An intermediate complexity AGCM simulations of climate response to a doubling of atmospheric carbon dioxide

Atmospheric response to doubled carbon dioxide concentration is estimated by analyzing 35-member ensemble mean made by an atmospheric general circulation model of intermediate complexity. Simulated changes in the mean fields are evaluated for winter (January-February-March) and summer (July-AugustSeptember) seasons. Results show that doubled CO2 concentration causes warming of around 2 °C at all levels in the model. At the surface, the largest temperature change is found over the polar areas; while at the higher levels considerable warming is found mostly over the continental parts. Atmospheric warming at the 300 hPa level is accompanied by cooling over the polar areas. At the levels above 300 hPa, temperature drops globally. Changes in jet stream occur at Northern Hemisphere with larger winter amplitudes. During the respective winter, stratiform precipitation significantly increases at the higher latitudes of both hemispheres and decreases mostly over the oceans. Over the Northern Hemisphere, convective precipitation is significantly increased during the summer. Over the southern part of tropical Pacific, stratiform and convective precipitation is decreased during the both seasons. Results also demonstrate that indirect impact of increased CO2 concentration (i.e. effects associated with changes in the lower boundary conditions) generally has a stronger contribution to the tropospheric warming than direct CO2 impact (i.e. the impact associated with absorption and emission of longwave radiation).