Abrupt climate change: causal constraints provided by the paleoclimate record

Important aspects of the physics involved in the climate changes of late Quaternary time elude us. The paleoclimatic record documents in detail that these changes were vast and in many cases happened abruptly. Every element of the global climate system was involved. While a case can be made that these reorganizations of the ocean's thermohaline circulation acted as the trigger for these jumps, no one has as yet been able to articulate exactly how it is that these reorganizations are capable of inducing such large changes in the way in which the atmosphere operates. In this review, I point out what to me are the key constraints placed by the records kept in ice and in sediments. The timing of these changes suggests that they have been paced by seasonality changes related to periodicities in the Earth's orbital elements and by a curious 1500-year cycle imprinted on sediments in the northern Atlantic. But as these pacemakers are inherently weak, in order for them to generate what we see in the record, there must exist powerful and likely nonlinear feedbacks in the system. It is my hope that by bringing together this information, I will provide an impetus for creative thinking with regard to physical scenarios capable of illuminating these feedbacks. Not only is this the crucial step in understanding our past, but it is also a necessary step if we are to properly evaluate the possible consequences of the ongoing greenhouse gas buildup. (C) 2000 Elsevier Science B.V. All rights reserved.