made emissions smaller than removals. Most ODSs are potent greenhouse 

gases. As the majority of ODSs have been phased out, demand for hydrochlorofluorocarbon 
2

(HCFC) and hydrofluorocarbon (HFC) substitutes for the substances controlled under the 

Montreal Protocol has increased; these are also greenhouse gases. HCFCs deplete much less 

ozone per kilogram emitted than chlorofluorocarbons (CFCs), while HFCs are essentially nonozone depleting gases. 

Ozone-Depleting Substances and Substitutes: Tropospheric Abundances 

and Emissions 

• The amended and adjusted Montreal Protocol continues to be successful at reducing 

emissions (Figure ES-1) and thereby abundances of most controlled ozone-depleting 

substances in the lower atmosphere (troposphere), as well as abundances of total 

chlorine and total bromine from these ozone-depleting substances. By 2008, the total 

tropospheric abundance of chlorine from ODSs and methyl chloride had declined to 3.4 

parts per billion (ppb) from its peak of 3.7 ppb. However, the rate of decline in total 

tropospheric chlorine by 2008 was only two-thirds as fast as was expected. This is 

because HCFC abundances increased more rapidly than expected, while CFCs decreased 

more slowly than expected. The discrepancy in CFC decreases is most likely because of 

emissions from “banks” in existing applications such as refrigerators, air conditioners, 

and foams. The rapid HCFC increases are coincident with increased production in 

developing countries, particularly in East Asia. The rate of decline of total tropospheric 

bromine from controlled ODSs was close to that expected and was driven by changes in 

methyl bromide.

• Declines in CFCs made the largest contribution to the observed decrease in total 

tropospheric chlorine during the past few years and are expected to continue to do 

so through the rest of this century. Observations show that CFC-12 tropospheric 

abundances have decreased for the first time. The decline of methyl chloroform 

(CH3CCl3) abundances made a smaller contribution to the decrease in total chlorine than 

described in past Assessments, because this short-lived substance has already been 

largely removed from the atmosphere. 

• Carbon tetrachloride (CCl4) tropospheric abundances have declined less rapidly 

than expected. Emissions derived from data reported to the United Nations 

Environment Programme (UNEP) are highly variable and on average appear smaller than 

those inferred from observed abundance trends. Although the size of this discrepancy is 

sensitive to uncertainties in our knowledge of how long CCl4 persists in the atmosphere 

(its “lifetime”), the variability cannot be explained by lifetime uncertainties. Errors in 

reporting, errors in the analysis of reported data, and/or unknown sources are likely 

responsible for the year-to-year discrepancies. 

• Observations near the tropical tropopause suggest that several very short-lived 

industrial chlorinated chemicals, not presently controlled under the Montreal 

Protocol (e.g., methylene chloride, CH2Cl2; chloroform, CHCl3; 1,2 dichloroethane, 

CH2ClCH2Cl; perchloroethylene, C2Cl4), reach the stratosphere. However, their 

contribution to stratospheric chlorine loading is not well quantified. 

• Bromine from halons stopped increasing in the troposphere during 2005–2008. As 

expected, abundances of halon-1211 decreased for the first time during 2005–2008, while 

halon-1301 continued to increase but at a slower rate than in the previous Assessment. 
3

• Tropospheric methyl bromide abundances continued to decline during 2005–2008, 

as expected due to reductions in industrial production, consumption, and emission. 

About half of the remaining methyl bromide consumption was for uses not controlled by 

the Montreal Protocol (quarantine and pre-shipment applications). 

• Tropospheric abundances and emissions of some HCFCs are increasing faster now 

than four years ago. Abundances of HCFC-22, the most abundant HCFC, increased 

more than 50% faster in 2007–2008 than in 2003–2004, while HCFC-142b abundances 

increased about twice as fast as in 2003–2004. HCFC-141b abundances increased at a 

similar rate to that observed in 2003–2004. Total emissions of HCFCs are projected to 

begin to decline during the coming decade due to measures already agreed to under the 

Montreal Protocol (Figure ES-1). 

• Tropospheric abundances and emissions of HFCs, used mainly as substitutes for 

CFCs and HCFCs, continue to increase. For example, abundances of HFC-134a, the 

most abundant HFC, have been increasing by about 10% per year in recent years. 

Abundances of other HFCs, including HFC-125, -143a, -32, and -152a, have also been 

increasing. Regional studies suggest significant HFC emissions from Europe, Asia, and 

North America. 

CFCs, HCFCs, HFCs, and Climate Change 

• The Montreal Protocol and its Amendments and Adjustments have made large 

contributions toward reducing global greenhouse gas emissions (Figure ES-1). In 

2010, the decrease of annual ODS emissions under the Montreal Protocol is estimated to 

be about 10 Gigatonnes of avoided CO2-equivalent1

 emissions per year, which is about 

five times larger than the annual emissions reduction target for the first commitment 

period (2008–2012) of the Kyoto Protocol. 

• The sum of the HFCs currently used as ODS replacements contributes about 0.4 

Gigatonnes of CO2-equivalent per year to total global CO2-equivalent emissions, 

while the HCFCs contribute about 0.7 Gigatonnes. CO2-equivalent emissions of 

HFCs are increasing by about 8% per year and this rate is expected to continue to grow, 

while the contribution from HCFCs is expected to start decreasing in the next decade. 

• Emissions of HFC-23, a byproduct of HCFC-22 production, contributed about 0.2 

Gigatonnes of CO2-equivalent per year in 2006–2008. HFC-23 is a particularly potent 

greenhouse gas with a lifetime of about 220 years. Its emissions have increased in the 

past decade despite global emissions reduction measures, including those covered by the 

Kyoto Protocol’s Clean Development Mechanism projects. 

Total Chlorine and Bromine and Implications for Ozone Depletion 

• Total chlorine has continued to decline from its 1990s peak values in both the 

troposphere and the stratosphere. Total tropospheric bromine is decreasing from 

                                                       

1 GWP-weighted emissions, also known as CO2-equivalent emissions, are defined as the amount of gas 

emitted multiplied by its 100-year Global Warming Potential (GWP). 
4

its peak values, which occurred comparatively recently, while stratospheric bromine 

is no longer increasing. 

• Relative declines in the sum of stratospheric chlorine and bromine from peak values 

are largest in midlatitudes and smallest in Antarctica (refer to Figure ESA-1 in the 

Appendix of this Executive Summary). These declines are not as pronounced as 

observed in their tropospheric abundances. Differences between declines in the 

troposphere and different regions of the stratosphere are primarily associated with the 

time required for air to move from the troposphere to those regions. The relative declines 

are smallest in Antarctica primarily because the transport times to polar regions are the 

largest.

Figure ES-1. Emissions of ODSs and their 

substitutes. Global emissions of ODSs (CFCs, halons, 

HCFCs, and others) and their non-ozone depleting 

substitutes (HFCs) from 1950 to 2050. Emissions are 

the total from developing and developed countries. The 

legends identify the specific groups of substances 

included in each panel. The high and low HFC labels 

identify the upper and lower limits, respectively, in global 

baseline scenarios. The blue hatched regions indicate 

the emissions that would have occurred, in the absence 

of the Montreal Protocol, with 2–3% annual production 

increases in all ODSs.

Top panel: Global mass-weighted emissions expressed 

as Megatonnes per year. The yellow dashed line shows 

HCFC emissions calculated without the provisions of the 

2007 accelerated HCFC phaseout under the Montreal 

Protocol. 

Middle panel: Global Ozone Depletion Potential–

weighted emissions expressed as Megatonnes of CFC11-equivalent per year. The emissions of individual 

gases are multiplied by their respective ODPs (CFC-11 = 

1) to obtain aggregate, equivalent CFC-11 emissions. 

The dashed line marks 1987, the year of the Montreal 

Protocol signing. 

Bottom panel: Global GWP-weighted emissions 

expressed as Gigatonnes of CO2-equivalent per year. 

The emissions of individual gases are multiplied by their 

respective GWPs (direct, 100-year time horizon; CO2 = 

1) to obtain aggregate, equivalent CO2 emissions. 

Shown for reference are emissions for the range of CO2

scenarios from the Intergovernmental Panel on Climate 

Change (IPCC) Special Report on Emission Scenarios

(SRES). The CO2 emissions for 1950–2007 are from 

global fossil fuel use and cement production. Beyond 

2007, the shaded region for CO2 reflects the maximum 

(A1B) and minimum (B2) SRES scenarios. The dashed 

line marks 2010, the middle year of the first commitment 

period of the Kyoto Protocol. Also shown is the 

magnitude of the reduction target of the first commitment 

period of the Kyoto Protocol, which is based on a 1990–

2010 projection of global greenhouse gas emission 

increases and the reduction target for participating 

countries. 

0

1

2

3

4

Megatonnes per year

Mass-weighted emissions

Without Montreal Protocol

HFCs

ODSs

HCFCs

CFCs-only

Without accelerated

HCFC phaseout

high

low

0.0

0.5

1.0

1.5

2.0

Megatonnes CFC-11-eq per year

ODP-weighted

emissions

Without Montreal

Protocol

ODSs

HCFCs

CFCs, Halons,

others

1950 1970 1990 2010 2030 2050

Year

0

10

20

30

Gigatonnes CO2-eq per year

HFCs

ODSs

HCFCs

CFCs, 

Halons,

others

GWP-weighted emissions

Without Montreal Protocol

IPCC-SRES CO2 range

Magnitude of

Kyoto Protocol

reduction target

 2.0 

high

low
5

OZONE AND CLIMATE: ANTARCTIC 

 The Antarctic ozone hole is the clearest manifestation of the effect of ODSs on the ozone 

layer. The depletion far exceeds natural variability and has occurred without exception since 

1980. The ozone hole also provides the most visible example of how ozone depletion affects 

surface climate. 

• Springtime Antarctic total column ozone losses (the ozone hole), first recognizable 

around 1980, continue to occur every year (Figure ES-2c). Although the ozone losses 

exhibit year-to-year variations that are primarily driven by year-to-year changes in 

meteorology, October mean column ozone within the vortex has been about 40% below 

1980 values for the past fifteen years. The average erythemal (“sunburning”) UV 

measured at the South Pole between 1991 and 2006 was 55–85% larger than the 

estimated values for the years 1963–1980. 

• Doubts raised since the previous Assessment regarding our understanding of the 

cause of the Antarctic ozone hole have been dispelled. New laboratory measurements 

on the key chemistry involved in polar ozone depletion have reaffirmed that past changes 

in ODSs are indeed the cause of the ozone hole. This is also supported by quantification 

of the chemicals responsible for the ozone hole via field observations. 

• There is increased evidence that the Antarctic ozone hole has affected the surface 

climate in the Southern Hemisphere. Climate models demonstrate that the ozone hole 

is the dominant driver of the observed changes in surface winds over the Southern 

Hemisphere mid and high latitudes during austral summer. These changes have 

contributed to the observed warming over the Antarctic Peninsula and cooling over the 

high plateau. The changes in the winds have also been linked to regional changes in 

precipitation, increases in sea ice around Antarctica, warming of the Southern Ocean, and 

a local decrease in the ocean sink of CO2. 

• The trends in the summertime winds in the Southern Hemisphere are not expected 

to persist over the next few decades. This is because of the expected offsetting 

influences on the surface winds of increasing greenhouse gases and the recovering ozone 

hole.

