TABLE 3.2 Key Physics and Processes Contributing to Climate Sensitivity (warming expected if carbon dioxide doubles from an unperturbed value of 278 ppmv to 556 ppmv)

Black-body radiation alone, ignoring the greenhouse effect of the unperturbed atmosphere

0.7ºC

Black-body radiation but also including the greenhouse effect in the unperturbed atmosphere

0.9ºC

As above, but also including well-documented feedbacks due to tropospheric water vapor changing at fixed relative humidity and changes in the lapse rate (vertical structure of the atmosphere), no clouds

1.5ºC

As above, also including clouds but keeping them fixed

1.8ºC

As above, including clouds and allowing the clouds to vary, along with other feedbacks such as snow and sea ice retreat, from IPCC AR4 suite of models

Best estimate 3.2ºC

Likely range 2.1-4.4ºC

of the unperturbed (background) atmosphere gives Δ T2X of about 0.9ºC in the absence of clouds.1 But abundant evidence and basic physics shows that atmospheric water vapor must increase in a globally warmer world, and multiple lines of evidence confirm that both the atmosphere and general circulation models conform to a feedback that acts approximately as if the relative humidity is kept fixed (Held and Soden, 2000; Pierrehumbert et al., 2007; Dessler and Sherwood, 2009). When this result is used to incorporate the water vapor feedback into calculations of Earth’s infrared emission to space, and the lapse rate feedback is also taken into account, we find that Δ T2X increases to 1.5ºC. This figure is helpful for understanding the physics contributing to climate sensitivity, but it is incomplete because it is only for clear sky conditions. In the real atmosphere, clouds contribute to Earth’s background greenhouse effect, and their possible changes represent a key feedback.

The feedbacks that modify the basic black-body feedback are at the heart of predicting future climate. The combined water vapor and lapse rate feedback increases climate sensitivity by affecting the infrared emission side of the balance. Snow and sea-ice retreat work instead on the solar absorption side, but they also increase the sensitivity. Clouds work on both the infrared and solar side, and their net influence on sensitivity can go either way.

A quantitative treatment of cloud, snow, relative humidity, and sea-ice feedbacks requires the use of general circulation models. The estimates of equilibrium climate sensitivity in this report will be based on simulations employing mixed layer ocean models, which are thought to closely mimic

1

These computations were carried out using idealized single-column models of the type described in Chapter 4.5.3 of Pierrehumbert (2010). See Methods section for details.



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