From Glossary of Meteorology
Most climate models are closely related to NWP models. In general, the equations for the atmosphere are those for momentum, continuity, the equation of state, the law of thermodynamics, and water vapor continuity, although simplified sets of governing equations may also be used. Depending on the climate timescale of interest, the atmospheric model may be coupled to similar models for the ocean (including the deep ocean for timescales greater than a year), the land surface and biosphere, land snow and ice, and sea ice. For longer timescales, variation in external forcing factors (solar radiation) may also be important. Because climate models are generally integrated for periods of many years, they have traditionally been run at lower resolution than NWP models to reduce computational costs.