10.1098/rsta.2003.1244 Sea-ice switches and abrupt climate change By Hezi Gildor 1 and Eli Tziperman 2 1 Lamont{Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA 2 Department of Environmental Sciences, Weizmann Institute of Science, Rehovot 76100, Israel Published online 22 July 2003 We propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea-ice cover. We explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism. We emphasize that because of the signi¯cant in°uence of sea ice on the climate system, it seems that high priority should be given to developing ways for reconstructing high-resolution (in space and time) sea-ice extent for past climate-change events. If proxy data can con¯rm that sea ice was indeed the major player in past abrupt climate-change events, it seems less likely that such dramatic abrupt changes will occur due to global warming, when extensive sea-ice cover will not be present. Keywords: climate; sea ice; thermohaline circulation; abrupt climate change; glacial cycles 1. Introduction Rapid and dramatic climate changes that occurred within a few decades abound in climate proxies (Taylor et al . 1993; Alley et al . 1993; Lehman & Keigwin 1992). A number of mechanisms for these abrupt climate changes have been sug- gested in the past (Adams et al . 1999). In particular, a threshold-like behaviour of the thermohaline circulation (THC) and THC instability and variability have been suggested as possible mechanisms for rapid climate instability (Weaver et al . 1991; Tziperman et al . 1994; Tziperman 1997; Marotzke 2000; Ganopolski & Rahmstorf 2001). It was demonstrated that variations in the THC can result from small, possibly stochastic, perturbations in freshwater °ux to the North Atlantic (Gri±es & Tziperman 1995) due to the excitation of a damped oscil- latory THC mode (see also Ganopolski & Rahmstorf 2001, 2002; Timmermann et al . 2002). However, we wish to make the case here that the THC by itself is not likely to be a satisfactory mechanism for observed past rapid climate changes. Rather, it is the dramatic rapid melts and expansions of sea-ice cover|through its strong cooling ice{albedo feedback, its insulating e®ect on local evaporation, and its role in diverting the storm track|that are a more likely explanation (Gildor & Tziperman 2000, 2001b; Adams et al . 1999; Broecker 2000; Dansgaard et al . 1989). One contribution of 14 to a Discussion Meeting `Abrupt climate change: evidence, mechanisms and implications’. Phil. Trans. R. Soc. Lond. A (2003) 361, 1935{1944 1935 c ° 2003 The Royal Society