To establish an equilibrium state, there must be radiative balance at the top of the atmosphere and energy balance at the surface and within the atmosphere. The earth's climate is determined not only by the atmosphere but also by the ocean, the land surface, the cryosphere, and the biosphere. Figure 5.2 schematically shows the earth's climate system and the complexity of interactions between components of the system. The earth's climate is regulated by interactive dynamic, physical, chemical, and biophysical processes. The characteristic time-scales for each component to recover its equilibrium state after being disturbed differ a great deal, ranging from one to two months for the atmosphere, a few months for the mixed layer of the ocean, several decades for the upper part of the ocean, a few thousand years for the deep ocean, to tens of thousands of years or more for the ice sheet.
Mutual interactions produce varying time-scales of climate variation.
However, our understanding of those interactions, obtained mainly through numerical experiments with climate models, is still very limited. Climate models numerically predict or determine the physical states of these components on the basis of physical laws. The representative physical state of each box is numerically predicted with the use of relevant physical laws. In order to predict transient, regional climate changes, the models used must allow in some way for the influences of the various components of the climate system and for the significant interactions between them. These provide an effective framework for testing new hypotheses about climate sensitivity and change.