• Observed Antarctic springtime column ozone does not yet show a statistically 

significant increasing trend (Figure ES-2c). Year-to-year variability, due to 

meteorology, is much larger than the expected response to the small ODS decreases in 

the Antarctic vortex to date. This is consistent with simulations using chemistry-climate 

models (CCMs). 

• The evolution of Antarctic springtime column ozone over the rest of the century is 

expected to be dominated by the decline in ODS abundance (Figure ES-2c). CCM 

simulations show that greenhouse gas changes have had, and will continue to have, a 

small impact on the ozone hole compared to the effects of the ODS changes. There are 

some indications that small episodic Antarctic ozone holes may occur even through the 

end of the century. In spring and early summer, Antarctica will continue to experience 

excess surface UV.
6

Figure ES-2. Schematic of the influence of ozonedepleting substances (ODSs) and climate change on 

the stratospheric ozone layer, and the influence of 

ozone changes on surface ultraviolet radiation. The 

red lines are based on observations to date. The blue 

dashed lines represent one commonly accepted scenario 

for the future. Shaded areas represent year-to-year 

variability and uncertainties in simulations of the past and 

future. The dashed vertical line at 1980, a year used as a 

benchmark for ozone and UV recovery, demarcates the 

situation before and after significant changes to the ozone 

layer. The curve for carbon dioxide (CO2), a greenhouse 

gas important to Earthʼs climate, is shown because its 

changes can affect stratospheric temperatures as well as 

wind patterns, both of which affect stratospheric ozone. 

(a) Combined effective abundance of ozone-depleting 

chlorine and bromine in the lower atmosphere 

(troposphere). The red line is a representation of the 

measured atmospheric abundances. The blue dashed 

line is the expected combined effective abundance of 

chlorine and bromine based on the most likely ODS 

scenario used in this report and current understanding 

of the workings of the atmosphere. Because of the 

Montreal Protocol, a continued decline is expected 

through the end of this century, with a return to the 

1980 benchmark value occurring around the middle of 

this century. A similar curve for the stratosphere would 

be shifted to the right (later dates) by a few years 

because of the time lag in the transport of substances 

from the surface to the stratosphere. 

(b) The atmospheric abundance of carbon dioxide, the major anthropogenic greenhouse gas that changes Earthʼs climate, 

including in the stratosphere; CO2 abundance is a proxy for climate change. The gray dotted/shaded area represents 

expectations of increasing future CO2 abundance based on different scenarios used in this Assessment. 

(c) The extent of the Antarctic ozone hole, as measured by the amount of ozone in the total overhead column averaged for 

the month of October. The ozone hole is the clearest indicator of ozone layer depletion by ODSs, and the ODSs in the 

atmosphere have been and are expected to continue to be the primary control on the extent and duration of the ozone 

hole. Antarctic ozone is expected to return to pre-1980 benchmark values in the late 21st century. The blue shaded area 

shows the estimated year-to-year variability of ozone for one scenario that includes changes in ODSs (panel a), CO2

(panel b, blue dashed line), and changes in nitrous oxide and methane (not shown), but does not capture all 

uncertainties. The gray dotted/shaded area shows the uncertainty due to different climate scenarios, but again does not 

capture all uncertainties. 

(d) The extent of northern midlatitude ozone depletion, as measured by the amount of ozone in the total overhead column 

between 30°N and 60°N averaged over each year; blue and gray shaded areas as in panel c. Panels c and d show the 

approximate relative magnitudes of the northern midlatitude ozone depletion and the Antarctic ozone hole. Influences of 

the quasi-biennial oscillation (QBO), volcanoes, and solar cycle have been removed from the observational data. The 

future projections do not include the influence of any volcanic eruptions or solar cycle variations. Natural variability 

makes it difficult to identify the projected return of northern midlatitude ozone levels to pre-1980 levels, but the 

expectation is that climate change will hasten this return by several decades, such that it will occur before the middle of 

the 21st century (before the return of stratospheric chlorine and bromine to the 1980 benchmark value, and before the 

return of Antarctic ozone, panel c). 

(e) Changes in clear-sky surface UV radiation at northern midlatitudes that accompany the ozone changes of the ODS 

scenario above. Because the ozone depletion in the northern midlatitudes has been small, the UV changes also have 

been small. The blue shaded area shows the year-to-year variability of surface UV for the ozone changes of panel d. 

Clouds, aerosols, and air pollution significantly affect surface UV, but it is difficult to project their future changes. The 

uncertainties in these changes, which are larger than the uncertainties due to ozone changes, are not represented in the 

figure. The expectation is that climate change will result in northern midlatitude clear-sky surface UV radiation levels well 

below 1980 values by the second half of this century. 

(c) Antarctic total ozone in October

(e) Northern midlatitude surface ultraviolet radiation

(b) Carbon dioxide

1980 now ~2100

(a) Ozone-depleting chlorine and bromine in the lower atmosphere

(d) Northern midlatitude total ozone
7

OZONE AND CLIMATE: GLOBAL AND ARCTIC 

 As a result of the controls introduced by the Montreal Protocol and its Amendments and 

Adjustments, it is expected that the decline in ODSs will lead to an increase in stratospheric 

ozone abundances. However, it will be challenging to attribute ozone increases to the decreases 

in ODSs during the next few years because of natural variability, observational uncertainty, and 

confounding factors, such as changes in stratospheric temperature or water vapor. A feature of 

this Assessment is the coordinated use by the community of chemistry-climate models (CCMs) 

with integrations covering the period from 1960–2100, which has allowed more detailed study of 

the long-term changes in the stratosphere and of the relative contributions of ODSs and 

greenhouse gases (GHGs). 

• Average total ozone values in 2006–2009 remain at the same level as the previous 

Assessment, at roughly 3.5% and 2.5% below the 1964–1980 averages respectively 

for 90°S–90°N and 60°S–60°N. Midlatitude (35°–60°) annual mean total column ozone 

amounts in the Southern Hemisphere [Northern Hemisphere] over the period 2006–2009 

have remained at the same level as observed during 1996–2005, at ~6% [~3.5%] below 

the 1964–1980 average. 

• The ozone loss in Arctic winter and spring between 2007 and 2010 has been 

variable, but has remained in a range comparable to the values prevailing since the 

early 1990s. Substantial chemical loss continues to occur during cold Arctic winters. 

• Robust linkages between Arctic stratospheric ozone depletion and tropospheric and 

surface climate trends have not been established, as expected from the smaller ozone 

depletion compared with the Antarctic. 

• Chemistry-climate models reproduce both the latitudinal and vertical structure of 

the observed ozone trends in both northern and southern midlatitudes during the 

past periods of increase of the ODSs, confirming our basic understanding of ozone 

change. Simulations agree with observations that the last decade has shown flattening of 

the time series of global total ozone.

• Analyses based on surface and satellite measurements show that erythemal UV 

irradiance over midlatitudes has increased since the late 1970s (Figure ES-2e). This 

is in qualitative agreement with the observed decrease in column ozone, although other 

factors (mainly clouds and aerosols) have influenced long-term changes in erythemal 

irradiance. Clear-sky UV observations from unpolluted sites in midlatitudes show that 

since the late 1990s, UV irradiance levels have been approximately constant, consistent 

with ozone column observations over this period. 

• New analyses of both satellite and radiosonde data give increased confidence in 

changes in stratospheric temperatures between 1980 and 2009. The global-mean 

lower stratosphere cooled by 1–2 K and the upper stratosphere cooled by 4–6 K between 

1980 and 1995. There have been no significant long-term trends in global-mean lower 

stratospheric temperatures since about 1995. The global-mean lower-stratospheric 

cooling did not occur linearly but was manifested as downward steps in temperature in 

the early 1980s and the early 1990s. The cooling of the lower stratosphere includes the 

tropics and is not limited to extratropical regions as previously thought. 
8

• The evolution of lower stratospheric temperature is influenced by a combination of 

natural and human factors that has varied over time. Ozone decreases dominate the 

lower stratospheric cooling since 1980. Major volcanic eruptions and solar activity have 

clear shorter-term effects. Models that consider all of these factors are able to reproduce 

this temperature time history. 

• Changes in stratospheric ozone, water vapor, and aerosols all radiatively affect 

surface temperature. The radiative forcing2 of climate in 2008 due to stratospheric 

ozone depletion (−0.05 ± 0.1 Watts per square meter, W/m2

) is much smaller than the 

positive radiative forcing due to the CFCs and HCFCs largely responsible for that 

depletion (about +0.3 W/m2). For context, the current forcing by CO2 is approximately 

+1.7 W/m2. Radiative calculations and climate modeling studies suggest that the 

radiative effects of variability in stratospheric water vapor (± ~0.1 W/m2

 per decade) can 

contribute to decadal variability in globally averaged surface temperature. Climate 

models and observations show that major volcanic eruptions (e.g., Mt. Pinatubo in 1991, 

roughly −3 W/m2) can cool the surface for several years.

• The global middle and upper stratosphere are expected to cool in the coming 

century, mainly due to CO2 increases. Stratospheric ozone recovery will slightly offset 

the cooling. HFCs could warm the tropical lower stratosphere and tropopause region by 

about 0.3°C if stratospheric abundances reach the 1 ppb level. 

• Emerging evidence from model simulations suggests that increasing greenhouse 

gases lead to an acceleration of the stratospheric circulation usually referred to as 

the Brewer-Dobson circulation. Such an acceleration could have important 

consequences, particularly decreases in column ozone in the tropics and increases in 

column ozone elsewhere. However, responsible mechanisms remain unclear and 

observational evidence for the circulation increase is lacking. 

• Global ozone is projected to increase approximately in line with the ODS decline, 

and the increase is accelerated by cooling of the upper stratosphere. Global ozone is 

not very sensitive to circulation changes, so high confidence can be placed in this 

projection.

• The evolution of ozone in the Arctic is projected to be more sensitive to climate 

change than in the Antarctic. The projected strengthening of the stratospheric BrewerDobson circulation is expected to significantly increase lower stratospheric ozone in the 

Arctic, augmenting the GHG-induced ozone increase from upper stratospheric cooling 

and hastening the return to 1980 levels. 

• GHG-induced temperature and circulation changes are projected to hasten the 

return of midlatitude total column ozone to 1980 levels by several decades, rising 

well above 1980 levels by the end of the century. The effect is most pronounced in 

northern midlatitudes (Figure ES-2d), where it would result in clear-sky surface UV 

radiation levels well below 1980 values by the second half of the century (Figure ES-2e). 

In southern midlatitudes, the effect of circulation changes is projected to be weaker and 

ozone is also influenced by depletion in the Antarctic, where the return to 1980 levels 

occurs much later.

                                                       

2 Positive radiative forcings generally warm the surface; negative radiative forcings generally cool the surface.
9

INFORMATION FOR POLICYMAKERS AND OPTIONS FOR POLICY FORMULATION 

 Cases related to the elimination of future emissions, production, and banks for various 

ozone-depleting substances (ODSs) can be formulated starting from a baseline future emission 

scenario. The baseline scenario here has been developed to account for past and present levels of 

ODSs along with emission projections. This scenario projects that stratospheric chlorine and 

bromine levels are likely to return to 1980 levels in midcentury for the midlatitudes and about 25 

years later in the Antarctic vortex. These additional cases are used to evaluate the impact of 

various hypothetical policy options. 

