WATER VAPOR AND THE DYNAMICS OF CLIMATE CHANGES

Abstract

Water vapor is not only Earth's dominant greenhouse gas. Through the release\nof latent heat when it condenses, it also plays an active role in dynamic\nprocesses that shape the global circulation of the atmosphere and thus climate.\nHere we present an overview of how latent heat release affects atmosphere\ndynamics in a broad range of climates, ranging from extremely cold to extremely\nwarm. Contrary to widely held beliefs, atmospheric circulation statistics can\nchange non-monotonically with global-mean surface temperature, in part because\nof dynamic effects of water vapor. For example, the strengths of the tropical\nHadley circulation and of zonally asymmetric tropical circulations, as well as\nthe kinetic energy of extratropical baroclinic eddies, can be lower than they\npresently are both in much warmer climates and in much colder climates. We\ndiscuss how latent heat release is implicated in such circulation changes,\nparticularly through its effect on the atmospheric static stability, and we\nillustrate the circulation changes through simulations with an idealized\ngeneral circulation model. This allows us to explore a continuum of climates,\nconstrain macroscopic laws governing this climatic continuum, and place past\nand possible future climate changes in a broader context.\n