Scientific Summary 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 by volume (ppmv) 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 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 storm track 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 (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.