Hansen's Regression to Zero

While reading Hansen’s latest mailout I came upon an intriguing reference that I followed up. I suspect this paper is as important as Douglass et al. in describing an important way the models do not agree with the observations. It may be more important, in redefining the role of the Sun in recent warming.

His mailing contains a massive revision of his estimate of the rate of warming down from 0.2C per decade to 0.15C per decade. Near the end of his mailing he notes:

Solar irradiance has a non-negligible effect on global temperature [see, e.g., Reference 7, which empirically estimates a somewhat larger solar cycle effect than that estimated by others who have teased a solar effect out of data with different methods].

Reference 7, a paper by Tung, K.K., J. Zhou, C.D. Camp, Constraining model transient climate response using independent observations of solar-cycle forcing and response, recently published in Geophys. Res. Lett. in 2008 is freely available. It uses the geographical distribution of global temperature in response to the 11 yr solar cycle to isolate the transient climate response (TCR). Noting previously that the 19 IPCC models have a very large range in TCR, it compares the empirically derived TCR with the TCR of all the major IPCC climate models. The result is damning:

[14] The TCRs of 19 coupled atmosphere-ocean GCMs in IPCC AR4 listed in Table 1 fall within the rather low range of 1.2–2.2 K with the exception of one, and thus fail the lower constraint of 2.5 K determined by ERA-40, GISS and HadCRUT3. The only exception is the Japanese MIROC (hi-res), with a TCR of 2.6 K. All models fail the higher constraint of 3.6 K determined by the NCEP data.

The emphasis was added by the authors. As I understand it, they are saying that GCM’s have grossly underestimated solar forcing. Like the tropospheric ‘hotspot’ due to GHG’s in Douglass et al., they are not even in the right ballpark. The problem, it seems, is with the rate of transfer of heat into the ocean.

[16] It is seen that most of the current generation of general circulation models assessed by IPCC AR4 have too low a transient climate response as compared with the observed range. This is consistent with the independent finding by Forest et al. [2006] that these models simulate too large an ocean heat uptake as compared to observations of ocean temperature changes during the period 1961–2003. See Raper et al. [2002] and Meehl et al. [2004] for different views on how ocean heat uptake affects TCR. This excessive heat into the oceans tends to reduce the transient climate response for the atmosphere, but does not affect the modeled equilibrium climate sensitivity, which was calculated with a slab ocean in thermal equilibrium with the atmosphere.

The last sentence notwithstanding, there is an argument that underestimating TCR must lead to overestimating GHG forcing in the recent past. This would be a confirmation of the AGW skeptical view that recent response to solar forcing has been grossly underestimated, and GHG forcing exaggerated.

For the paper to be acknowledged by James Hansen himself is intriguing. Perhaps backpedaling is his way of avoiding jousting with jesters (climate skeptics).