To make the models as reliable as possible, data assimilation is used, a method for transferring observations of various variables into mathematical models.
We create reanalyses – reconstructions – of past climatic conditions. Ongoing reanalyses will be able to provide information about the climate 1,000 years back in time. In addition to data from satellites and recent direct measurements, data from core samples, tree rings, pollen, and plankton deposits are used. Such retrospectives in time help us to understand natural climate variations and to find other influencing factors. Reanalyses can also be used to improve climate models, which in turn will improve the quality of climate projections.