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 clim ate. The chemical ozone response is of opposite sign to temperature ch anges, 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. Change s of up to 15% are seen, including both photochemistry and transport. Good agreement is found between the authors' results and those in othe r models for tropical latitudes where the stratospheric temperature re sponses are similar. However, in the extratropics, there are large dif ferences between present results and those of the other models due to differences in tropospheric warming and tropospheric forcing of the st ratospheric residual circulation. A net decrease in column ozone at mi dlatitudes is seen in this climate model, in contrast to the other mod els that showed an increase in column ozone everywhere. These ozone re ductions lead to an increase of 10% in UV radiation reaching the surfa ce 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 clou ds 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 t he depth in percent of the current Antarctic ozone hole forms in the A rctic due to both chemistry and transport changes resulting from a red uction of sudden warmings seen in the doubled CO2 atmosphere.