Forced Climate Variability During the Last Millennium with the Earth System Model CLIMBER-2

Abstract

During the last millennium, variations in solar luminosity, volcanic activity, anthropogenic land use change and greenhouse emissions are the major forcings of the Earth climate. Recent reconstructions of these forcings are employed in CLIMBER-2, a coupled atmosphere—ocean—biosphere model of intermediate complexity, to estimate their effect on the climate system. Solar-forced CLIMBER-2 simulations produce changes in the annual-mean surface temperature and in the North Atlantic meridional overturning in reasonable agreement with simulations by a comprehensive atmosphere—ocean general circulation model. The solar-forced simulation reproduces the centennial climate variability reconstructed from paleoclimatic data, for the relatively warm and wet ‘Medieval Warm Period’ compared to the cold and dry ‘Little Ice Age’. However, in the southern European region the simulated precipitation increased during the Little Ice Age in consistence with reports on advances of glaciers and on flooding events. The rather low temperatures in the second half of the 19th century are attributed mainly to deforestation and little to solar activity and volcanic activity. The subsequent warming in the 20th century can be simulated reasonably only when the effect from the growing emission of greenhouse gases is included.