TIMING CONSTRAINTS ON THE MECHANISMS OF DEGLACIATION: A POSSIBLE TROPICAL ROLE

The mechanism which amplifies insolation forcing to generate the rapid collapse of ice sheets at the end of glacial periods is not well understood. The phasing of events during such deglaciations, and the absolute timing of the penultimate deglaciation, both argue against a direct northern-hemisphere mechanism. These data will be discussed and compared to similar timing constraints at other deglaciations (Termination 1 and 3), including new U/Th age constraints. These timing constraints suggest that deglaciation is driven by processes in the tropics or the southern hemisphere, rather than the northern hemisphere. Although southern-hemisphere mechanisms involving CO2 are plausible, this talk will focus on the possible role of the tropics. Model results with the ECHAM3 atmospheric GCM suggest that a warming of the tropical ocean during glacial times generates a substantial warming over the northern-American continent in the region where glacial ice sheets form. This high-latitude response to tropical change is due to alteration in the vertical profiles of temperature and moisture in the extratropical atmosphere. This effect is exaggerated by the topography of the northern-American ice sheet during the glacial. An opposite, cool, perturbation to interglacial tropical temperatures does not cause significant cooling over North America, largely because of the absence of ice-sheet topography in the interglacials. This asymmetry represents a new mechanism to explain the saw-tooth nature of glacial-interglacial climate change. This atmospheric moisture mechanism is distinct from previously suggested tropical linkages involving atmospheric circulation change. These two tropical mechanisms for warming of the northern-American region are consistent with the timing constraints and phasing of events at the deglaciations.