Environmental Engineering Reference
In-Depth Information
FIGURE 3.4 A histogram of TCR from a set of 19 models from 1% per year experiments described in Ch. 8
of WG1/AR4. The TCR values have also been converted into the warming since the mid-19th century due
to well-mixed greenhouse gases by multiplying by 2/3.
otherwise have taken place” (IPCC, 2007c). In particular, a negative forcing
of 0.8 W/m
2
due to non-WMGG forcing, or a total forcing of 1.9 W/m
2
,
would be consistent with the 20th century warming if TCR were roughly
1.5°C—while a negative forcing of 1.2 W/m
2
, or total forcing of 1.5 W/m
2
,
would be consistent with a TCR of about 1.9°C. Total aerosol forcing of
this magnitude is consistent, for example, with the estimate of Quaas et al.
(2008), but the range of estimates is large.
Best estimates of TCR typically range from 1.5-2°C. For example, Stott
et al. (2006) use fingerprinting techniques to adjust the results from GCMs
to improve fits to the spatial structure of observations and obtain a best es-
timate close to 2°C. Knutti and Tomassini (2008) use a simple model to fit
the 20th century temperature record and ocean heat uptake, using a Bayes-
ian analysis to determine optimal parameters and uncertainties. Their best
estimate for TCR is 1.5-1.6°C, with a sharp cutoff at 1 and with a fairly long
