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Seminar Abstract

David Battisti - February 5, 2002


Mechanisms Responsible for Abrupt Climate Change: Past and Future

On millennial time scales, the paleodata suggests marked variability in the climate of Europe and Northern Africa that appears to be concomitant with changes in the strength of the thermohaline circulation in the N. Atlantic. Indeed, it has been suggested that the Little Ice Age and the Medieval warm period were the most recent manifestations of the extremes of this millennial-scale thermohaline-climate oscillation, and that this "mode" of climate variability extends throughout the majority of Northern Hemisphere and, perhaps, the globe. During glacial epochs, the global extent of these >climate excursions is unambiguous and the climate changes are large in amplitude and often abrupt in time.

The leading hypothesis for the millennial-scale thermohaline-climate "mode" invokes a switch in the the overturning circulation in the (N. Atlantic) ocean, say from "on" to nearly-off, which then causes an abrupt, large cooling over the whole northern hemisphere; the millennial time scales come from the adjustment time of the global ocean.

In this talk I will present observations and calculations that suggests the overturning circulation in the N. Atlantic today has only a modest effect on the wintertime climate of Europe, and an even smaller effect elsewhere. Our results, when applied to the conventional view of the millennial scale climate variability, suggests that the the ocean thermohaline circulation changes may be a response to the abrupt climate change, rather than a driver. In support of this view, calculations will be presented that illustrate the potency for small changes in the tropics as the driver of large, abrupt global climate changes during glacial times.

Time permitting, I will discuss a new hypothesis for abrupt climate change during glacial times (featuring the tropics as the driver), and I will discuss why I think it is unlikely that increasing greenhouse gases will lead to an abrupt climate change.

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