The Current Wisdom is a series of monthly articles in which Patrick J. Michaels, director of the Center for the Study of Science, reviews interesting items on global warming in the scientific literature that may not have received the media attention that they deserved, or have been misinterpreted in the popular press.
Our periodic compilations of low equilibrium climate sensitivity (ECS) estimates have become a big hit.
In our on-going effort to keep up with the science, today we update our previous summary with two additional recently published lower-than-IPCC climate sensitivity estimates—one made by Troy Masters and another by Alexander Otto and colleagues (including several co-authors not typically associated with global warming in moderation, or “lukewarming”). There is also a third paper currently in the peer-review process.
The new additions yield a total of at least 16 experiments published in the peer-reviewed scientific literature beginning in 2011 that have found that the most likely value of the ECS to be well below the (previously?) “mainstream” estimate from the U.N.’s Intergovernmental Panel on Climate Change (IPCC). Since the negative impacts from global warming/climate change scale with the magnitude of the temperature rise, lower projections of future warming should lead to lower projections of future damages. We say “should” because one way around this, as the federal government has figured out, is to ignore all the new science indicating less expected future warming when calculating future damages, and inexplicably doubling the damages estimated to be caused by a given increment of carbon dioxide (a.k.a., social cost of carbon).
Here is a quick summary of the two new papers:
Examining the output of climate models run under increases in human emissions of greenhouse gas and aerosols, Troy Masters noted a robust relationship between the modeled rate of heat uptake in the global oceans and the modeled climate sensitivity. With this relationship in hand, he then turned to the observations to determine what the observed rate of oceanic heat uptake has been during the past 50 years or so. From the observed behavior, he was able to determine the climate sensitivity, and found it to be substantially less than that in the vast majority of the climate models. He found that the most likely value of the ECS from the observations was 1.98°C with a 90 percent range extending from 1.2°C to 5.15°C. He notes that the high end is driven by uncertainties in the oceanic heat uptake data earlier in the record.
Otto and colleagues used a simple energy budget model to relate observed global temperature changes to changes in the radiation climatology and the heat uptake in the earth system as humans have heaped various substances into the atmosphere. They conclude that the at best estimate for ECS is 2.0°C with a 90 percent range from 1.2°C to 3.9°C.
Both studies come with a long list of caveats relating to data quality, etc., that are common to all studies trying to estimate the ECS.