a1 Senior Scientist, Center for Climatic Research, Institute for Environmental Studies, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA.
Research over the past century has shown that the rates and magnitudes of climatic change constitute a continuum. Changes have now been identified in the climatic record that range in duration from interannual through decades and centuries to the multi-millennial time-scale. Examples range from the drought years of the 1930 and 1970 decades to the ponderous comings and goings of the ice-ages. More recently it has become clear that some changes can be quite rapid. In recent decades great progress has been made in identifying the causes of climatic variation.
The present understanding of the causes of climatic change emphasizes continental drift (or ‘plate tectonics’) at the million-years' scale, with pulses of plate movement producing significant bursts of volcanic activity that may act on the millennial or century scale. At the multi-millennial scale there is growing agreement that the variations in irradiance of the Earth, resulting from slow changes in the Sun-Earth geometry (the so-called Milankovitch variations), exercise the operative control on the timing of ice-ages and interglacials. At the decadal and interannual scales there is less agreement; but there is at least a body of research which suggests that significant volcanic activity is a contributing factor. There is considerable agreement—but little direct evidence—that anthropogenic causes such as increased carbon dioxide and other Man-made or-enhanced trace gases in the atmosphere, will be important in the coming decades.
Cultural responses might be expected to differ across this continuum. To assess the expected response to a climatic variation, one must know at least the shape of the response surface.
There is probably a critical threshold combination of climatic change magnitude and duration. Human cultures seem to be adapted to frequently-occurring short ‘aberrations’ from the expected climate. Some evidence indicates, on the other hand, that relatively small changes of climates (of the order of a century in duration) have been associated over the past 8,000 years with cultural changes that proved large enough to lead to different names being assigned in perhaps half of the cultural termini identified. A climate model which includes the effect of volcanic aerosols, suggests that most of the climatic changes associated with these globally synchronous cultural termini are related to peaks of volcanic activity. Some apparently catastrophic events have been recognized in this connection.
There remains the problem of assessing, in realistic terms, the impact of large-magnitude climatic variations on modern human societies. Of particular concern is the effect of climatic events associated with very large-scale short-term insertions of aerosols into the atmosphere. It is likely that non-equilibrium models of the atmosphere, with specified sea-surface temperatures, would give realistic results if refined to the degree that they could replicate events of lesser magnitude which have occurred in the past century. At present there appear to be no models in which the formulation of the radiative effect of aerosols or gases gives a good match with observed radiative effects. It seems that much more research, including field experiments, will be needed if science is to supply reliable advice to society on the nature of coming climatic changes.
* Based on the Keynote Address delivered to the Conference on Civilization and Rapid Climatic Change, held in Calgary, Alberta, Canada, during 21–24 August 1987.