Information for Policymakers 

• The Montreal Protocol has both protected the ozone layer and provided substantial 

co-benefits by reducing climate change (see Figure ES-1, bottom two panels). It has 

protected the stratospheric ozone layer by phasing out production and consumption of 

ozone-depleting substances. Simulations show that unchecked growth in the emissions of 

ODSs would have led to global ozone depletion in the coming decades very much larger 

than current levels. Solar UV radiation at the surface would also have increased 

substantially.

• Projections of hydrofluorocarbon (HFC) growth in scenarios that assume no 

controls suggest that by 2050, Global Warming Potential–weighted emissions from 

these substances could be comparable to the GWP-weighted emissions of 

chlorofluorocarbons (CFCs) at their peak in 1988 (see Figure ES-1, bottom panel). 

The highest projection assumes that developing countries use HFCs with GWPs 

comparable to those currently in use.

• The accelerated hydrochlorofluorocarbon (HCFC) phase-out agreed to by the 

Parties to the Montreal Protocol in 2007 is projected to reduce ozone depletion and 

to help reduce climate forcing (see Figure ES-1). This acceleration is expected to 

reduce cumulative HCFC emissions by about 0.7 million Ozone Depletion Potential–

tonnes between 2011 and 2050 and would bring forward the year equivalent effective 

stratospheric chlorine (EESC) returns to 1980 levels by 4–5 years. The accelerated 

HCFC phasedown is projected to reduce greenhouse gas emissions by about 0.5 

Gigatonnes of carbon dioxide (CO2)-equivalent per year averaged over 2011 through 

2050. The projected benefit would be determined by the climate impact of the 

replacements. In comparison, global anthropogenic emissions of CO2 were greater than 

30 Gigatonnes per year in 2008. 

• Since the previous Assessment, new fluorocarbons have been suggested as possible 

replacements for potent HCFC and HFC greenhouse gases. For example, HFC1234yf (Ozone Depletion Potential (ODP) = 0; 100-year GWP = 4) is proposed to replace 

HFC-134a (ODP = 0; 100-year GWP = 1370) in mobile air conditioning. To fully assess 

the environmental impacts, each proposed substance would need to be evaluated for its 

ODP, GWP, atmospheric fate, safety, and toxicity. Preliminary analyses indicate that 

global replacement of HFC-134a with HFC-1234yf at today’s level of use is not expected 

to contribute significantly to tropospheric ozone formation or produce harmful levels of 

the degradation product TFA (trifluoroacetic acid). It is well established that TFA is a 
10

ubiquitous component of the environment, but uncertainties remain regarding its natural 

and anthropogenic sources, long-term fate, and abundances. 

 Due to the success of the Montreal Protocol and its Amendments and Adjustments in 

reducing the production, emissions, and abundances of controlled ODSs, emissions from other 

compounds and activities not controlled by the Montreal Protocol have become relatively more 

important to stratospheric ozone. 

• Increasing abundances of radiatively important gases, especially carbon dioxide 

(CO2) and methane (CH4), are expected to significantly affect future stratospheric 

ozone through effects on temperature, winds, and chemistry. CO2 increased in the 

atmosphere at 2.1 parts per million per year from 2005–2008, while CH4 increased by 

about 6.7 parts per billion per year from 2006–2008. 

• Nitrous oxide (N2O) is known to both deplete global ozone and warm the climate. 

The current ODP-weighted anthropogenic emission is larger than that of any ODS. 

• Deliberate large injections of sulfur-containing compounds into the stratosphere, 

which have been suggested as a climate intervention approach (geoengineering), 

would alter the radiative, dynamical, and chemical state of the stratosphere and 

could be expected to have substantial unintended effects on stratospheric ozone 

levels. 

Options for Policy Formulation 

 Additional cases have been developed to show the impact of further control measures on 

various substances. Table ES-1 below shows the percentage reductions in integrated chlorine 

and bromine levels and integrated GWP-weighted emissions, relative to the baseline scenario, 

that can be achieved in these hypothetical cases. 

• Halons and CFCs: Leakage from banks is the largest source of current ODPweighted emissions of ODSs. A delay in the capture and destruction of estimated CFC 

banks from 2011 to 2015 is currently thought to reduce the possible ozone and climate 

benefits that could be achieved by about 30%. 

• Carbon tetrachloride (CCl4): Elimination of future CCl4 emissions after 2010 would 

have an EESC impact comparable to the capture and destruction of CFC and halon banks. 

This is a much larger effect than was estimated in the previous Assessment because of a 

revision in the estimated emissions. 

• HCFCs: The recent growth in reported HCFC production in developing countries was 

larger than projected in the previous Assessment. This alone would have resulted in a 

larger projected HCFC production in the new baseline scenario compared to the previous 

Assessment, but is expected to be more than compensated for by the accelerated HCFC 

phasedown agreed to by the Parties to the Montreal Protocol in 2007. 

• Elimination of all emissions of chlorine- and bromine-containing ODSs after 2010: 

This would bring forward the return of EESC to 1980 levels by about 13 years. The 

elimination of these ODS emissions would have a climate impact equivalent to about a 
11

0.7 Gigatonnes of CO2-equivalent per year reduction from 2011 through 2050, on 

average. The sum of current banks of CFCs plus HCFCs contributes about the same 

amount to these CO2-equivalent emissions as future HCFC production. 

• Methyl bromide: Two methyl bromide cases were examined. Case 1: A phaseout of 

quarantine and pre-shipment emissions beginning in 2011 would accelerate the return of 

EESC to 1980 levels by 1.5 years, relative to a case of maintaining emissions at 2004–

2008 average levels. Case 2: Continuing critical use exemptions at the approved 2011 

level indefinitely would delay the return of EESC to 1980 levels by 0.2 year. 

Table ES-1. Hypothetical cases. Reductions in integrated chlorine and bromine levels (as measured by 

equivalent effective stratospheric chlorine, EESC) and integrated GWP-weighted emissions, relative to 

the baseline scenario, that can be achieved in hypothetical cases developed to show the impact of further 

control measures on various substances.

Substance or Group of 

Substances

Reductions (%) in 

Integrated EESC 

(equivalent effective 

stratospheric chlorine)

Reduction in Cumulative 

GWP-Weighted Emissions 

from 2011 to 2050

(Gigatonnes of CO2-

equivalent)

Bank capture and 

destruction in 2011 and 

2015:

2011 2015 2011 2015

CFCs 11 7.0 7.9 5.5

Halons 14 9.1 0.4 0.3

HCFCs 4.8 5.3 1 4.9 5.5 1

Production elimination 

after 2010:

HCFCs 8.8 13.2

CH3Br for quarantine and 

pre-shipment

6.7 0.002

Total emissions elimination 

after 2010:

CCl4

2 7.6 0.9

CH3CCl3

0.1 0.004

HFCs 0.0 Up to 170 3

1 The impact of a 2015 HCFC bank recovery is larger than a 2011 bank recovery because this calculation assumes 

destruction of the bank in only a single year, and because the bank in 2015 is larger than the bank in 2011 owing 

to continued annual production that is larger than the annual bank release. 

2

 Banks are assumed to be zero. Emissions include uncertain sources such as possible fugitive emissions and 

unintended byproduct emissions. 

3 Strongly dependent on future projections and does not include HFC-23 emissions. Currently HFCs are not 

controlled under the Montreal Protocol, but are included in the basket of gases of the Kyoto Protocol.
12

APPENDIX: 

SCIENTIFIC SUMMARIES OF THE CHAPTERS 

CHAPTER 1: OZONE-DEPLETING SUBSTANCES AND RELATED CHEMICALS 

• The amended and adjusted Montreal Protocol continues to be successful at reducing 

emissions and atmospheric abundances of most controlled ozone-depleting 

substances (ODSs). 

Tropospheric Chlorine 

• Total tropospheric chlorine from long-lived chemicals (~3.4 parts per billion (ppb) 

in 2008) continued to decrease between 2005 and 2008. Recent decreases in 

tropospheric chlorine (Cl) have been at a slower rate than in earlier years (decreasing at 

14 parts per trillion per year (ppt/yr) during 2007–2008 compared to a decline of 21 

ppt/yr during 2003–2004) and were slower than the decline of 23 ppt/yr projected in the 

A1 (most likely, or baseline) scenario of the 2006 Assessment. The tropospheric Cl 

decline has recently been slower than projected in the A1 scenario because 

chlorofluorocarbon-11 (CFC-11) and CFC-12 did not decline as rapidly as projected and 

because increases in hydrochlorofluorocarbons (HCFCs) were larger than projected. 

• The contributions of specific substances or groups of substances to the decline in 

tropospheric Cl have changed since the previous Assessment. Compared to 2004, by 

2008 observed declines in Cl from methyl chloroform (CH3CCl3) had become smaller, 

declines in Cl from CFCs had become larger (particularly CFC-12), and increases in Cl 

from HCFCs had accelerated. Thus, the observed change in total tropospheric Cl of −14 

ppt/yr during 2007–2008 arose from: 

• −13.2 ppt Cl/yr from changes observed for CFCs 

• −6.2 ppt Cl/yr from changes observed for methyl chloroform 

• −5.1 ppt Cl/yr from changes observed for carbon tetrachloride 

• −0.1 ppt Cl/yr from changes observed for halon-1211 

• +10.6 ppt Cl/yr from changes observed for HCFCs 

• Chlorofluorocarbons (CFCs), consisting primarily of CFC-11, -12, and -113, 

accounted for 2.08 ppb (about 62%) of total tropospheric Cl in 2008. The global

atmospheric mixing ratio of CFC-12, which accounts for about one-third of the current 

atmospheric chlorine loading, decreased for the first time during 2005–2008 and by mid2008 had declined by 1.3% (7.1 ± 0.2 parts per trillion, ppt) from peak levels observed 

during 2000–2004. 

• Hydrochlorofluorocarbons (HCFCs), which are substitutes for long-lived ozonedepleting substances, accounted for 251 ppt (7.5%) of total tropospheric Cl in 2008. 
13

HCFC-22, the most abundant of the HCFCs, increased at a rate of about 8 ppt/yr 

(4.3%/yr) during 2007–2008, more than 50% faster than observed in 2003–2004 but 

comparable to the 7 ppt/yr projected in the A1 scenario of the 2006 Assessment for 

2007–2008. HCFC-142b mixing ratios increased by 1.1 ppt/yr (6%/yr) during 2007–

2008, about twice as fast as was observed during 2003–2004 and substantially faster than 

the 0.2 ppt/yr projected in the 2006 Assessment A1 scenario for 2007–2008. HCFC-141b 

mixing ratios increased by 0.6 ppt/yr (3%/yr) during 2007–2008, which is a similar rate 

observed in 2003–2004 and projected in the 2006 Assessment A1 scenario. 

• Methyl chloroform (CH3CCl3) accounted for only 32 ppt (1%) of total tropospheric 

Cl in 2008, down from a mean contribution of about 10% during the 1980s. 

• Carbon tetrachloride (CCl4) accounted for 359 ppt (about 11%) of total 

tropospheric Cl in 2008. Mixing ratios of CCl4 declined slightly less than projected in 

the A1 scenario of the 2006 Assessment during 2005–2008. 

Stratospheric Chlorine and Fluorine 

• The stratospheric chlorine burden derived by ground-based total column and spacebased measurements of inorganic chlorine continued to decline during 2005–2008. 

