  The climate system is a forced, dissipative, nonlinear, complex and
heterogeneous system that is out of thermodynamic equilibrium. The system
exhibits natural variability on many scales of motion, in time as well as
space, and it is subject to various external forcings, natural as well as
anthropogenic. This paper reviews the observational evidence on climate
phenomena and the governing equations of planetary-scale flow, as well as
presenting the key concept of a hierarchy of models as used in the climate
sciences. Recent advances in the application of dynamical systems theory, on
the one hand, and of nonequilibrium statistical physics, on the other, are
brought together for the first time and shown to complement each other in
helping understand and predict the system's behavior. These complementary
points of view permit a self-consistent handling of subgrid-scale phenomena as
stochastic processes, as well as a unified handling of natural climate
variability and forced climate change, along with a treatment of the crucial
issues of climate sensitivity, response, and predictability.
