those in the Great Plains.67 Similar evidence supports extreme droughts in Patagonia during these common periods, suggesting a common global cause for these droughts. Even longer tree ring records from California, spanning several millennia, indicate periods of below-average rainfall lasting on the order of a millennium.68 This record also indicates a recurrence of short-term droughts of greater magnitude and duration than any in the instrumental record of the past 100 years. Were such droughts to recur during present times in California, drastic consequences would ensue for a vulnerable and populous region.
While the atmospheric boundary conditions are essential for extending the timescales of climate variability, changes in the atmospheric composition and radiation budget, owing to internal, coupled, or external mechanisms, are directly responsible for determining the atmospheric temperature distributions in space and time. Consequently, variations in composition and radiation budget, and the direct and indirect consequences of these variations, are of critical importance.
Atmospheric composition can change because of many factors. Among those of considerable current interest are changes due to the influence of humans, most particularly through the anthropogenic increase in atmospheric carbon dioxide, chlorofluorocarbons (CFCs) and other halocarbons, methane, nitrogen oxides (NOx), and aerosols. Some of these gases, such as CFCs and halocarbons, have no known natural sources, so their influences are the direct result of anthropogenic change.
Each of these compounds plays a direct role in the atmospheric equilibrium temperature, and this direct influence is fairly predictable given any particular emissions scenario. However, more importantly, the indirect influence of increased gas and aerosol concentrations is not well known. For example, as discussed above, an increased direct warming is thought to drive an increase in the rate of the hydrological cycle, an influence that will have an uncertain net effect on the amount and vertical distribution of water vapor in the atmosphere. Since water vapor is the primary greenhouse gas, being able to predict its response to a direct forcing is essential.
In addition to the direct anthropogenic forcing, there are possible feedbacks in the atmosphere involving the natural system that must be understood. Most notable is the response of continental biomass, which can alter the rate and storage times of carbon in vegetation and thus either exacerbate CO2, methane, and NOx emissions or provide them an enhanced sink. Similarly, direct changes