Global Mean Temperature Responses - Climate Stabilization Targets - page 93

2010). It is also useful for the canonical 1% peryr experiment, so we can set
TCR = F 2X /( β + γ ), where F 2X equals the forcing due to doubling of CO 2 . We
can then use the same proportionality constant, 1 /( β + γ ), to interpret 20th
century warming and 21st century projections given only the evolution of
the forcing F over time. As an example, Gregory and Forster (2008) show
how the same proportionality constant between forcing and global mean
temperature holds in a particular GCM for all of the 21st century forcing
scenarios utilized by the AR4 and pictured in Figure 3.3 (IPCC, 2007c). The
lack of separation of these different scenarios in the first half of the 21st
century is not primarily due to some inertia in the physical climate system,
but rather to the fact that the net radiative forcings in the various scenarios
do not substantially diverge until the latter half of the century.
FIGURE 3.3 In stabilization scenarios, such as A1B (green) and B1 (blue) after 2100, or the “constant com-
position commitment” (yellow) in which the forcing is held fixed at the values in 2000, the warming grows
slowly despite the constant forcing. Rescaling the TCR by the forcing will underestimate the surface warm-
ing in the stabilization period by an amount that grows with time, as the system slowly makes its transition
to its equilibrium response. The average ratio of TCR to the equilibrium sensitivity in the models utilized by
the AR4 (using values in Table 8.2 in Chaper 8 of the WG1 report) is 0.55. So the slow growth in the stabiliza-
tion period, in which the forcing is somehow maintained at a constant level, continues for many centuries
beyond that indicated in this figure, until the additional warming in these periods becomes comparable
to that in the preceding periods of increasing forcing.