In a recent posting I said I would be commenting on a paper
by Zhou and Tung (Zhou, J., and K. Tung, 2012: Deducing Multi-decadal
Anthropogenic Global Warming Trends Using Multiple Regression Analysis. J.
Atmos. Sci.doi:10.1175/JAS-D-12-0208.1, in press.)

When I came across this paper I had mixed feelings. The
paper says very similar things to those have I have been saying since January 2012:
that the underlying rate of temperature increase is less than IPCC models
assume, due to the influence of the Atlantic Multidecadal Oscillation (AMO). I
was pleased to get further corroboration in a peer reviewed paper. On the other
hand I was peeved as a paper I submitted earlier this year was not accepted.

Their approach is similar to that of Foster and Rahmstorf.
See

Forster and Rahmstorf developed a multiple linear regression
model using total solar radiation, aerosols, ENSO and a linear trend as independent
variables and 5 alternate temperature records as the dependent variable. The period
analysed was 1979 to 2010; the only period common to all 5 temperature series.
They concluded that for that the period the underlying temperature trend was
0.014 to 0.18 °C per year. The paper was welcomed in many quarters as countering
the claim that the rate of temperature increase had had been falling off or
even stationary for the last decade or more of that period.

The Zhou and Tung paper adopts a similar approach but they have
substituted the ENSO with the AMO. They conclude that the rate of temperature
increase since the start of the 20

^{th}century, which they ascribe to anthropogenic effects, has been less that that estimated by Foster and Rahmstorf. They give 0.0068 °C for the 100 year trend, 0.0080 °C for the 75 year trend, 0.0083 °C for the 50 year trend and 0.0070 °C for the 25 year trend. These figures are about half of those of Foster and Rahmstorf.
They consider the suggestion of Booth et al that the AMO is
anthropogenic and reject it.

My own equivalent figures are 0.0050 °C per year from 1856
to the present, 0.0067 °C for the 100
year rate and 0.011 for the 30 year
rate. These values are similar to those of Zhou and Tung with one exception: I
get an accelerating rate of increase which reflects the growing concentration of
GHGs.

The conclusion of both their work and mine is the same:
climate models, which simulate all the increase in temperature as anthropogenic
and driven by GHGs, are overestimating the increase in temperature by a factor
of two. A corollary to both sets of ideas is that if, as seems likely, the AMO
is regular then it is likely to restrict temperature increase for the next few
decades while the AMO is decreasing.

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