Sample Forecasts of Future Temperature Change

In an effort to understand the complexities of the earth’s climate, scientists run computer simulations. In these simulations, equations represent the physical processes of the climate—such as seasonal changes in sunlight, evaporation and condensation of water, warming of the atmosphere by ocean waters, and so on.

Every simulation begins with assumptions. In generating the forecasts shown on this graph, researchers assumed that there would be no action to restrict future greenhouse gas emissions. But each line represents a different set of assumptions regarding future economic development and fundamental climate processes. These different assumptions generate very different future possibilities.

The researchers who created this graph explain that two-thirds of the overall difference between the extreme forecasts is due to uncertainty related to basic climate processes, such as the influence of clouds and ocean circulation. The rest of the difference reflects uncertainty in future emissions of carbon dioxide, which depend on population growth, economic growth, and technological changes.