Environmental Engineering Reference
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tail on the high sensitivity end. The cutoff at 1 is common to most estimates,
related to the fact that lower values of TCR requires positive forcing from
unknown forcing agents or very large contributions to the century long trend
from internal variability unlike any produced by current GCMs.
The temperature record over the past 30 years provides a potentially
useful constraint on TCR (Gregory and Forster, 2008; Murphy et al., 2009).
The anthropogenic aerosol forcing, although it has probably grown over
much of the industrial era, was likely slowing if not fully leveling off over
this period, leaving the well-mixed greenhouse gas forcing more dominant.
Additionally, solar forcing, which is well constrained by satellite measure-
ment of total solar irradiance, contributed very little to the trend over this
period. Removing estimates of volcanic and ENSO signals from the global
mean temperature record results in a fairly linear residual (e.g., Lean and
Rind, 2009; Thompson et al., 2009). The residual yields a warming of about
0.48 K, or 0.16 K/decade, over the 30-year period. The WMGG forcing over
this period is close to 1 W/m
2
. Rescaling implies a TCR of 1.8°C, similar to
the estimate of Gregory and Forster. Murphy et al. (2009), estimating TCR
directly from TOA fluxes and ocean heat uptake over a similar time period,
paint a picture consistent with relatively flat aerosol forcing in recent de-
cades and a TCR of around 1.8.
The difficulty in using this relatively short period for estimating TCR
is that internal climate variations are capable of modifying this trend sub-
stantially, so we cannot assume that the observed trend is entirely forced.
Gregory and Forster (2008) estimate uncertainty by using the variability in
30-year trends of global mean temperature from a particular GCM. Their
result is 1.3-2.3°C as the 90% confidence interval around their best estimate
of 1.8°C. This is about the same range as in the CMIP3 models, but with
a very different source of uncertainty. The Gregory and Forster estimate of
uncertainty assumes that we have no information as to whether the contri-
bution from internal variability was positive or negative during this period,
although the CMIP3 range of values is due to uncertain physics in the mod-
els, especially cloud feedbacks.
There is a body of work on multi-decadal variability, especially in the
North Atlantic, which suggests that internal variability has contributed posi-
tively to the temperature trends over the past 30 years. If correct, this would
lower estimates of TCR that are based on comparison to temperature trends
in recent decades. The North Atlantic is likely to be the source of much of
the multi-decadal variability, and a variety of oceanographic and coupled
model studies (Zhang, 2008; Knight, 2009; Latif et al., 2009; Polyakov et al.,
2009) indicate that the North Atlantic has been in a warm phase over much