This burden agrees within ±0.3 ppb (±8%) with the amounts expected from surface data 

when the delay due to transport is considered. The uncertainty in this burden is large 

relative to the expected chlorine contributions from shorter-lived source gases and 

product gases of 80 (40–130) ppt. Declines since 1996 in total column and stratospheric 

abundances of inorganic chlorine compounds are reasonably consistent with the observed 

trends in long-lived source gases over this period. 

• Measured column abundances of hydrogen fluoride increased during 2005–2008 at 

a smaller rate than in earlier years. This is qualitatively consistent with observed 

changes in tropospheric fluorine (F) from CFCs, HCFCs, hydrofluorocarbons (HFCs), 

and perfluorocarbons (PFCs) that increased at a mean annual rate of 40 ± 4 ppt/yr (1.6 ± 

0.1%/yr) since late 1996, which is reduced from 60–100 ppt/yr observed during the 1980s 

and early 1990s. 

Tropospheric Bromine 

• Total organic bromine from controlled ODSs continued to decrease in the 

troposphere and by mid-2008 was 15.7 ± 0.2 ppt, approximately 1 ppt below peak 

levels observed in 1998. This decrease was close to that expected in the A1 scenario of 

the 2006 Assessment and was driven by declines observed for methyl bromide (CH3Br) 

that more than offset increased bromine (Br) from halons. 

• Bromine from halons stopped increasing during 2005–2008. Mixing ratios of halon1211 decreased for the first time during 2005–2008 and by mid-2008 were 0.1 ppt below 

levels observed in 2004. Halon-1301 continued to increase in the atmosphere during 

2005–2008 but at a slower rate than observed during 2003–2004. The mean rate of 

increase was 0.03–0.04 ppt/yr during 2007–2008. A decrease of 0.01 ppt/yr was 

observed for halon-2402 in the global troposphere during 2007–2008. 
14

• Tropospheric methyl bromide (CH3Br) mixing ratios continued to decline during 

2005–2008, and by 2008 had declined by 1.9 ppt (about 20%) from peak levels 

measured during 1996–1998. Evidence continues to suggest that this decline is the 

result of reduced industrial production, consumption, and emission. This industryderived emission is estimated to have accounted for 25–35% of total global CH3Br 

emissions during 1996–1998, before industrial production and consumption were 

reduced. Uncertainties in the variability of natural emissions and in the magnitude of 

methyl bromide stockpiles in recent years limit our understanding of this anthropogenic 

emissions fraction, which is derived by comparing the observed atmospheric changes to 

emission changes derived from reported production and consumption. 

• By 2008, nearly 50% of total methyl bromide consumption was for uses not 

controlled by the Montreal Protocol (quarantine and pre-shipment applications). 

From peak levels in 1996–1998, industrial consumption in 2008 for controlled and noncontrolled uses of CH3Br had declined by about 70%. Sulfuryl fluoride (SO2F2) is used 

increasingly as a fumigant to replace methyl bromide for controlled uses because it does 

not directly cause ozone depletion, but it has a calculated direct, 100-year Global 

Warming Potential (GWP100) of 4740. The SO2F2 global background mixing ratio 

increased during recent decades and had reached about 1.5 ppt by 2008. 

Stratospheric Bromine 

• Total bromine in the stratosphere was 22.5 (19.5–24.5) ppt in 2008. It is no longer 

increasing and by some measures has decreased slightly during recent years. 

Multiple measures of stratospheric bromine monoxide (BrO) show changes consistent 

with tropospheric Br trends derived from observed atmospheric changes in CH3Br and 

the halons. Slightly less than half of the stratospheric bromine derived from these BrO 

observations is from controlled uses of halons and methyl bromide. The remainder 

comes from natural sources of methyl bromide and other bromocarbons, and from 

quarantine and pre-shipment uses of methyl bromide not controlled by the Montreal 

Protocol. 

Very Short-Lived Halogenated Substances (VSLS) 

 VSLS are defined as trace gases whose local lifetimes are comparable to, or shorter than, 

tropospheric transport time scales and that have non-uniform tropospheric abundances. In 

practice, VSLS are considered to be those compounds having atmospheric lifetimes of less than 

6 months. 

• The amount of halogen from a very short-lived source substance that reaches the 

stratosphere depends on the location of the VSLS emissions, as well as atmospheric 

removal and transport processes. Substantial uncertainties remain in quantifying the 

full impact of chlorine- and bromine-containing VSLS on stratospheric ozone. Updated 

results continue to suggest that brominated VSLS contribute to stratospheric ozone 

depletion, particularly under enhanced aerosol loading. It is unlikely that iodinated gases 

are important for stratospheric ozone loss in the present-day atmosphere. 
15

• Based on a limited number of observations, very short-lived source gases account 

for 55 (38–80) ppt chlorine in the middle of the tropical tropopause layer (TTL). 

From observations of hydrogen chloride (HCl) and carbonyl chloride (COCl2) in this 

region, an additional ~25 (0–50) ppt chlorine is estimated to arise from VSLS 

degradation. The sum of contributions from source gases and these product gases 

amounts to ~80 (40–130) ppt chlorine from VSLS that potentially reaches the 

stratosphere. About 40 ppt of the 55 ppt of chlorine in the TTL from source gases is from 

anthropogenic VSLS emissions (e.g., methylene chloride, CH2Cl2; chloroform, CHCl3; 

1,2 dichloroethane, CH2ClCH2Cl; perchloroethylene, C2Cl4), but their contribution to 

stratospheric chlorine loading is not well quantified. 

• Two independent approaches suggest that VSLS contribute significantly to 

stratospheric bromine. Stratospheric bromine derived from observations of BrO 

implies a contribution of 6 (3–8) ppt of bromine from VSLS. Observed, very short-lived 

source gases account for 2.7 (1.4–4.6) ppt Br in the middle of the tropical tropopause 

layer. By including modeled estimates of product gas injection into the stratosphere, the 

total contribution of VSLS to stratospheric bromine is estimated to be 1–8 ppt. 

• Future climate changes could affect the contribution of VSLS to stratospheric 

halogen and its influence on stratospheric ozone. Future potential use of 

anthropogenic halogenated VSLS may contribute to stratospheric halogen in a similar 

way as do present-day natural VSLS. Future environmental changes could influence both 

anthropogenic and natural VSLS contributions to stratospheric halogens.

Equivalent Effective Stratospheric Chlorine (EESC)

 EESC is a sum of chlorine and bromine derived from ODS tropospheric abundances 

weighted to reflect their potential influence on ozone in different parts of the stratosphere. The 

growth and decline in EESC varies in different regions of the atmosphere because a given 

tropospheric abundance propagates to the stratosphere with varying time lags associated with 

transport. Thus the EESC abundance, when it peaks, and how much it has declined from its peak 

vary in different regions of the atmosphere. 

Figure ESA-1. Stratospheric 

EESC derived for the 

midlatitude and polar 

stratospheric regions relative 

to peak abundances, plotted as 

a function of time. Peak 

abundances are ~1950 ppt for the 

midlatitude stratosphere and 

~4200 ppt for the polar 

stratosphere. Percentages shown 

to the right indicate the observed 

change in EESC by the end of 

2008 relative to the change 

needed for EESC to return to its 

1980 abundance. A significant 

portion of the 1980 EESC level is 

from natural emissions. 

0%

50%

60%

70%

80%

90%

100%

110%

1980 1985 1990 1995 2000 2005 2010

EESC abundance 

(relative to the peak)

Midlatitude stratosphere

Polar stratosphere

-10%

-28%

% return to 

1980 level

by the end

of 2008

Date of peak in the 

troposphere

1980 levels
16

• EESC has decreased throughout the stratosphere. 

• By the end of 2008, midlatitude EESC had decreased by about 11% from its peak 

value in 1997. This drop is 28% of the decrease required for EESC in midlatitudes 

(red curve in figure) to return to the 1980 benchmark level. 

• By the end of 2008, polar EESC had decreased by about 5% from its peak value in 

2002. This drop is 10% of the decrease required for EESC in polar regions (blue 

curve in figure) to return to the 1980 benchmark level. 

• During the past four years, no specific substance or group of substances dominated 

the decline in the total combined abundance of ozone-depleting halogen in the 

troposphere. In contrast to earlier years, the long-lived CFCs now contribute similarly 

to the decline as do the short-lived CH3CCl3 and CH3Br. Other substances contributed 

less to this decline, and HCFCs added to this halogen burden over this period. 

Emission Estimates and Lifetimes 

• While global emissions of CFC-12 derived from atmospheric observations decreased 

during 2005–2008, those for CFC-11 did not change significantly over this period. 

Emissions from banks account for a substantial fraction of current emissions of the CFCs, 

halons, and HCFCs. Emissions inferred for CFCs from global observed changes did not 

decline during 2005–2008 as rapidly as projected in the A1 scenario of the 2006 

Assessment, most likely because of underestimates of bank emissions. 

• Global emissions of CCl4 have declined only slowly over the past decade.

• These emissions, when inferred from observed global trends, were between 40 and 80 

Gigagrams per year (Gg/yr) during 2005–2008 given a range for the global CCl4

lifetime of 33–23 years. By contrast, CCl4 emissions derived with a number of 

assumptions from data reported to the United Nations Environment Programme 

(UNEP) ranged from 0–30 Gg/yr over this same period. 

• In addition, there is a large variability in CCl4 emissions derived from data reported to 

UNEP that is not reflected in emissions derived from measured global mixing ratio 

changes. This additional discrepancy cannot be explained by scaling the lifetime or 

by uncertainties in the atmospheric trends. If the analysis of data reported to UNEP is 

correct, unknown anthropogenic sources may be partly responsible for these observed 

discrepancies. 

• Global emissions of HCFC-22 and HCFC-142b derived from observed atmospheric 

trends increased during 2005–2008. HCFC-142b global emissions increased 

appreciably over this period, compared to a projected emissions decline of 23% from 

2004 to 2008. By 2008, emissions for HCFC-142b were two times larger than had been 

projected in the A1 scenario of the 2006 Assessment. These emission increases were 

coincident with increasing production of HCFCs in developing countries in general and 

in East Asia particularly. It is too soon to discern any influence of the 2007 Adjustments 

to the Montreal Protocol on the abundance and emissions of HCFCs. 

• The sum of CFC emissions (weighted by direct, 100-year GWPs) has decreased on 

average by 8 ± 1%/yr from 2004 to 2008, and by 2008 amounted to 1.1 ± 0.3 
17

Gigatonnes of carbon dioxide–equivalent per year (GtCO2-eq/yr). The sum of GWPweighted emissions of HCFCs increased by 5 ± 2%/yr from 2004 to 2008, and by 2008 

amounted to 0.74 ± 0.05 GtCO2-eq/yr. 

• Evidence is emerging that lifetimes for some important ODSs (e.g., CFC-11) may be 

somewhat longer than reported in past assessments. In the absence of corroborative 

studies, however, the CFC-11 lifetime reported in this Assessment remains unchanged at 

45 years. Revisions in the CFC-11 lifetime would affect estimates of its global emission 

derived from atmospheric changes and calculated values for Ozone Depletion Potentials 

(ODPs) and best-estimate lifetimes for some other halocarbons. 

