Climate since A.D. 1500: Introduction

It is now common knowledge that today's climate is unlikely to prevail into the 21st century. An increase in “greenhouse gases”, as a result of human activity, is likely to perturb the world's energy balance, leading to higher surface temperatures and a redistribution of precipitation patterns (National Research Council 1982; Bolin et al. 1986; I.P.C.C., 1990). The magnitude of any climatic change, and its distribution both geographically and seasonally, is estimated by the use of computer models of the general circulation. Simulations of equilibrium climatic conditions with early twentieth century CO₂ levels are generally compared with those resulting from doubling of CO₂ to obtain the differences that might be expected in the future (Schlesinger 1984). More recently, transient climate models have been developed to simulate the gradually changing climate as CO₂ and the other greenhouse gases increase from year to year. The focus of all these experiments is, of course, to isolate the impact of human activities on climate over the next century or so. However, whatever the anthropogenic climatic effects may be in the future, they will be superimposed on a climatic system which also responds to “natural” forcing factors. Unless we improve our understanding of what these factors are, and how the climate system has responded to them in the past, there is little prospect of interpreting, or anticipating, future climatic changes. Therefore, in order to understand how climate may vary in the future we must understand how and why it has varied in the past. With such knowledge we may be able to place our contemporary climate in a longer term perspective and identify any underlying trends or periodicities in climate upon which future climatic changes might be superimposed. With such knowledge we may be able to isolate the causes of past climatic fluctuations, causes which may continue to operate in the future and influence the course of forthcoming climatic events (Bradley 1990).