Environmental Engineering Reference
In-Depth Information
in the coming centuries? The answer to these questions in large measure
rests on the net amount of carbon dioxide released during the fossil fuel
era, however long that may last. The discussion of Earth System Sensitivity
and other considerations pertaining to very long-term climate change will
be deferred to Chapter 6.
3.2 EQUILIBRIUM CLIMATE SENSITIVITY
Climate sensitivity is calculated by determining how much Earth's sur-
face and atmosphere need to warm in order to radiate away enough energy
to space to make up for the reduction in energy loss out of the top of the
atmosphere caused by the increase of CO
2
or other anthropogenic green-
house gases. In equilibrium, there is no net transfer of energy into or out
of the oceans, so the equilibrium sensitivity can be treated in terms of the
top-of-atmosphere energy balance.
The top-of-atmosphere balance is the rate at which energy escapes to
space in the form of infrared minus the rate at which energy is absorbed
in the form of sunlight, both expressed per square meter of Earth's sur-
face. Figure 3.1 shows schematically how the balance depends on surface
temperature, for a case which is initially in equilibrium at temperature T
0
(where the blue solid or dashed line crosses the horizontal axis). If CO
2
is increased, leading to a reduction in outgoing radiation by an amount
Δ
F, Earth must warm up so as to restore balance. The amount of warming
required is determined by the slope of the line describing the increase in
energy imbalance with temperature. A lower slope results in a higher climate
sensitivity, as illustrated by the dashed slanted lines in Figure 3.1. Climate
sensitivity is often described in terms of the warming
Δ
T
2X
that would result
from a standardized radiative forcing
Δ
F
2X
corresponding to a doubling of
CO
2
from its pre-industrial value. In the following we use the value
Δ
F
2X
=
3.7 W/m
2
diagnosed from general circulation models to express the slope
in terms of
Δ
T
2X
(IPCC, 2007a).
The most basic feedback affecting planetary temperature is the black-
body radiation feedback, which is the tendency of a planet to lose heat
to space by infrared radiation at a greater rate as the surface and atmo-
sphere are made warmer while holding the composition and structure of
the atmosphere fixed (see Table 3.2). This feedback was first identified by
Fourier (1827; see Pierrehumbert, 2004). For a planet with a mean surface
temperature of 14°C (about the same as Earth's averaged over 1951-1980)
the black-body feedback alone would yield
Δ
T
2X
= 0.7°C if the planet's
atmosphere had no greenhouse effect of any kind. Such a planet would have