Other Trace Gases That Directly Affect Ozone and Climate 

• The methane (CH4) global growth rate was small, averaging 0.9 ± 3.3 ppb/yr 

between 1998–2006, but increased to 6.7 ± 0.6 ppb/yr from 2006–2008. Analysis of 

atmospheric data suggests that this increase is due to wetland sources in both the high 

northern latitudes and the tropics. The growth rate variability observed during 2006–

2008 is similar in magnitude to that observed over the last two decades. 

• In 2005–2008 the average growth rate of nitrous oxide (N2O) was 0.8 ppb/yr, with a 

global average tropospheric mixing ratio of 322 ppb in 2008. A recent study has 

suggested that at the present time, Ozone Depletion Potential–weighted anthropogenic 

emissions of N2O are the most significant emissions of a substance that depletes ozone. 

• Long-term changes in carbonyl sulfide (COS) measured as total columns above the 

Jungfraujoch (46.5°N) and from surface flasks sampled in the Northern Hemisphere 

show that atmospheric mixing ratios have increased slightly during recent years 

concurrently with increases in “bottom-up” inventory-based emissions of global 

sulfur. Results from surface measurements show a mean global surface mixing ratio of 

493 ppt in 2008 and a mean rate of increase of 1.8 ppt/yr during 2000–2008. New 

laboratory, observational, and modeling studies indicate that vegetative uptake of COS is 

significantly larger than considered in the past. 

Other Trace Gases with an Indirect Influence on Ozone 

• The carbon dioxide (CO2) global average mixing ratio was 385 parts per million 

(ppm) in 2008 and had increased during 2005–2008 at an average rate of 2.1 

ppm/yr. This rate is higher than the average growth rate during the 1990s of 1.5 ppm/yr 

and corresponds with increased rates of fossil fuel combustion. 

• Hydrofluorocarbons (HFCs) used as ODS substitutes continued to increase in the 

global atmosphere. HFC-134a is the most abundant HFC; its global mixing ratio 

reached about 48 ppt in 2008 and was increasing at 4.7 ppt/yr. Other HFCs have been 

identified in the global atmosphere at <10 ppt (e.g., HFC-125, -143a, -32, and -152a) and 

were increasing at ≤1 ppt/yr in 2008. 

• Emissions of HFC-23, a byproduct of HCFC-22 production, have increased over the 

past decade even as efforts at minimizing these emissions were implemented in both 

developed and developing countries. These emission increases are concurrent with 
18

rapidly increasing HCFC-22 production in developing countries and are likely due to 

increasing production of HCFC-22 in facilities not covered by the Kyoto Protocol’s 

Clean Development Mechanism projects. Globally averaged HFC-23 mixing ratios 

reached 21.8 ppt in 2008, with a yearly increase of 0.8 ppt/yr (3.9%/yr). 

• The sum of emissions (weighted by direct, 100-year GWPs) of HFCs used as ODS 

replacements has increased by 8–9%/yr from 2004 to 2008, and by 2008 amounted 

to 0.39 ± 0.03 GtCO2-eq/yr. Regional studies suggest significant contributions of HFC134a and -152a emissions during 2005–2006 from Europe, North America, and Asia. 

Emissions of HFC-23, most of which do not arise from use of this substance as an ODS 

replacement, added an additional 0.2 Gt CO2-eq/year, on average, during 2006–2008. 

• Sulfur hexafluoride (SF6) and nitrogen trifluoride (NF3): Global averaged mixing 

ratios of SF6 reached 6.4 ppt in 2008, with a yearly increase of 0.2 ppt/yr. NF3 was 

detected in the atmosphere for the first time, with a global mean mixing ratio in 2008 of 

0.45 ppt and a growth rate of 0.05 ppt/yr, or 11%/yr. 

Direct Radiative Forcing 

 The abundances of ODSs as well as many of their replacements contribute to radiative 

forcing of the atmosphere. These climate-related forcings have been updated using the current 

observations of atmospheric abundances and are summarized in Table ESA-1. This table also 

contains the primary Kyoto Protocol gases as reference. 

• Over these 5 years, radiative forcing from the sum of ODSs and HFCs has increased 

but, by 2008, remained small relative to the forcing changes from CO2 (see Table 

ESA-1). 

Table ESA-1. Direct radiative forcings of ODSs and other gases, and their recent changes. 

Specific Substance or Group 

of Substances

Direct Radiative Forcing 

(2008), milliWatts per 

square meter (mW/m2)

Change in Direct Radiative 

Forcing (2004–2008), 

mW/m2

CFCs * 268 −7

HCFCs * 45 7

HFCs # , a 15 5

HFC-23 # 4 0.7

CO2

# 1740 139

CH4

# 500 4

N2O # 170 12

PFCs # 5.4 0.5

SF6

# 3.4 0.5

Sum of Montreal Protocol 

gases

313 0

Sum of Kyoto Protocol gases 2434 161 *

 Montreal Protocol Gases (CFCs and HCFCs). 

#

 Kyoto Protocol Gases (CO2, CH4, N2O, HFCs, PFCs, and SF6). a

 Only those HFCs for which emissions arise primarily through use as ODS replacements (i.e., not HFC-23). 
19

CHAPTER 2: STRATOSPHERIC OZONE AND SURFACE ULTRAVIOLET 

RADIATION 

Global Ozone Observations and Interpretation 

 As a result of the Montreal Protocol, ozone is expected to recover from the effect of 

ozone-depleting substances (ODSs) as their abundances decline in the coming decades. The 

2006 Assessment showed that globally averaged column ozone ceased to decline around 1996, 

meeting the criterion for the first stage of recovery. Ozone is expected to increase as a result of 

continued decrease in ODSs (second stage of recovery). This chapter discusses recent 

observations of ozone and ultraviolet radiation in the context of their historical records. Natural 

variability, observational uncertainty, and stratospheric cooling necessitate a long record in order 

to attribute an ozone increase to decreases in ODSs. Table ESA-2 summarizes ozone changes 

since 1980. 

 The primary tools used in this Assessment for prediction of ozone are chemistry-climate 

models (CCMs). These CCMs are designed to represent the processes determining the amount 

of stratospheric ozone and its response to changes in ODSs and greenhouse gases. Eighteen 

CCMs have been recently evaluated using a variety of process-based comparisons to 

measurements. The CCMs are further evaluated here by comparison of trends calculated from 

measurements with trends calculated from simulations designed to reproduce ozone behavior 

during an observing period. 

Total Column Ozone 

• Average total ozone values in 2006–2009 have remained at the same level for the 

past decade, about 3.5% and 2.5% below the 1964–1980 averages respectively for 

90°S–90°N and 60°S–60°N. Average total ozone from CCM simulations behaves in a 

manner similar to observations between 1980 and 2009. The average column ozone for 

1964–1980 is chosen as a reference for observed changes for two reasons: 1) reliable 

ground-based observations sufficient to produce a global average are available in this 

period; 2) a significant trend is not discernible in the observations during this period. 

• Southern Hemisphere midlatitude (35°S–60°S) annual mean total column ozone 

amounts over the period 2006–2009 have remained at the same level as observed 

during 1996–2005, approximately 6% below the 1964–1980 average. Simulations by 

CCMs also show declines of the same magnitude between 1980 and 1996, and minimal 

change after 1996, thus both observations and simulations are consistent with the 

expectations of the impact of ODSs on southern midlatitude ozone. 

• Northern Hemisphere midlatitude (35°N–60°N) annual mean total column ozone 

amounts over the period 2006–2009 have remained at the same level as observed 

during 1998–2005, approximately 3.5% below the 1964–1980 average. A minimum 

about 5.5% below the 1964–1980 average was reached in the mid-1990s. Simulations by 

CCMs agree with these measurements, again showing the consistency of data with the 

expected impact of ODSs. The simulations also indicate that the minimum in the mid1990s was primarily caused by the ozone response to effects of volcanic aerosols from 
20

the 1991 eruption of Mt. Pinatubo. 

• The latitude dependence of simulated total column ozone trends generally agrees 

with that derived from measurements, showing large negative trends at Southern 

Hemisphere mid and high latitudes and Northern Hemisphere midlatitudes for the 

period of ODS increase. However, in the tropics the statistically significant range of 

trends produced by CCMs (−1.5 to −4 Dobson units per decade (DU/decade)) does not 

agree with the trend obtained from measurements (+0.3 ± 1 DU/decade). 

Ozone Profiles 

• Northern Hemisphere midlatitude (35°N–60°N) ozone between 12 and 15 km 

decreased between 1979 and 1995, and increased between 1996 and 2009. The 

increase since the mid-1990s is larger than the changes expected from the decline in ODS 

abundances. 

• Northern Hemisphere midlatitude (35°N–60°N) ozone between 20 and 25 km 

declined during 1979–1995 and has since ceased to decline. Observed increases 

between 1996 and 2008 are statistically significant at some locations but not globally. 

• Northern Hemisphere midlatitude (35°N–60°N) ozone between 35 and 45 km 

measured using a broad range of ground-based and satellite instruments ceased to 

decline after the mid-1990s, consistent with the leveling off of ODS abundances. All 

datasets show a small ozone increase since that time, with varying degrees of statistical 

significance but this increase cannot presently be attributed to ODS decrease because of 

observational uncertainty, natural ozone variability, and stratospheric cooling. CCMs 

simulate the ozone response to changes in ODSs and increases in greenhouse gases; 

analysis of CCM results suggests that longer observational records are required to 

separate these effects from each other and from natural variability. 

• In the midlatitude upper stratosphere (35–45 km) of both hemispheres, the profile 

ozone trends derived from most CCMs from 1980 to 1996 agree well with trends 

deduced from measurements. The agreement in both magnitude and shape of the ozone 

trends provides evidence that increases in ODSs between 1980 and 1996 are primarily 

responsible for the observed behavior. 

• In the tropical lower stratosphere, all simulations show a negative ozone trend just 

above the tropopause, centered at about 18–19 km (70–80 hectoPascals, hPa), due to 

an increase in upwelling. The simulated trends in the lower tropical stratosphere are 

consistent with trends deduced for 1985–2005 from Stratospheric Aerosol and Gas 

Experiment (SAGE II) satellite data, although uncertainties in the SAGE II trends are 

large. The near-zero trend in tropical total ozone measurements is inconsistent with the 

negative trend found in the integrated SAGE I + SAGE II stratospheric profiles. The 

tropospheric ozone column does not increase enough to resolve this discrepancy. 
21

Polar Ozone Observations and Interpretation 

• The Antarctic ozone hole continued to appear each spring from 2006 to 2009. This 

is expected because decreases in stratospheric chlorine and bromine have been moderate 

over the last few years. Analysis shows that since 1979 the abundance of total column 

ozone in the Antarctic ozone hole has evolved in a manner consistent with the time 

evolution of ODSs. Since about 1997 the ODS amounts have been nearly constant and 

the depth and magnitude of the ozone hole have been controlled by variations in 

temperature and dynamics. The October mean column ozone within the vortex has been 

about 40% below 1980 values for the past fifteen years. 

• Arctic winter and spring ozone loss has varied between 2007 and 2010, but 

remained in a range comparable to the values that have prevailed since the early 

1990s. Chemical loss of about 80% of the losses observed in the record cold winters of 

1999/2000 and 2004/2005 has occurred in recent cold winters. 

• Recent laboratory measurements of the chlorine monoxide dimer (ClOOCl) 

dissociation cross section and analyses of observations from aircraft and satellites 

have reaffirmed the fundamental understanding that polar springtime ozone 

depletion is caused primarily by the ClO + ClO catalytic ozone destruction cycle, 

with significant contributions from the BrO + ClO cycle. 

