Rapid Climatic Warming in the past: reconstruction and modelling

Two independent methods are used: simulations with the ECHAM4 atmospheric general circulation model and reconstructions based on geological and palaeoecological data. In both climatic transitions, January temperatures increased by as much as 20 degrees Celcius from -25 degrees a -15 degrees Celcius to -5 degrees a +5 degrees Celcius. During July the changes were smaller: temperatures increased on the average by 3-5 degrees from 10 degrees -15 degrees Celcius to 13-17 degrees Celcius during both warming phases. In both transitions an increase in precipitation was balanced by a comparable increase in evapotranspiration so that the effective precipitation remained rather stable or increased slightly.

The strong increase in January temperatures was forced by changes in the N Atlantic Ocean, as the variations in sea surface temperatures and the position of the sea ice margin determined the temperature change over land. The increase in July temperatures was mainly driven by the small increase in insolation and the deglaciation of Scotland and Scandinavia. The latter effects were much less than the changes in the N Atlantic Ocean. In addition, the magnitude of the two climate shifts was geographically changing as a result of different distances to the Atlantic Ocean and the land ice. Moreover, the timing of the major warming phases is spatially different, as this timing is mainly determined by the relative distance to the sea ice and land ice margins.