  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. Contrary to widely held beliefs, atmospheric circulation statistics can
change non-monotonically with global-mean surface temperature, in part because
of dynamic effects of water vapor. For example, the strengths of the tropical
Hadley circulation and of zonally asymmetric tropical circulations, 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 climates. 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 circulation model. This allows us to explore a continuum of climates,
constrain macroscopic laws governing this climatic continuum, and place past
and possible future climate changes in a broader context.