• Polar stratospheric clouds (PSCs) over Antarctica occur more frequently in early 

June and less frequently in September than expected based on the previous satellite 

Table ESA-2.  Summary of ozone changes estimated from observations.

Column ozone 12–15 km 20–25 km 35–45 km Comment

Data sources Ground-based, 

satellite

Ozonesondes Ozonesondes,

satellites, 

FTIR

Satellites,

Umkehrs, FTIR 

Northern 

midlatitudes 

1980–1996

Declined by 

about 6%

Declined by 

about 9%

Declined by 

about 7%

Declined by 

about 10%

1992–1996 

column and lower 

stratosphere data 

affected by Mt.

Pinatubo

Northern 

midlatitudes 

1996–2009

Increased from 

the minimum 

values by about 

2% by 1998 and 

remained at the 

same level 

thereafter 

Increased by 

about 6%

Increased by

about 2.5%

Increased by 1

to 2%, but 

uncertainties are 

large

Southern 

midlatitudes 

1980–1996

Declined by 6% No 

information

Declined by 

about 7%

Declined by 

about 10%

Southern 

midlatitudes 

1996–2009

Remained at 

approximately 

the same level 

No 

statistically 

significant 

changes

No statistically 

significant 

changes

Increased by 1

to 3%, but 

uncertainties are 

large
22

PSC climatology. This result is obtained from measurements by a new class of satellite 

instruments that provide daily vortex-wide information concerning PSC composition and 

occurrence in both hemispheres. The previous satellite PSC climatology was developed 

from solar occultation instruments that have limited daily coverage. 

• Calculations constrained to match observed temperatures and halogen levels

produce Antarctic ozone losses that are close to those derived from data. Without 

constraints, CCMs simulate many aspects of the Antarctic ozone hole, however they do 

not simultaneously produce the cold temperatures, isolation from middle latitudes, deep 

descent, and high amounts of halogens in the polar vortex. Furthermore, most CCMs 

underestimate the Arctic ozone loss that is derived from observations, primarily because 

the simulated northern winter vortices are too warm. 

Ultraviolet Radiation 

 Ground-based measurements of solar ultraviolet (UV) radiation (wavelength 280–400 

nanometers) remain limited both spatially and in duration. However, there have been advances 

both in reconstructing longer-term UV records from other types of ground-based measurements 

and in satellite UV retrievals. Where these UV datasets coincide, long-term changes agree, even 

though there may be differences in instantaneous, absolute levels of UV. 

• Ground-based UV reconstructions and satellite UV retrievals, supported in the later 

years by direct ground-based UV measurements, show that erythemal 

(“sunburning”) irradiance over midlatitudes has increased since the late 1970s, in 

qualitative agreement with the observed decrease in column ozone. The increase in 

satellite-derived erythemal irradiance over midlatitudes during 1979–2008 is statistically 

significant, while there are no significant changes in the tropics. Satellite estimates of 

UV are difficult to interpret over the polar regions.

• In the Antarctic, large ozone losses produce a clear increase in surface UV 

radiation. Ground-based measurements show that the average spring erythemal 

irradiance for 1990–2006 is up to 85% greater than the modeled irradiance for 1963–

1980, depending on site. The Antarctic spring erythemal irradiance is approximately 

twice that measured in the Arctic for the same season. 

• Clear-sky UV observations from unpolluted sites in midlatitudes show that since the 

late 1990s, UV irradiance levels have been approximately constant, consistent with 

ozone column observations over this period. 

• Surface UV levels and trends have also been significantly influenced by clouds and 

aerosols, in addition to stratospheric ozone. Daily measurements under all 

atmospheric conditions at sites in Europe and Japan show that erythemal irradiance has 

continued to increase in recent years due to net reductions in the effects of clouds and 

aerosols. In contrast, in southern midlatitudes, zonal and annual average erythemal 

irradiance increases due to ozone decreases since 1979 have been offset by almost a half 

due to net increases in the effects of clouds and aerosols. 
23

CHAPTER 3: FUTURE OZONE AND ITS IMPACT ON SURFACE UV 

 Globally averaged total column ozone has declined over recent decades due to the release 

of ozone-depleting substances (ODSs) into the atmosphere. Now, as a result of the Montreal 

Protocol, ozone is expected to recover from the effects of ODSs as ODS abundances decline in 

the coming decades. However, a number of factors in addition to ODSs have led to and will 

continue to lead to changes in ozone. Discriminating between the causes of past and projected 

ozone changes is necessary, not only to identify the progress in ozone recovery from ODSs, but 

also to evaluate the effectiveness of climate and ozone protection policy options. 

Factors Affecting Future Ozone and Surface Ultraviolet Radiation 

• At least for the next few decades, the decline of ODSs is expected to be the major 

factor affecting the anticipated increase in global total column ozone. However, 

several factors other than ODS will affect the future evolution of ozone in the 

stratosphere. These include changes in (i) stratospheric circulation and temperature due 

to changes in long-lived greenhouse gas (GHG) abundances, (ii) stratospheric aerosol 

loading, and (iii) source gases of highly reactive stratospheric hydrogen and nitrogen 

compounds. Factors that amplify the effects of ODSs on ozone (e.g., stratospheric 

aerosols) will likely decline in importance as ODSs are gradually eliminated from the 

atmosphere. 

• Increases in GHG emissions can both positively and negatively affect ozone. Carbon 

dioxide (CO2)-induced stratospheric cooling elevates middle and upper stratospheric 

ozone and decreases the time taken for ozone to return to 1980 levels, while projected 

GHG-induced increases in tropical upwelling decrease ozone in the tropical lower 

stratosphere and increase ozone in the extratropics. Increases in nitrous oxide (N2O) and 

methane (CH4) concentrations also directly impact ozone chemistry but the effects are 

different in different regions. 

• The Brewer-Dobson circulation (BDC) is projected to strengthen over the 21st

century and thereby affect ozone amounts. Climate models consistently predict an 

acceleration of the BDC or, more specifically, of the upwelling mass flux in the tropical 

lower stratosphere of around 2% per decade as a consequence of GHG abundance 

increases. A stronger BDC would decrease the abundance of tropical lower stratospheric 

ozone, increase poleward transport of ozone, and could reduce the atmospheric lifetimes 

of long-lived ODSs and other trace gases. While simulations showing faster ascent in the 

tropical lower stratosphere to date are a robust feature of chemistry-climate models 

(CCMs), this has not been confirmed by observations and the responsible mechanisms 

remain unclear. 

• Substantial ozone losses could occur if stratospheric aerosol loading were to increase 

in the next few decades, while halogen levels are high. Stratospheric aerosol increases 

may be caused by sulfur contained in volcanic plumes entering the stratosphere or from 

human activities. The latter might include attempts to geoengineer the climate system by 

enhancing the stratospheric aerosol layer. The ozone losses mostly result from enhanced 

heterogeneous chemistry on stratospheric aerosols. Enhanced aerosol heating within the 

stratosphere also leads to changes in temperature and circulation that affect ozone. 
24

• Surface ultraviolet (UV) levels will not be affected solely by ozone changes but also 

by the effects of climate change and by air quality change in the troposphere. These 

tropospheric effects include changes in clouds, tropospheric aerosols, surface reflectivity, 

and tropospheric sulfur dioxide (SO2) and nitrogen dioxide (NO2). The uncertainties in 

projections of these factors are large. Projected increases in tropospheric ozone are more 

certain and may lead to reductions in surface erythemal (“sunburning”) irradiance of up 

to 10% by 2100. Changes in clouds may lead to decreases or increases in surface 

erythemal irradiance of up to 15% depending on latitude.

Expected Future Changes in Ozone 

 Full ozone recovery from the effects of ODSs and return of ozone to historical levels are 

not synonymous. In this chapter a key target date is chosen to be 1980, in part to retain the 

connection to previous Ozone Assessments. Noting, however, that decreases in ozone may have 

occurred in some regions of the atmosphere prior to 1980, 1960 return dates are also reported. 

 The projections reported on in this chapter are taken from a recent compilation of CCM 

simulations. The ozone projections, which also form the basis for the UV projections, are 

limited in their representativeness of possible futures since they mostly come from CCM 

simulations based on a single GHG emissions scenario (scenario A1B of Emissions Scenarios. A 

Special Report of Working Group III of the Intergovernmental Panel on Climate Change, 

Cambridge University Press, 2000) and a single ODS emissions scenario (adjusted A1 of the 

previous (2006) Ozone Assessment). 

 Throughout this century, the vertical, latitudinal, and seasonal structure of the ozone 

distribution will be different from what it was in 1980. For this reason, ozone changes in 

different regions of the atmosphere are considered separately. 

• The projections of changes in ozone and surface clear-sky UV are broadly consistent 

with those reported on in the 2006 Assessment. 

• The capability of making projections and attribution of future ozone changes has 

been improved since the 2006 Assessment. Use of CCM simulations from an increased 

number of models extending through the entire period of ozone depletion and recovery 

from ODSs (1960–2100) as well as sensitivity simulations have allowed more robust 

projections of long-term changes in the stratosphere and of the relative contributions of 

ODSs and GHGs to those changes. 

• Global annually averaged total column ozone is projected to return to 1980 levels 

before the middle of the century and earlier than when stratospheric halogen 

loading returns to 1980 levels. CCM projections suggest that this early return is 

primarily a result of GHG-induced cooling of the upper stratosphere because the effects 

of circulation changes on tropical and extratropical ozone largely cancel. Global (90°S–

90°

N) annually averaged total column ozone will likely return to 1980 levels between 

2025 and 2040, well before the return of stratospheric halogens to 1980 levels between 

2045 and 2060.

• Simulated changes in tropical total column ozone from 1960 to 2100 are generally 

small. The evolution of tropical total column ozone in models depends on the balance 

between upper stratospheric increases and lower stratospheric decreases. The upper 
25

stratospheric increases result from declining ODSs and a slowing of ozone destruction 

resulting from GHG-induced cooling. Ozone decreases in the lower stratosphere mainly 

result from an increase in tropical upwelling. From 1960 until around 2000, a general 

decline is simulated, followed by a gradual increase to values typical of 1980 by 

midcentury. Thereafter, although total column ozone amounts decline slightly again 

toward the end of the century, by 2080 they are no longer expected to be affected by 

ODSs. Confidence in tropical ozone projections is compromised by the fact that 

simulated decreases in column ozone to date are not supported by observations, 

suggesting that significant uncertainties remain. 

• Midlatitude total column ozone is simulated to evolve differently in the two 

hemispheres. Over northern midlatitudes, annually averaged total column ozone is 

projected to return to 1980 values between 2015 and 2030, while for southern 

midlatitudes the return to 1980 values is projected to occur between 2030 and 2040. The 

more rapid return to 1980 values in northern midlatitudes is linked to a more pronounced 

strengthening of the poleward transport of ozone due to the effects of increased GHG 

levels, and effects of Antarctic ozone depletion on southern midlatitudes. By 2100, 

midlatitude total column ozone is projected to be above 1980 values in both hemispheres. 

