While weather is the state of the atmosphere at any specific time, climate is a long-term average of the weather over a large area. Climate data is typically obtained by measuring weather variables and other related indicators such as pollen and ice cover over many years. Climate models provide information about "average" weather conditions over long time spans. Neither climate data nor climate models show what the weather is like on a specific day, but they do indicate what it might typically be like.

Typhoon Violet near Japan, imaged by NSCAT scatterometer.

Earth's climate depends on several large-scale factors: energy coming from the Sun, the composition of the atmosphere, the extent and health of the biosphere (vegetation primarily), the temperature and circulation patterns of the ocean, and the extent of ice cover. Just as in weather forecasting, discrepancies in measurements or actual changes in any of these factors can have large effects down the road when running computer climate models. Accurate measurements of all of these climate "contributors" and an understanding of their interactions are necessary to forecast Earth's future climate. Insight into climate trends makes it possible to plan and prepare for the future.