Climate Change and the Middle Atmosphere. Part IV: Ozone Response to Doubled CO2

Parameterized stratospheric ozone photochemistry has been included in the Goddard Institute for Space Studies (GISS) GCM to investigate the coupling between chemistry and climate change for the doubled CO2 climate. The chemical ozone response is of opposite sign to temperature changes, so that radiative cooling in the upper stratosphere results in increased ozone, while warming reduces ozone in the lower stratosphere. The increased overhead column reduces the amount of UV reaching the lower stratosphere, resulting in further ozone decreases there. Changes of up to 15% are seen, including both photochemistry and transport. Good agreement is found between the authors’ results and those in other models for tropical latitudes where the stratospheric temperature responses are similar. However, in the extratropics, there are large differences between present results and those of the other models due to differences in tropospheric warming and tropospheric forcing of the stratospheric residual circulation. A net decrease in column ozone at midlatitudes is seen in this climate model, in contrast to the other models that showed an increase in column ozone everywhere. These ozone reductions lead to an increase of 10% in UV radiation reaching the surface at northern midlatitudes. The authors find significantly less of an increase in the high-latitude ozone column than in the other models. When parameterized heterogeneous chemistry on polar stratospheric clouds is also included, while maintaining current chlorine loading, it is found that the Antarctic ozone hole becomes significantly larger and of longer duration. In addition, an ozone hole of approximately half the depth in percent of the current Antarctic ozone hole forms in the Arctic due to both chemistry and transport changes resulting from a reduction of sudden warmings seen in the doubled CO2 atmosphere.