Scientific Summary Chapter 4: Stratospheric Ozone Changes and Climate
Since the last Assessment, new research has better quantified the impact of stratospheric ozone changes on climate. Additional model and observational analyses are assessed which examine the influence of stratospheric ozone changes on stratospheric temperatures and circulation, Southern Hemisphere tropospheric circulation and composition, surface climate, oceans, and sea ice.
Stratospheric ozone changes are the dominant driver of observed globally averaged long-term temperature changes in the lower stratosphere. Between 1979 and 1995 global mean lower stratospheric temperature decreased by about 1 K but has since remained approximately constant.
Models broadly reproduce the evolution of global mean lower stratospheric temperature change. Stratospheric ozone changes are the dominant driver of these changes, with volcanic aerosol driving episodic warming, and greenhouse gas increases having only a minor contribution.
Observed mid- and upper-stratospheric temperatures decreased from 1979 to 2005, but the magnitude of the cooling is uncertain. A newly reprocessed data set of satellite measurements exhibits substantially different cooling trends compared to the existing data set. Models indicate that increasing greenhouse gases, as well as ozone changes, both made comparable contributions to observed cooling in the mid and upper stratosphere.
There was little overall change in global lower stratospheric water vapor concentration between 2000 and 2012, based on satellite measurements, which show a decrease between 2000 and 2004 followed by an increase to 2012.
The observed cooling of the Antarctic lower stratosphere since 1979 during austral spring is consistent with the average simulated cooling in models forced with observed ozone depletion. There is a large range in the magnitude of the simulated cooling, with models that underestimate the ozone depletion also underestimating the temperature trends.
Climate models consistently predict a long-term increase in the strength of the Brewer-Dobson circulation due to greenhouse gas increases, with important impacts on stratospheric and tropospheric composition.
The predicted increase in the strength of the Brewer-Dobson circulation extends throughout the depth of the stratosphere.
Observations of changes in temperature, ozone, and trace gases over the past three to five decades are suggestive of increased upwelling in the tropical lower stratosphere, consistent with a strengthening of the shallow branch of the Brewer-Dobson circulation predicted by models. There is large uncertainty in changes in the deep branch of the Brewer-Dobson circulation inferred from observations in the mid and upper stratosphere.
Stratospheric ozone recovery and an acceleration of the Brewer-Dobson circulation in the future would both tend to increase the global tropospheric ozone burden. The projected net changes in tropospheric ozone and other compounds vary regionally and are scenario and model dependent.
Stratospheric temperature changes due to Antarctic ozone depletion are very likely the dominant driver of the observed changes in Southern Hemisphere tropospheric circulation in summer over recent decades, with associated surface climate and ocean impacts.
The contribution of Antarctic ozone depletion to the observed increase in the Southern Annular Mode index in austral summer is substantially larger in most models than the contribution from greenhouse gas increases over the past three to five decades. An increase in this index corresponds to a decrease in atmospheric pressure at high latitudes, an increase at midlatitudes, and a poleward shift of the midlatitude jet. The role of ozone depletion is largest in summer. Observations and models suggest smaller Southern Annular Mode trends in other seasons.
Stratospheric ozone depletion has likely contributed to the observed expansion of the Southern Hemisphere Hadley Cell in austral summer.
Climate models simulate a poleward shift of the Southern Hemisphere midlatitude maximum in precipitation and a moistening of the subtropics in response to stratospheric ozone depletion in austral summer. There is some evidence of a consistent pattern of trends in observations.
Observational and modeling studies present a broadly consistent picture of the ocean’s response to surface wind stress changes, which have likely been substantially caused by stratospheric ozone changes, with intensification of the subtropical ocean gyres and the meridional overturning circulations, and a subsurface warming. The impact of these wind stress changes on oceanic carbon uptake remains uncertain. The role of ocean eddies, which modify the ocean circulation and temperature response to wind stress changes, is better understood than at the time of the last Ozone Assessment, but remains a source of uncertainty.
The influence of stratospheric ozone depletion on Antarctic sea ice increases reported in the last Ozone Assessment is not supported by a number of new coupled modeling studies. These suggest that ozone depletion drives a decrease in Southern Hemisphere sea ice extent and thus did not lead to the small observed increase. However, there is low confidence in this model result because of large uncertainties in the simulation of Antarctic sea ice.
No robust link between stratospheric ozone changes and Northern Hemisphere tropospheric climate has been found, consistent with the conclusions of the previous Ozone Assessment.
There is further evidence that in austral summer over the next 50 years, Antarctic stratospheric ozone recovery and increases in greenhouse gases will have opposite effects on the Southern Hemisphere tropospheric circulation, with associated surface climate and ocean impacts.
Ozone recovery is expected to drive a weakening and equatorward shift of the midlatitude jet, while increases in greenhouse gases are expected to drive a strengthening and poleward shift of the jet. Under a low greenhouse gas emissions scenario, ozone recovery is expected to dominate the effect of greenhouse gas increases on Southern Hemisphere tropospheric circulation in austral summer to give a weakening and equatorward shift of the midlatitude jet over the next 50 years, whereas in a high emissions scenario the jet is projected to continue to strengthen and shift poleward.
An equatorward shift in the Southern Hemisphere Hadley Cell boundary and extratropical rainfall in summer is simulated in response to ozone recovery. These changes offset a scenario-dependent fraction of projected greenhouse-gas induced changes in these variables.
Simulations from multiple models indicate that if the concentrations of ozone-depleting substances (ODSs) had continued to increase in the absence of the Montreal Protocol, the enhanced ozone depletion from uncontrolled ODSs would be expected to have led to substantial additional cooling in the Antarctic polar stratosphere, with associated changes in Southern Hemisphere circulation and rainfall patterns.
New estimates of global mean ozone radiative forcing due to emissions of ozone-depleting substances, which account for stratospheric ozone change and its indirect effect on tropospheric ozone, indicate a stronger surface cooling effect than that due to stratospheric ozone changes alone.
The overall global mean ozone radiative forcing from the effects of ODS emissions on both tropospheric and stratospheric ozone is assessed to be -0.15 (-0.3 to 0) watts per square meter (W m-2) in 2011. Approximately three quarters of this results from ozone changes in the stratosphere.
Models indicate that ODS-induced stratospheric ozone depletion has acted to decrease tropospheric ozone. This ODS-driven decrease in tropospheric ozone contributes to the overall negative ozone
radiative forcing, although the magnitude is uncertain.
The radiative forcing due to observed decreases in stratospheric ozone concentration alone is estimated to be -0.05 W m-2 (-0.15 to 0.05) W m-2 in 2011. A rapid adjustment to radiative forcing may also arise from cloud changes, resulting from the circulation changes driven by ODS-induced ozone depletion. The radiative effect of this cloud adjustment may be of a larger magnitude than the non-adjusted forcing.
Uncertainty in future lower stratospheric ozone trends in the tropics precludes a confident assessment of the sign of future stratospheric ozone radiative forcing. Current models give a range of stratospheric ozone radiative forcing of -0.05 to +0.25 W m-2 in 2100 under a high greenhouse gas emissions scenario, which is generally suggestive of a slight warming contribution relative to present.
