Climate prediction is a response to the question, "What will the climate be like in the decades and centuries to come?"
This image shows measurements in the infrared channel. Clouds appear in white, while surfaces appear in green and blue. Composite made up from observations by the meteorological geostationary satellite fleet GOES-09, -10 and -12, Meteosat 5 and 7. © EUMETSAT 2005
It could be tempting to establish a parallel with weather forecasting, but the problems involved are radically different. The job of weather forecasts is to say what the weather will be like in a few days' time: the main difficulty lies in determining the state of the atmosphere with suffi cient precision. The job of climate prediction is to say what the climate will be like a long time from now.
This depends on two factors, one physical, the other economic. For a given greenhouse gas concentration in the atmosphere, a climate model can be used to compute the corresponding climate. This is the physical factor. However, greenhouse gas concentrations are linked to human activity. To know what the climate will be like in 100 years, you also need to know what economic activity there will be at that time: industrial and agricultural production, transport, energy sources and so on. This requires having population and geopolitical forecasts.
On the basis of all these forecasts, economists suggest scenarios, each one of which is associated with a particular change in greenhouse gas emissions. On the basis of an economic scenario and a climate model it is thus possible to calculate what the associated climate will be like. This is called climate prediction. In this complex process, satellites play a key role.
By comparing their precise, adapted observations with climate models, we can reduce the uncertainty attached to the physical factor of climate prediction. In addition, this tool will be increasingly used by economic and political players for decisionmaking.
J.-Y. G.
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