Climate of the Past Discussions www.clim-past-discuss.net 1814-9340 1814-9359 3 4 2007 10.5194/cpd-3-999-2007 http://www.clim-past-discuss.net/3/999/2007/ http://www.clim-past-discuss.net/3/999/2007/cpd-3-999-2007.html http://www.clim-past-discuss.net/3/999/2007/cpd-3-999-2007.pdf 999 1020 2007-08-24 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 on 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 for the early Holocene and for the 21st century are strongly linked, allowing to reduce this uncertainty. Using the limited information presently available for the early Holocene, simulations presenting very large changes for 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 are not consistent with recent observations. Using this evidence based on observations during both the early Holocene and the last decades, the most realistic projection indicates a nearly disappearance of the sea ice at the end of the 21st century for a moderate increase in atmospheric greenhouse gas concentrations. For a faster increase in those concentrations, the Arctic Ocean would become almost ice-free in summer as early as 2060 AD. 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