Environmental Engineering Reference
In-Depth Information
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
0.7°C
Black-body radiation but also including the greenhouse effect in the unperturbed
atmosphere
0.9°C
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
1.5°C
As above, also including clouds but keeping them fixed
1.8°C
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 3.2°C
Likely range 2.1-4.4°C
of the unperturbed (background) atmosphere gives
Δ
T
2X
of about 0.9°C 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 incorpo-
rate 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
T
2X
increases to 1.5°C. 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.
