ABSTRACT
The geological record provides evidence that has been used to reconstruct changes in the Earth's climate during the past 3 billion years or so, albeit with considerably greater uncertainty for earlier geological times than for more recent geological times. Geological evidence can also be used to reconstruct past variations in the concentration of C02 (carbon dioxide) and CH4 (methane), which are two important heat-trapping gases. The ice preserved in the Antarctic and Greenland ice caps contains air bubbles that were sealed off within a few hundred years of the accumulation of the snow, thereby providing samples of the atmosphere for the last 250,000 years. From these air bubbles we can measure changes in the concentrations of CO2 and CH4. Other, less certain evidence has been used to reconstruct the variation in the atmospheric CO2 concentration as far back as 570 million years (e.g. Berner, 1994). Past variations in the Earth's climate and in the composition of the atmosphere provide a long-term perspective against which the human-induced changes in the atmosphere, and projected changes in the atmosphere and in climate, can be compared. In this chapter this long-term perspective is presented.
