A recently released study suggests man-made carbon dioxide emissions may not cause nearly as much warming as many climate models project, in what could be another sign of what’s being called “luke warming.”
University of Dublin meteorologist J. Ray Bates’ recent study found “that to disregard the low values of effective climate sensitivity (1°C)… is a standpoint that needs to be reconsidered.” His conclusion challenges findings from the United Nations that doubling human emissions would cause anywhere from 1.5 to 4.5 degrees Celsius of warming.
It’s a highly controversial finding since the study argues climate sensitivity — a measure of how much warming would occur with a doubling of atmospheric CO2 — may be much lower than suggested by the UN’s Intergovernmental Panel on Climate Change.
Bates’ paper adds to the growing literature suggesting climate sensitivity is likely on the low end of the spectrum suggested by the IPCC, meaning global warming would not be nearly as intense as suggested by environmental activists and politicians.
“As a lukewarmer, I am always enthused by new works that tend to add more evidence to that conclusion,” Chip Knappenberger, a climate scientist at the libertarian Cato Institute, told The Daily Caller News foundation. “This work definitely does so, as it now stands.”
Moreover, Bates’ study seems to reaffirm conclusions reached by Dr. Richard Lindzen, a climate scientist at the Massachusetts Institute of Technology (MIT), who found in 2011 less than 1 degree Celsius of warming would occur from doubling atmospheric CO2 concentrations.
But Knappenberger, who describes himself as a “lukewarmer,” believes climate sensitivity is lower than activists argue, says there are uncertainties in such low climate sensitivity measurements that need to be ironed out.
“There are a lot of uncertainties behind some of the critical quantities that go into the sensitivity calculation in this work,” he said. “Until they can get rectified, it’s hard to know how much faith to put into the final result.”
Knappenberger’s own research has shown climate models have been overestimating global warming for the last six decades.
The debate over climate sensitivity may seem arcane, but knowing how much warming may occur with a doubling of CO2 is crucial for climate model predictions. Many climate scientists assume climate sensitivity to be 3 degrees Celsius — the central estimate given by IPCC in its 2007 report.
The 2007 IPCC report found climate sensitivity is “likely to be in the range 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C,” adding that “values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.”
As more studies came out suggesting lower climate sensitivity, the IPCC was forced to alter its climate sensitivity estimates. The IPCC’s 2013 climate report did not give a central climate sensitivity estimate since climate models couldn’t agree.
Studies continue to point to lower climate sensitivity than the IPCC suggested in 2007.
A 2014 study by independent climate scientist Nic Lewis and Georgia Tech climate scientist Judith Curry found that doubling carbon dioxide concentrations would only cause 1.64 degrees Celsius of warming in the long term.
“Our results… are in line with those of several recent studies based on observed centennial warming and strongly suggest complex global climate models used for warming projections are oversensitive to carbon dioxide concentrations,” Lewis said.
Curry wrote in her blog that “the uncertainties in external forcing are substantial, particularly for aerosols.”
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