out with coupled atmosphere-ocean models. There have not yet been any successful simulations of the pattern and magnitude of the Younger Dryas or of recurrent Dansgaard/Oeschger events with coupled ocean-atmosphere general circulation models. And there have not been any integrations of coupled ocean-atmosphere models over sufficiently long time scales to determine the natural level of millennial variability in such models, particularly in glacial conditions. Despite considerable success in modeling some aspects of past abrupt climate changes, there is much work still to be done.

Fluctuations in the atmospheric carbon dioxide concentration (Petit et al., 1999) have played a crucial role in climate change on the glacial-inter-glacial time scale (Pollard and Thompson, 1997). According to general circulation model results (Manabe and Broccoli, 1985), without the reduced greenhouse effect arising from low carbon dioxide in glacial times, the Southern Hemisphere would have experienced little cooling despite the massive growth of Northern Hemisphere ice sheets. However, carbon dioxide fluctuations are not radiatively significant on the time scale of the Younger Dryas or Dansgaard/Oeschger events, and up to industrial times carbon dioxide had only insignificant fluctuations throughout the Holocene. Methane is an important greenhouse gas and did fluctuate markedly in the course of many abrupt-change events, with changes in low-latitude and high-latitude sources (Brook et al., 1999). The low-latitude source changes are generally thought to be indicative of changes in tropical hydrology, so methane serves as an important indicator of the involvement of the tropics in abrupt climate change (Chappellaz et al., 1997). However, the magnitudes of methane changes were too small to yield appreciable radiative forcings, and the observation that temperature changes in Greenland preceded methane changes (Severinghaus et al., 1998) is not generally consonant with driving by a methane greenhouse effect.

Water vapor is an important greenhouse gas, but it is different from carbon dioxide and methane in that its concentration is determined primarily by atmospheric temperature and circulation patterns rather than by sources and sinks (Held and Soden, 2000; Pierrehumbert, 1999). Most of the atmosphere is highly undersaturated in water vapor and thus could hold considerably more than it does at present. The undersaturation is particularly prominent in the tropics, so reorganizations of the atmospheric water-vapor distribution stand as a possibility for amplifying and globalizing abrupt climate change. There are intriguing possibilities for interaction between water-vapor feedback and dust fluctuations, inasmuch as dust and other aerosols can serve as cloud condensation nuclei (Durkee et al., 2000).

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