Introduction

Paleoclimatology is the study of climate prior to the period of instrumental measurements. Instrumental records span only a tiny fraction (<10 7) of the Earth's climatic history and so provide a totally inadequate perspective on climatic variation and the evolution of climate today. A longer perspective on climatic variability can be obtained by the study of natural phenomena which are climate-dependent, and which incorporate into their structure a measure of this dependency. Such phenomena provide a proxy record of climate and it is the study of proxy data that is the foundation of paleoclimatology. As a more detailed and reliable record of past climatic fluctuations is built up, the possibility of identifying causes and mechanisms of climatic variation is increased. Thus, paleoclimatic data provide the basis for testing hypotheses about the causes of climatic change. Only when the causes of past climatic fluctuations are understood will it be possible to fully anticipate or forecast climatic variations in the future (Bradley and Eddy, 1991).

Studies of past climates must begin with an understanding of the types of proxy data available and the methods used in their analysis. One must be aware of the difficulties associated with each method used and of the assumptions each entails. With such a background, it may then be possible to synthesize different lines of evidence into a comprehensive picture of former climatic fluctuations, and to test hypotheses about the causes of climatic change. This book deals with the different types of proxy data and how these have been used in paleoclimatic reconstructions. The organization is methodological, but through discussion of examples, selected from major contributions in each field, an overview of the climatic record during the late Quaternary (the last ~1 Ma) is also provided. The climate of earlier periods can be studied using some of the methods discussed here (particularly those in Chapters 6, 7, and 9) but the farther back in time one goes, the greater are the problems of dating, preservation, disturbance, and hence interpretation. For a thorough discussion of climate over a much longer period, the reader is referred to Frakes etal. (1992).

Although our perspective on the past is obviously somewhat myopic, the Quaternary was a period of major environmental changes that were possibly greater than at any other time in the last 60 million years (Fig. 1.1). Nevertheless, there is no doubt that an understanding of climatic variation and change during the Quaternary period is necessary not only to appreciate many features of the natural environment today, but also to comprehend fully our present climate. Different components of the climate system change and respond to external factors at different rates (see Section 2.2); in order to understand the role such components play in the evolution of climate it is necessary to have a record considerably longer than the time it takes for them to undergo significant changes. For example, the growth and decay of continental ice sheets may take tens of thousands of years; in order to understand the factors leading up to such events and the effects such events subsequently have on climate, it is necessary to have a record considerably longer than the cryospheric (snow and ice) changes which have taken place. Furthermore, as major periods of global ice build-up and decay appear to have occurred on a quasi-periodic basis during at least the late Quaternary, a much longer record than the mean duration of this period (~105 yr) is necessary to determine the causative factors, and to appreciate how those factors play a role in climate today. A detailed paleoclimatic record, spanning at least the late Quaternary period, is therefore fundamental to comprehension of modern climate, and the causes of climatic variation and change (Kutzbach, 1976). Furthermore, unless the natural variability of climate is understood, it will be extremely difficult to identify with confidence any anthropogenic effects on climate.

Computer models can be used to estimate the spatial and temporal pattern of climate change as greenhouse gas concentrations increase in the atmosphere. This provides a "target" of expected change against which contemporary observations can be compared. If the climate system evolves towards such a target, one could then argue that anthropogenic effects have been detected on a global scale (Santer et al., 1996). But natural variability, unless fully represented in model simulations, may confound such detection efforts. Whatever anthropogenic effects there are on climate, they will be superimposed on the underlying background of "natural" climate variability, which may be varying on all timescales in response to different forcing factors. Paleoclimatic research provides the essential understanding of climate system variability, and its relationship to both forcing mechanisms and feedbacks, which may amplify or reduce the direct consequences of particular forcings.

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