Modern research has produced computer climate models that are based largely on weather-prediction models. Climate models, however, include simulations of the ocean and land surface and other components of the climate system, as well as the atmosphere. Climate models are also allowed to run for a longer simulated time, several months or even years, instead of the few days needed to produce a weather forecast. This variation in time scales is one of the fundamental differences between climate and weather. Climate modeling is computer simulation of a high order. We can change a number in the computer program and make the Earth spin faster or backwards, we can turn off the Sun, we can make the seasons disappear, or we can change atmospheric carbon dioxide. Of course, we are doing all this with a make-believe or virtual Earth. Climate modeling by computer simulation is an exciting area of research. It can provide valuable insights into how the climate system works. However, the results are approximate solutions of the climate problem, not exact depictions.