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 diversity and complexity of processes operating in the climate system mean that, even with continued progress in understanding climate feedbacks and monitoring global climate forcing and temperature changes, the exact sensitivity of the climate system may remain uncertain (Roe and Baker, 2007).
The concept of climate sensitivity technically only applies to equilibrium climate states, that is, the total warming after the oceans, cryosphere, and biosphere have had ample time to fully adjust to the imposed forcing. In reality, the strength of climate forcings and feedbacks are continuously varying, and it takes the climate system—especially the oceans—a long time to warm up in response to a positive climate forcing. In addition, many estimates of climate sensitivity do not include climate feedbacks associated with processes that operate on decadal to centennial time scales, such as the disappearance of glaciers, changes in vegetation distribution, or changes to the carbon cycle on land and in the oceans; several recent studies that consider some of these processes have suggested that Earth's climate sensitivity may be substantially higher than the aforementioned "best estimate" (Hansen et al., 2008; Sokolov et al., 2009). Nevertheless, estimates of climate sensitivity are a useful metric for evaluating the causes of observed climate change and estimating how much the Earth will ultimately warm in response to past, present, and future human activities. Climate feedbacks and climate sensitivity also remain an important area for future research (see Research Needs at the end of this chapter).
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