closely related to the balance between incoming and outgoing energy at the top of the atmosphere.
Global mean surface temperature varies in response to events outside the climate system that affect the global energy balance (NRC 2005). The external forcings considered to be of greatest importance for climate over the last 2,000 years are changes in atmospheric concentrations of carbon dioxide and other greenhouse gases, aerosol concentrations, volcanic activity, and solar radiation. Changes in land use (clearing of forests, increasing the coverage of cultivated land, and desertification) may also contribute to climate variability, but their influence is difficult to quantify (Ruddiman 2003). Human activities have caused increases in the atmospheric concentrations of greenhouse gases and aerosols, which first became appreciable in the 19th century.
The climate system also exhibits internal variability that would occur even in the absence of external forcing. A familiar example of internal climate variability on a year-to-year scale is El Niño, which is a consequence of interactions between the tropical Pacific Ocean and the global atmosphere. Interactions among the more massive, slowly varying components of the climate system could give rise to internal variability of the climate system on timescales of decades to centuries that may be largely unrelated to the external forcings on those timescales.
The change in global mean surface air temperature that occurs in response to a persistent external forcing of 1 watt per square meter over the Earth’s surface is defined as the sensitivity of the climate system (NRC 2003a). An alternative unit, used extensively in this report, is the temperature increase (in °C) that would occur in response to a doubling of the preindustrial atmospheric carbon dioxide concentration. Climate sensitivity is determined by the laws of physics and can be estimated using the methods described in the next section. The fluctuations in global mean surface temperature that occurred in response to past natural forcings provide a check on estimates of climate sensitivity. Other things being equal, the higher the sensitivity, the larger the future warming that can be expected in response to future greenhouse forcing. The strength of the various external forcings can be quantified and compared; knowledge gained from understanding the response to one kind of forcing is applicable to predicting the response to other kinds of forcing.
As in other physical systems, high climate sensitivity is indicative of the prevalence of positive climate feedbacks (NRC 2003a). The most important positive feedback in the climate system involves the increase in the concentration of atmospheric water vapor as the Earth warms. Changes in concentrations of water vapor, a greenhouse gas in its own right, amplify the warming or cooling that occurs in response to changes in concentrations of other greenhouse gases. Another positive feedback involves the decrease in the fractional area covered by snow and ice as temperatures warm, which decreases the reflectivity of the Earth as a whole. Other feedbacks involve changes in cloudiness, lapse rate, the atmospheric circulation, and land surface properties as the Earth warms or cools. The combined effect of the various positive and negative feedbacks determines the sensitivity of the climate system and the sensitivity, in turn, determines how much the Earth will warm in response to a prescribed increase in the atmospheric concentration of carbon dioxide or changes in other external forcings.
The sensitivity of the climate system can be estimated in several different ways. The direct response to a doubling of preindustrial atmospheric carbon dioxide concen-