SOROOSH SOROOSHIAN AND DOUGLAS G. MARTINSON
Establishing a baseline of natural climate variability over decade-to-century time scales requires a perspective that can be obtained only from a better knowledge of past variability, particularly that which precedes the pre-industrial era. Information revealing these past climate conditions is contained in historical records and "proxy" indicators. The historical records of climate (other than systematic weather observations, which began in the late 1800s), while invaluable because of their scope and often uniquely relevant perspective, are usually limited to the last several hundred years (see Chapter 2). The proxy indicators represent any piece of evidence that can be used to infer climate. Typically, proxy evidence includes the characteristics and constituent compositions of annual layers in polar ice caps, trees, and corals; material stored in ocean and lake sediments (including biological, chemical, and mineral constituents); records of lake levels; and certain historical documents.
Such proxy indicators can provide a wealth of information on past atmospheric compositions, tropospheric aerosol loads, volcanic eruptions, air and sea temperatures, precipitation and drought patterns, ocean chemistry and productivity, sea-level changes, former ice-sheet extent and thickness, and variations in solar activity—among other things. These records are particularly appropriate for detecting three manifestations of climate variability:
Periodic or near-periodic variations (the latter are those that become evident only after examination of considerable data through which a clear statistical signal stubbornly emerges);
Large and pronounced climate signals, such as severe and sustained droughts, drastically altered precipitation patterns, anomalously warm or cold periods, or floods; and
Gradual trends, infrequent shifts, or other characteristics of natural variability that are difficult to recognize without the benefit of a long, continuous (or near-continuous) record.
Because the bulk of these proxy indicators are recorded naturally, their time span is potentially unlimited; their resolution and accuracy are limited only by the fidelity of the recorder itself. These "natural archives" are simply there for the taking—awaiting discovery, recovery, means of extraction, and interpretation. Consequently, proxy indicators represent a potential treasure trove of unique past climate information. The use of these data can present difficult problems of interpretation, particularly in light of the scanty spatial and temporal sampling, but can enhance our ability to reconstruct global changes. This chapter provides a state-of-the-art look at several aspects of this relatively new topic with respect to the climate of the last few millennia.
The use of proxy indicators in the study of modern climate draws on a broad range of disciplines and techniques. The potential of tree rings was recognized in the