SCOTT J. LEHMAN1
New sediment records from the southeastern Norwegian Sea document a series of abrupt changes in poleward flow of warm surface waters during the last deglaciation (15,000-8,000 yr BP), some of which produced shifts in sea surface temperature of more than 5°C in less than 40 years. These findings confirm the timing, magnitude, and rates of circum-Atlantic climate change implied by ice-core data and predicted by numerical models, and support the theory that the intensity of thermohaline overturning controlled air temperatures around and downwind of the northern North Atlantic.
Unlike the deglacial interval, the last 8,000 years or so have been marked by relatively stable climate conditions in the North Atlantic region. However, as pointed out earlier by Broecker (1987), the conditions that conferred stability over this interval are not yet understood; thus, it cannot be assumed that such conditions will persist in the face of the gradual changes already forecast as a response to anthropogenic perturbation of the climate system.
A key question confronting scientists and policy makers is the degree to which the 0.3°C to 0.6°C rise in globally averaged surface air temperatures observed since the mid-1800s (e.g., Jones and Wigley, 1986) can be ascribed to humankind's impact on atmospheric concentrations of CO2 and other trace-gases. Although the direction and magnitude of the observed temperature change is compatible with many model predictions, the extent to which it may be embraced or overprinted by natural climate variations is not yet known. Indeed, the need to document and understand natural climate variations in this context has motivated many of the contributions to this volume. This paper, on the other hand, views the issue of natural climate variation from a different perspective. Here I present evidence from the end of the last glacial period for natural shifts in circulation of the North Atlantic Ocean that produced regional changes in air and sea temperatures of 5 to 10°C in less than 50 years, which are much too large and sudden to be confused with gradual changes of the type seen in the instrumental record. Rather, the point is that comparable ocean-driven changes might be an unforeseen response to gradual anthropogenic forcing