Originally posted at Bishop Hill on Aug 22, 2014
Nic Lewis emails:
I am sorry to see that Piers Forster has given what I consider to be an incorrect statement to the Guardian (p.19) regarding the new Chen & Tung Science paper about the Atlantic storing excess heat. He is quoted as saying: “Most importantly, this paper is another nail in the coffin of the idea that the hiatus is evidence that our projections of long-term climate change need revising down.”
The paper very much supports the view of those scientists, including myself, who consider that natural variability in the ocean significantly affects surface warming trends on a multidecadal basis. It does not support the view that a reduction in the rate of increase in radiative forcing caused by aerosols, solar variations, reduction in stratospheric water vapour, etc. is the prime cause of the hiatus.
The press release for the new study states: “Rapid warming in the last two and a half decades of the 20th century, they proposed in an earlier study, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface.” The new study supports Tung’s previous (2013) conclusions by providing evidence from sub-surface ocean heat data, by filling in details of the cycle and by shedding light on the mechanism involved. If only half the warming over 1976-2000 (linear trend 0.18°C/decade) was indeed anthropogenic, and the IPCC AR5 best estimate of the change in anthropogenic forcing over that period (linear trend 0.33Wm-2/decade) is accurate, then the transient climate response (TCR) would be little over 1°C. That is probably going too far, but the 1.3-1.4°C estimate in my and Marcel Crok’s report A Sensitive Matter is certainly supported by Chen and Tung’s findings.
Since the CMIP5 models used by the IPCC on average adequately reproduce observed global warming in the last two and a half decades of the 20th century – a period over which uncertainty in aerosol forcing changes is small relative to the increase in greenhouse gas forcing – without any contribution from multidecadal ocean variability, it follows that those models (whose mean TCR is slightly over 1.8°C) must be substantially too sensitive. And since the IPCC projections of long-term climate change are based on those models, they must therefore need revising down, contrary to Piers’ claim.
BTW, the longer term anthropogenic warming trends (50, 75 and 100 year) to 2011, after removing the solar, ENSO, volcanic and AMO signals given in Fig. 5 B of Tung’s earlier study (freely accessible via the link), of respectively 0.083, 0.078 and 0.068°C/decade also support low TCR values (varying from 0.91°C to 1.37°C), upon dividing by the linear trends exhibited by the IPCC AR5 best estimate time series for anthropogenic forcing. My own work gives TCR estimates towards the upper end of that range, still far below the average for CMIP5 models.