2010), Water vapor and the dynamics of climate changes

Water vapor is not only Earth’s dominant greenhouse gas. Through the release of latent heat when it condenses, it also plays an active role in dynamic processes that shape the global circulation of the atmosphere and thus climate. Here we present an overview of how latent heat release affects atmosphere dynamics in a broad range of climates, ranging from extremely cold to extremely warm. Con-trary to widely held beliefs, atmospheric circulation statis-tics can change non-monotonically with global-mean sur-face temperature, in part because of dynamic effects of water vapor. For example, the strengths of the tropical Hadley circulation and of zonally asymmetric tropical cir-culations, as well as the kinetic energy of extratropical baroclinic eddies, can be lower than they presently are both in much warmer climates and in much colder cli-mates. We discuss how latent heat release is implicated in such circulation changes, particularly through its effect on the atmospheric static stability, and we illustrate the circulation changes through simulations with an idealized general circulationmodel. This allows us to explore a con-tinuum of climates, constrainmacroscopic laws governing this climatic continuum, and place past and possible future climate changes in a broader context. 1