Climate is the long-term pattern of weather in a particular area. Weather can change from hour to hour, day to day, month to month or even from year to year. For periods of 30 years or more, however, distinct weather patterns occur. A desert might experience a rainy week, but over the long term, the region receives very little rainfall. It has a dry climate.

Global warming is often linked to the burning of fossil fuels—coal, oil, and natural gas—by industries and cars. Warming is also linked to the destruction of tropical forests. Such human activity has increased the amount of carbon dioxide in the atmosphere by more than 20 percent in the past 100 years.

The climate has changed many times during Earth’s history, but the changes have occurred slowly, over thousands of years. Only in the past several centuries have human activities begun to influence climate—and scientists are still working to understand what the consequences will be.