struction of hydrologic variables with annual resolution. In this paper, we first review highlights of climate history as gleaned from the paleo record. We then provide an example of application of tree-ring data to the study of hydrologic variability in the western United States.

The greatest changes have occurred over the longest time scales: millions to hundreds of millions of years. For much of its four-billion-year history, the earth has been relatively warm and free of ice sheets such as those now found in Greenland and Antarctica. Yet, there have been major changes that happened relatively rapidly. For example, there is evidence of major cooling about 37 million years ago, when a long ice-free period ended, and 2.4 million years ago. This second change marked the start of the generally cooler epoch (the Quaternary) that has continued to the present. The best documented processes leading to such major long-term shifts have to do with changes in the distribution of continents and oceans, the rise and decline of major mountain ranges, and variations in sea level. Changes in the relative fluxes between the biological and geological components of the carbon cycle have also played a part by modifying the carbon dioxide concentration of the atmosphere. Such changes in the processes regulating global climate may be thought of as changes in the boundary conditions of the climate system. Large changes in these boundary conditions have taken place on time scales of hundreds of thousands, millions, and tens of millions of years. It is possible that some have occurred more rapidly, but many of the dating techniques used do not possess the resolution necessary to reveal rapid change in the distant past.

In the relatively cool world of the most recent 2.4 million years, the distribution of continents and relief has been much as at present. Since about 875,000 years ago, the climate has undergone repeated major excursions between long ice ages (about 110,000 years each) and much shorter periods of warmer climate (5,000 to 15,000 years). Such a warmer period commenced about 15,000 years ago.

The main evidence for these periodic climate fluctuations