mate by modifying cloud properties (Shaviv, 2002; Svensmark, 1998, 2006) or through a variety of other mechanisms (Gray et al., 2005). Cosmic rays are influenced by solar activity, so it is difficult to study the effect of cosmic rays in isolation. However, direct observations of cosmic ray fluxes do not show any net change over the last several decades (Benestad, 2005), and a plausible physical mechanism linking changes in cosmic rays to changes in climate has not been demonstrated. Hence, cosmic rays are not regarded as an important climate forcing (Forster et al., 2007).

The influence of climate forcings on Earth’s temperature is modulated by the effects of feedbacks in the climate system. One example of a positive feedback is the ice-reflectivity feedback: If a positive climate forcing leads to a slight warming that melts ice, especially (white, highly reflective) sea ice floating on the (dark, highly absorptive) ocean surface, the surface of the Earth will reflect less sunlight back to space, and the increased absorption of solar radiation reinforces the initial warming. On the other hand, if warming were to cause an increase in the amount of low-lying clouds, which tend to cool the Earth by reflecting solar radiation back to space (especially when they occur over ocean areas), this would tend to offset some of the initial warming—a negative feedback. Other important feedbacks involve changes in evaporation, other kinds of clouds, land-surface properties, the vertical profile of temperature in the atmosphere, and the circulation of the atmosphere and oceans—all of which operate on different time scales and interact with one another and with other environmental changes in addition to responding directly to changes in temperature.

The net effect of all feedback processes determines the sensitivity of the climate system, or the response of the system to a given set of forcings (NRC, 2003b). Climate sensitivity is typically expressed as the temperature change expected if atmospheric CO₂ levels were fixed at twice their preindustrial concentration, with all other forcings neglected (or 560 ppm of CO₂, which corresponds to a climate forcing of 3.7 W/m²), and then remained there until the climate system reaches equilibrium. A variety of methods have been used to estimate climate sensitivity, including statistical analysis of climate forcing and observed temperature changes, analyses based on estimates of forcing and temperature variations from paleoclimatic records (see below), energy balance models, and climate models of varying complexity (e.g., Annan et al., 2005; Hegerl et al., 2006; Knutti et al., 2006; Murphy et al., 2004; Wigley et al., 2005). The IPCC’s latest comprehensive assessment of climate sensitivity based on these techniques indicates that the expected warming due to a doubling of CO₂ is between 3.6°F and 8.1°F (2.0°C and 4.5°C), with a best estimate of 5.4°F (3.0°C) (Hegerl et al., 2007). Unfortunately, the