Why do we need global climate models and scenarios?
Even though the climate system is extremely complex, current scientific understanding has led to several global climate models that do a reasonable job of reproducing past climate averaged over very large areas. So long as the basic physics in the climate system remain the same, this means that climate change can be partly predictable, particularly over large spatial scales such as continents and the Globe.
Human activities change the Earth’s surface cover and the amounts of greenhouse gases in the atmosphere—two key inputs into the climate system. But scientists judge that the global climate models also can be helpful in identifying potential climate change due to human activity on a large scale. However, the ability to do so relies on our ability to predict population change, economic change, technological development, and other relevant characteristics of future human activity. In practice, scientists recognize the uncertainties in predicting these human factors (as well as the potential errors in current scientific understanding of the complex climate system). Thus they do not produce predictions of future climate change. Instead, climate projections are determined for future scenarios that are based on plausible ranges of how many people there might be, and where and how they might live. Rather than predictions of what will happen, these are “what-if” projections: what would be expected to happen if the specified conditions actually occur.
Small changes in climate could, but will not necessarily, have a large impact on weather events, and on the intensity of extremes. CARA aims to help communities assess how vulnerable they might be to the impacts of extreme weather events, especially if they would occur with greater frequency and/or intensity.