Environmental Engineering Reference
In-Depth Information
The ratio of TCR to the equilibrium response is smaller than one would
expect from the linear analysis described above, using the mean value for
these models of the heat uptake efficiency
γ
(Dufresne and Bony, 2008)
and the radiative restoring
β
implied by the equilibrium climate sensitivity
of these models. The explanation is that the strength of the restoring force
provided by energy fluxes escaping to space,
β
, is found to decrease in cli-
mate models as the deep oceans equilibrate (Williams et al., 2008; Winton
et al., 2010). In the CMIP3 archive the value of the radiative restoring
relevant on the time scale of the 20th and 21st centuries is larger than the
value relevant for the equilibrium response—by 30% on average, but with
considerable model-to-model variations. This weakening of the radiative
restoring once the deep oceans equilibrate is likely associated with changes
in the horizontal structure of the warming. In the period of increasing forc-
ing, the warming of the subpolar oceans is held back by strong coupling to
the deep oceans. After stabilization, the deep oceans slowly warm and polar
regions are thereby allowed to warm more rapidly, relative to the average
warming of Earth's surface. The radiative restoring is weaker for warming
of the subpolar surface than for warming elsewhere because the coupling
between the surface and tropospheric layers from which most of the radia-
tion escaping to space is emitted is relatively weak in subpolar latitudes.
The result is a reduction in the strength of the radiative restoring per degree
warming of Earth's surface as a whole.
The forcing due to well-mixed greenhouse gases (WMGGs) from the
mid-19th century till 2010 is estimated to be 2.7 W/m
2
by NOAA's annual
greenhouse gas index (
http://www.esrl.noaa.gov/gmd/aggi/
), which is about
two-thirds of the forcing due to doubling of CO
2
of about 3.7 W/m
2
. There-
fore, one can estimate the warming due to the WMGGs since the mid-19th
century, for a given value of TCR, as roughly two-thirds of TCR.
The distribution of TCR values generated by the CMIP3 models, as tabu-
lated in Chapter 8 of the WG1/AR4 report (Randall et al., 2007), is displayed
as a histogram in Figure 3.4. The mean, or median of this distribution, and
the spread of values about this mean, are of interest in describing the values
emerging from the best efforts of the major modeling groups around the
world. The mean is between 1.8°C and 1.9°C and is somewhat larger than
the median, which is close to 1.6°C.
To explain the 20th century warming (0.7-0.8°C) with WMGGs would
require Earth to reside at the extreme low end of this distribution. As stated
in the SPM of WG1/AR4, “it is likely that increases in greenhouse gases con-
centrations alone would have caused more warming than observed because
volcanic and anthropogenic aerosols have offset some warming that would
