SCIENTIFIC SUMMARY
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.
• 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 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 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 2100. In contrast,
by 2100, surface clear-sky UV in the Arctic is projected to be 0–10% below 1960 levels