On the response of the middle atmosphere to anthropogenic forcing.

Anthropogenic forcing of the atmosphere by greenhouse gases (GHG) and ozone-depleting substances has provided an unintended test of the robustness of current understanding of the physics and chemistry of the middle atmosphere, that is, the stratosphere and mesosphere. We explore this topic by examining how well anthropogenic changes can be simulated by modern, comprehensive numerical models. Specifically, we discuss the simulations of trends in global mean temperature; the development of the ozone hole and its impact on the dynamics of the Southern Hemisphere, both in the stratosphere and troposphere; trends in the stratospheric Brewer-Dobson circulation; and the response of the quasi-biennial oscillation (QBO) to increasing burdens of CO2 . We find that, in most of these cases, numerical simulation is able to reproduce observed changes and provide physical insights into the relevant mechanisms. Simulation of the QBO is on a less firm footing. Although many numerical models can now generate realistic QBOs, future projections of its behavior under the increasing burdens of GHG are inconsistent and even contradictory.