ZHOU AND TUNG
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.
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 20th 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.