• October-mean Antarctic total column ozone is projected to return to 1980 levels 

after midcentury, later than in any other region, and yet earlier than when 

stratospheric halogen loading is projected to return to 1980 levels. The slightly 

earlier return of ozone to 1980 levels (2045–2060) results primarily from upper 

stratospheric cooling and resultant increases in ozone. The return of polar halogen 

loading to 1980 levels (2050–2070) in CCMs is earlier than in empirical models that 

exclude the effects of GHG-induced changes in circulation. Our confidence in the drivers 

of changes in Antarctic ozone is higher than for other regions because (i) ODSs exert a 

strong influence on Antarctic ozone, (ii) the effects of changes in GHG abundances are 

comparatively small, and (iii) projections of ODS emissions are more certain than those 

for GHGs. Small Antarctic ozone holes (areas of ozone <220 Dobson units, DU) could 

persist to the end of the 21st century.

• March-mean Arctic total column ozone is projected to return to 1980 levels two to 

three decades before polar halogen loading returns to 1980 levels, and to exceed 

1980 levels thereafter. While CCM simulations project a return to 1980 levels between 

2020 and 2035, most models tend not to capture observed low temperatures and thus 

underestimate present-day Arctic ozone loss such that it is possible that this return date is 

biased early. Since the strengthening of the Brewer-Dobson circulation through the 21st

century leads to increases in springtime Arctic column ozone, by 2100 Arctic ozone is 

projected to lie well above 1960 levels.

Uncertainties in Projections 

• Conclusions dependent on future GHG levels are less certain than those dependent 

on future ODS levels since ODS emissions are controlled by the Montreal Protocol. 

For the six GHG scenarios considered by a few CCMs, the simulated differences in 

stratospheric column ozone over the second half of the 21st century are largest in the 
26

northern midlatitudes and the Arctic, with maximum differences of 20–40 DU between 

the six scenarios in 2100. 

• There remain sources of uncertainty in the CCM simulations. These include the use 

of prescribed ODS mixing ratios instead of emission fluxes as lower boundary 

conditions, the range of sea surface temperatures and sea ice concentrations, missing 

tropospheric chemistry, model parameterizations, and model climate sensitivity. 

• Geoengineering schemes for mitigating climate change by continuous injections of 

sulfur-containing compounds into the stratosphere, if implemented, would 

substantially affect stratospheric ozone, particularly in polar regions. Ozone losses 

observed following large volcanic eruptions support this prediction. However, sporadic 

volcanic eruptions provide limited analogs to the effects of continuous sulfur emissions. 

Preliminary model simulations reveal large uncertainties in assessing the effects of 

continuous sulfur injections. 

Expected Future Changes in Surface UV 

 While a number of factors, in addition to ozone, affect surface UV irradiance, the focus in 

this chapter is on the effects of changes in stratospheric ozone on surface UV. For this reason, 

clear-sky surface UV irradiance is calculated from ozone projections from CCMs. 

• Projected increases in midlatitude ozone abundances during the 21st century, in the 

absence of changes in other factors, in particular clouds, tropospheric aerosols, and 

air pollutants, will result in decreases in surface UV irradiance. Clear-sky erythemal 

irradiance is projected to return to 1980 levels on average in 2025 for the northern 

midlatitudes, and in 2035 for the southern midlatitudes, and to fall well below 1980 

values by the second half of the century. However, actual changes in surface UV will be 

affected by a number of factors other than ozone. 

• In the absence of changes in other factors, changes in tropical surface UV will be 

small because changes in tropical total column ozone are projected to be small. By 

the middle of the 21st century, the model projections suggest surface UV to be slightly 

higher than in the 1960s, very close to values in 1980, and slightly lower than in 2000. 

The projected decrease in tropical total column ozone through the latter half of the 

century will likely result in clear-sky surface UV remaining above 1960 levels. Average 

UV irradiance is already high in the tropics due to naturally occurring low total ozone 

columns and high solar elevations. 

• The magnitude of UV changes in the polar regions is larger than elsewhere because 

ozone changes in polar regions are larger. For the next decades, surface clear-sky UV 

irradiance, particularly in the Antarctic, will continue to be higher than in 1980. Future 

increases in ozone and decreases in clear-sky UV will occur at slower rates than those 

associated with the ozone decreases and UV increases that occurred before 2000. In 

Antarctica, surface clear-sky UV is projected to return to 1980 levels between 2040 and 

2060, while in the Arctic this is projected to occur between 2020 and 2030. By 2100, 

October surface clear-sky erythemal irradiance in Antarctica is likely to be between 5% 

below to 25% above 1960 levels, with considerable uncertainty. This is consistent with 

multi-model-mean October Antarctic total column ozone not returning to 1960 levels by 
27

2100. In contrast, by 2100, surface clear-sky UV in the Arctic is projected to be 0–10% 

below 1960 levels. 

CHAPTER 4: STRATOSPHERIC CHANGES AND CLIMATE 

• Stratospheric climate trends since 1980 are better understood and characterized than in 

previous Assessments and continue to show the clear influence of both human and 

natural factors.

• New analyses of both satellite and radiosonde data give increased confidence 

relative to previous Assessments of the complex time/space evolution of 

stratospheric temperatures between 1980 and 2009. The global-mean lower 

stratosphere cooled by 1–2 K and the upper stratosphere cooled by 4–6 K from 1980 to 

about 1995. There have been no significant long-term trends in global-mean lower 

stratospheric temperatures since about 1995. The global-mean lower-stratospheric 

cooling did not occur linearly but was manifested as downward steps in temperature in 

the early 1980s and the early 1990s. The cooling of the lower stratosphere included the 

tropics and was not limited to extratropical regions as previously thought. 

• The complex evolution of lower stratospheric temperature is influenced by a 

combination of natural and human factors that has varied over time. Ozone 

decreases dominate the lower stratospheric cooling over the long term (since 1980). 

Major volcanic eruptions and solar activity have clear shorter-term effects. Since the 

mid-1990s, slowing ozone loss has contributed to the lack of temperature trend. Models 

that consider all of these factors are able to reproduce this complex temperature time 

history. 

• The largest lower stratospheric cooling continues to be found in the Antarctic ozone 

hole region during austral spring and early summer. The cooling due to the ozone 

hole strengthened the Southern Hemisphere polar stratospheric vortex compared with the 

pre-ozone hole period during these seasons. 

• Tropical lower stratospheric water vapor amounts decreased by roughly 0.5 parts 

per million (ppm) around 2000 and remained low through 2009. This followed an 

apparent but uncertain increase in stratospheric water vapor amounts from 1980–2000. 

The mechanisms driving long-term changes in stratospheric water vapor are not well 

understood. 

• Stratospheric aerosol concentrations increased by between 4 to 7% per year, 

depending on location, from the late 1990s to 2009. The reasons for the increases in 

aerosol are not yet clear, but small volcanic eruptions and increased coal burning are 

possible contributing factors. 

 

• There is new and stronger evidence for radiative and dynamical linkages between 

stratospheric change and specific changes in surface climate. 

• Changes in stratospheric ozone, water vapor, and aerosols all radiatively affect 

surface temperature. The radiative forcing of climate in 2008 due to stratospheric 

ozone depletion (−0.05 ± 0.1 Watts per square meter, W/m2) is much smaller than the 
28

positive radiative forcing due to the chlorofluorocarbons (CFCs) and 

hydrochlorofluorocarbons (HCFCs) largely responsible for that depletion (+0.31 ± 0.03 

W/m2

). Radiative calculations and climate modeling studies suggest that the radiative 

effects of variability in stratospheric water vapor (roughly ±0.1 W/m2 per decade) can 

contribute to decadal variability in globally averaged surface temperature. Climate 

models and observations show that the negative radiative forcing from a major volcanic 

eruption such as Mt. Pinatubo in 1991 (roughly −3 W/m2

) can lead to a surface cooling 

that persists for about two years. 

• Observations and model simulations show that the Antarctic ozone hole caused 

much of the observed southward shift of the Southern Hemisphere middle latitude 

jet in the troposphere during summer since 1980. The horizontal structure, 

seasonality, and amplitude of the observed trends in the Southern Hemisphere 

tropospheric jet are only reproducible in climate models forced with Antarctic ozone 

depletion. The southward shift in the tropospheric jet extends to the surface of the Earth 

and is linked dynamically to the ozone hole-induced strengthening of the Southern 

Hemisphere stratospheric polar vortex. 

• The southward shift of the Southern Hemisphere tropospheric jet due to the ozone 

hole has been linked to a range of observed climate trends over Southern 

Hemisphere mid and high latitudes during summer. Because of this shift, the ozone 

hole has contributed to robust summertime trends in surface winds, warming over the 

Antarctic Peninsula, and cooling over the high plateau. Other impacts of the ozone hole 

on surface climate have been investigated but have yet to be fully quantified. These 

include: observed increases in sea-ice area averaged around Antarctica; a southward shift 

of the Southern Hemisphere stormtrack and associated precipitation; warming of the 

subsurface Southern Ocean at depths up to several hundred meters; and decreases of 

carbon uptake over the Southern Ocean. 

• In the Northern Hemisphere, robust linkages between Arctic stratospheric ozone 

depletion and the tropospheric and surface circulation have not been established, 

consistent with the comparatively small ozone losses there. 

• The influence of stratospheric changes on climate will continue during and after 

stratospheric ozone recovery. 

• The global middle and upper stratosphere are expected to cool in the coming 

century, mainly due to carbon dioxide (CO2) increases. The cooling due to CO2 will 

cause ozone levels to increase in the middle and upper stratosphere, which will slightly 

reduce the cooling. Stratospheric ozone recovery will also reduce the cooling. These 

ozone changes will contribute a positive radiative forcing of climate (roughly +0.1 W/m2) 

compared to 2009 levels, adding slightly to the positive forcing from continued increases 

in atmospheric CO2 abundances. Future hydrofluorocarbon (HFC) abundances in the 

atmosphere are expected to warm the tropical lower stratosphere and tropopause region 

by roughly 0.3 K per part per billion (ppb) and provide a positive radiative forcing of 

climate. 

• Chemistry-climate models predict increases of stratospheric water vapor, but 

confidence in these predictions is low. Confidence is low since these same models: 1) 

have a poor representation of the seasonal cycle in tropical tropopause temperatures 
29

(which control global stratospheric water vapor abundances) and 2) cannot reproduce 

past changes in stratospheric water vapor abundances. 

• Future recovery of the Antarctic ozone hole and increases in greenhouse gases are 

expected to have opposite effects on the Southern Hemisphere tropospheric middle 

latitude jet. Over the next 50 years, the recovery of the ozone hole is expected to reverse 

the recent southward shift of the Southern Hemisphere tropospheric jet during summer. 

However, future increases in greenhouse gases are expected to drive a southward shift in 

the Southern Hemisphere tropospheric jet during all seasons. The net effect of these two 

forcings on the jet during summer is uncertain.

• Climate simulations forced with increasing greenhouse gases suggest a future 

acceleration of the stratospheric Brewer-Dobson circulation. Such an acceleration 

would lead to decreases in column ozone in the tropics and increases in column ozone 

elsewhere by redistributing ozone within the stratosphere. The causal linkages between 

increasing greenhouse gases and the acceleration of the Brewer-Dobson circulation 

remain unclear. 

• Future stratospheric climate change will affect tropospheric ozone abundances. In 

chemistry-climate models, the projected acceleration of the Brewer-Dobson circulation 

and ozone recovery act together to increase the transport of stratospheric ozone into the 

troposphere. Stratospheric ozone redistribution will also affect tropospheric ozone by 

changing the penetration of ultraviolet radiation into the troposphere, thus affecting 

photolysis rates. 

