Climate of the Past www.clim-past.net 1814-9324 1814-9332 3 4 2007 10.5194/cp-3-683-2007 http://www.clim-past.net/3/683/2007/ http://www.clim-past.net/3/683/2007/cp-3-683-2007.html http://www.clim-past.net/3/683/2007/cp-3-683-2007.pdf 683 692 2007-12-20 Information on the early Holocene climate constrains the summer sea ice projections for the 21st century H. Goosse hgs@astr.ucl.ac.be E. Driesschaert T. Fichefet M.-F. Loutre Université Catholique de Louvain, Institut d'Astronomie et de Géophysique G. Lemaître, Chemin du Cyclotron, 2, 1348 Louvain-la-Neuve, Belgium The summer sea ice extent strongly decreased in the Arctic over the last decades. This decline is very likely to continue in the future but uncertainty of projections is very large. An ensemble of experiments with the climate model LOVECLIM using 5 different parameter sets has been performed to show that summer sea ice changes during the early Holocene (8 kyr BP) and the 21st century are strongly linked, allowing for the reduction of this uncertainty. Using the limited number of records presently available for the early Holocene, simulations presenting very large changes over the 21st century could reasonably be rejected. On the other hand, simulations displaying low to moderate changes during the second half of the 20th century (and also over the 21st century) are not consistent with recent observations. Using this very complementary information based on observations during both the early Holocene and the last decades, the most realistic projection with LOVECLIM indicates a nearly disappearance of the sea ice in summer at the end of the 21st century for a moderate increase in atmospheric greenhouse gas concentrations. 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