CHAPTER 5: A FOCUS ON INFORMATION AND OPTIONS FOR POLICYMAKERS 

 Ozone Depletion Potentials (ODPs) and Global Warming Potentials (GWPs) are metrics 

frequently used to quantify the relative impacts of substances on ozone depletion and climate 

forcing. In Chapter 5, both ODPs and GWPs have been updated. The direct GWPs for some 

compounds presented here have not appeared previously in WMO/UNEP or Intergovernmental 

Panel on Climate Change (IPCC) assessments. Indirect GWPs have also been reevaluated. 

INFORMATION FOR POLICYMAKERS 

• The Montreal Protocol is working. It has protected the stratospheric ozone layer 

from much higher levels of depletion by phasing out production and consumption of 

ozone-depleting substances (ODSs). Simulations show that unchecked growth in the 

emissions of ODSs would have led to ozone depletion globally in the coming decades 

much larger than has been observed. Solar UV-B radiation at the surface would also 

have increased substantially. 

• The Montreal Protocol and its Amendments and Adjustments have made large 

contributions toward reducing global greenhouse gas emissions. Because many 

ODSs are potent greenhouse gases, the Montreal Protocol has successfully avoided larger 

climate forcing. In 2010, the decrease of annual ODS emissions under the Montreal 

Protocol is estimated to be about 10 Gigatonnes (Gt) of carbon dioxide–equivalent 
30

(GtCO2-eq) per year, which is about five times larger than the annual emissions reduction 

target for the first commitment period (2008–2012) of the Kyoto Protocol. 

• The accelerated hydrochlorofluorocarbon (HCFC) phaseout agreed to by the 

Parties to the Montreal Protocol in 2007 is projected to reduce cumulative HCFC 

emissions by 0.6–0.8 million ODP-tonnes between 2011 and 2050 and bring forward 

the year equivalent effective stratospheric chlorine (EESC) returns to 1980 levels by 

4–5 years. In terms relevant to climate, the accelerated HCFC phaseout is projected 

to reduce emissions by 0.4–0.6 GtCO2-eq per year averaged over 2011 through 2050. 

The actual climate benefit will be determined, in part, by the climate impact of the 

compounds used to replace the HCFCs. In comparison, global anthropogenic emissions 

of CO2 were greater than 30 Gt per year in 2008. 

• EESC at midlatitudes is projected to return to 1980 levels in 2046 for the baseline 

(A1) scenario, 2–3 years earlier than projected in the previous Assessment. This 

revision is primarily due to an improved understanding of lower stratospheric chlorine 

and bromine release from ODSs, along with contributions from smaller projected HCFC 

emissions, and despite larger projected emissions of carbon tetrachloride (CCl4) and a 

smaller 1980 mixing ratio of methyl bromide (CH3Br). 

• EESC in the Antarctic vortex is projected to return to 1980 levels around 2073 for 

the baseline (A1) scenario, 7–8 years later than projected in the previous 

Assessment. This is primarily due to an improved understanding of lower stratospheric 

chlorine and bromine release from ODSs, with smaller contributions from changes in the 

emissions of CCl4 and HCFCs and a smaller 1980 mixing ratio of CH3Br. The return to 

1980 levels in the Antarctic vortex is about 26 years later than the return of midlatitude 

EESC to 1980 levels. 

• Due to the ongoing success of the Montreal Protocol and its Amendments and 

Adjustments in reducing the production, emissions, and abundances of controlled 

ODSs, other compounds and activities not controlled by the Montreal Protocol are 

becoming relatively more important to stratospheric ozone levels. 

• Increasing abundances of radiatively important gases that are not controlled by the 

Montreal Protocol, especially CO2, methane (CH4), and nitrous oxide (N2O), are 

expected to significantly affect future stratospheric ozone levels (see also Chapter 3). 

Under many IPCC future scenarios, it is projected that these gases will cause globally 

averaged ozone changes larger than those resulting from any of the ODS reduction cases 

explored in this chapter. 

• A nitrous oxide (N2O) ODP of 0.017 has been calculated. The anthropogenic ODPweighted emission of N2O is larger than that of any current halogenated ODS 

emission. The ODP of N2O is more uncertain than it is for halogenated substances, but it 

has been known since 1970 that N2O depletes stratospheric ozone. Reductions in N2O 

emissions would also reduce climate forcing. 

• Since the previous Assessment, new fluorocarbons have been suggested as possible 

replacements for potent HCFC and hydrofluorocarbon (HFC) greenhouse gases. 

For example, HFC-1234yf (CF3CF=CH2) (ODP = 0; 100-year GWP = 4) is proposed to 

replace HFC-134a (CH2FCF3) (ODP = 0; 100-year GWP = 1370) in motor vehicle 
31

(mobile) air conditioning. Each new fluorocarbon proposed as a replacement will require 

an evaluation for ODP, GWP, atmospheric fate, safety, and toxicity for a thorough 

understanding of its potential environmental impact. Preliminary analyses of the 

atmospheric fate of HFC-1234yf indicate that global replacement of HFC-134a with 

HFC-1234yf at today’s level of use is not expected to contribute significantly to 

tropospheric ozone formation or harmful levels of the degradation product TFA 

(trifluoroacetic acid). It is well established that TFA is a ubiquitous natural component of 

the hydrosphere, but uncertainties remain regarding its natural and anthropogenic 

sources, long-term fate, and abundances. 

OPTIONS FOR POLICYMAKERS 

 A new baseline scenario for ODSs is presented in Chapter 5 that reflects our current 

understanding of atmospheric mixing ratios, production levels, and bank sizes. Elimination of 

future emissions, production, and banks of various ODSs are applied to this scenario to evaluate 

the maximum impacts of various hypothetical phaseouts (see Table ESA-3). The year EESC 

returns to 1980 levels, and integrated EESC changes, are two metrics used in the evaluation. The 

calculations of the years when EESC returns to the 1980 level in these hypothetical cases do not 

consider other effects such as changing atmospheric transport and lifetimes. An elimination of 

anthropogenic N2O emissions is also considered and compared to some ODS cases using 

globally averaged total ozone. In addition to the hypothetical cases discussed below, the impacts 

on stratospheric ozone of other activities, such as the use of automotive biofuels, commercial 

subsonic aircraft, and rocket launches, are considered in Chapter 5. These other activities are not 

expected to substantially affect stratospheric ozone now or in the near future. 

• Projections suggest that unmitigated HFC growth could result in GWP-weighted 

emissions up to 8.8 GtCO2-eq per year by 2050, comparable to the GWP-weighted 

emissions of chlorofluorocarbons (CFCs) at their peak in 1988. The highest of these 

projections assumes that developing countries use HFCs with GWPs comparable to those 

currently used in the same applications in developed countries. The projected radiative 

forcing in 2050 from these compounds (up to 0.4 W/m2) can be reduced by using 

compounds with lower GWPs. 

• Options available for limiting future halocarbon emissions will have less impact on 

future ozone levels than what has already been accomplished by the Montreal 

Protocol. 

• Leakage of CFCs and leakage of halons from the banks are the largest sources of 

current ODP-weighted emissions of ODSs. A delay of four years, from 2011 to 2015, 

in the capture and destruction of the estimated CFC banks is currently thought to reduce 

the potential ozone and climate benefits from these actions by about 30%. The 

percentage impact of a four-year delay in the capture and destruction of the halon banks 

is similar. 

• Elimination of future CCl4 emissions is now projected to have a larger impact on 

integrated EESC than was projected in the previous Assessment. Recent observed 

CCl4 mixing ratios have declined more slowly than previously projected. Extrapolation 
32

Table ESA-3. Summary of hypothetical cases for accelerating the recovery of the ozone layer and 

reducing carbon-equivalent emissions. The table below shows the reductions in integrated EESC and 

integrated CO2-eq emissions relative to the baseline (A1) scenario that can be achieved in several 

hypothetical cases. The EESC excess above 1980 levels is integrated from 2011 until the time EESC 

returns to the 1980 level (before 2050). Any potential contribution from very short-lived substances is 

neglected.

Substance or Group of 

Substances

Reductions (%) in 

Integrated EESC 

(equivalent effective 

stratospheric chlorine)

Reduction in Cumulative 

GWP-Weighted Emissions 

from 2011 to 2050

(Gt of CO2-equivalent)

Bank capture and 

destruction in 2011 and 

2015:

2011 2015 2011 2015

CFCs 11 7.0 7.9 5.5

Halons 14 9.1 0.4 0.3

HCFCs 4.8 5.3 1 4.9 5.5 1

Production elimination 

after 2010:

HCFCs 8.8 13.2

CH3Br for quarantine and 

pre-shipment

6.7 0.002

Total emissions elimination 

after 2010:

CCl4

2 7.6 0.9

CH3CCl3

0.1 0.004

HFCs 0.0 Up to 170 3

1 The impact of a 2015 HCFC bank recovery is larger than a 2011 bank recovery because this calculation assumes 

destruction of the bank in only a single year, and because the bank in 2015 is larger than the bank in 2011 owing 

to continued annual production that is larger than the annual bank release. 2 Banks are assumed to be zero. Emissions include uncertain sources such as possible fugitive emissions and 

unintended byproduct emissions. 3 Strongly dependent on future projections and does not consider HFC-23 emissions. HFCs are not controlled by 

the Montreal Protocol, but are included in the basket of gases of the Kyoto Protocol. 

of this trend leads to larger future projected emissions in the baseline scenario and thus to 

the increased projected impact of the elimination of emissions. 

• The estimated impact on integrated EESC resulting from elimination of future 

HCFC production is slightly smaller than in the previous Assessment. The recent 

growth in reported HCFC production in developing countries was larger than projected in 

the previous Assessment. This alone would have resulted in a larger projected HCFC 

production in the new baseline scenario compared to the previous Assessment, but is 

projected to be more than compensated for by the accelerated HCFC phaseout agreed to 

by the Parties to the Montreal Protocol in 2007. Projections suggest that total emissions 
33

of HCFCs will begin to decline in the coming decade due to measures already agreed to 

under the Montreal Protocol. 

• The elimination of all emissions of chlorine- and bromine-containing ODSs after 

2010 would shift the year EESC reaches the 1980 level by about 13 years, from 2046 

to 2033. In terms relevant to climate, this would reduce emissions of these substances by 

about 0.7 GtCO2-eq per year averaged over 2011 through 2050. Future production of 

HCFCs and the sum of the current banks of CFCs plus HCFCs contribute about equally 

to this number. In comparison, global anthropogenic emissions of CO2 were greater than 

30 Gt per year in 2008. 

• A phaseout of methyl bromide emissions from quarantine and pre-shipment (QPS) 

applications beginning in 2011 would shift the year EESC reaches the 1980 level 

earlier by 1.5 years compared to continued use at current levels. Continuing Critical 

Use Exemptions (CUEs) indefinitely at the approved 2011 level would delay the return of 

EESC to 1980 levels by 0.2 years. 

• Elimination of anthropogenic emissions of very short-lived substances (VSLS) could 

shift the year EESC reaches the 1980 level earlier by almost 3 years, if anthropogenic 

VSLSs contribute 40 parts per trillion of EESC to the stratosphere. It remains unclear, 

however, how VSLS emissions reductions at different surface locations would affect their 

contribution to stratospheric chlorine. VSLSs are not controlled by the Montreal 

Protocol.