As our March 2020 video on the Climate Sensitivity Question explains, a key issue in climate science is how much warming can be expected from doubling the amount of CO2 in the air. The estimate is called the Equilibrium Climate Sensitivity or ECS, and for decades the IPCC and others said the number was between 1.5 and 4.5 degrees C, with a best estimate of 3 degrees C, based on simulations from climate models. But as we showed in our video, in recent years a series of papers estimating ECS from real-world observations suggested it was much lower than inside computers, more like 2 degrees C. In the IPCC’s most recent report, they stopped relying on climate models and instead turned to a 2020 study that combined ancient and modern evidence on warming rates using novel statistical methods and concluded ECS was between 2.5 and 4.0 degrees C, with a most-likely value of 3.1 C. So when challenged the IPCC doubled down, or up, raising the ECS best estimate a bit, shrunk the ECS range and pushed up the bottom end by a full degree. Unfortunately a new paper just published in the peer-reviewed literature has shown that the study the IPCC relied on made some math errors and also relied on obsolete data. In fact they didn’t even use the updated data the IPCC itself used elsewhere in its report. And guess what happens when these errors are fixed.
While the discussion of ECS might seem to be dense and esoteric, it has enormous policy implications. As economists have shown, if ECS is down around 2 degrees C the whole basis for imposing costly climate policy falls apart. And apparently it does.
According to the study’s author, mathematician Nic Lewis, fixing the errors in the statistical methods actually increased the ECS estimate slightly, from 3.1 to 3.2 C. But bringing the data up to date had the opposite effect, and altogether the ECS best estimate dropped to 2.2 C, with a likely range from 1.8 to 2.7 C. So whereas the IPCC said it’s unlikely ECS is below 2.5 C, Lewis’ recalculation shows there is only a 26 percent probability it exceeds 2.5 C. And while the IPCC all but ruled out ECS being under 2.0 C, Lewis concluded there is a 36 percent probability that it is.
There’s more. Lewis further noted that if the analysis were re-done dropping the 1860s, for which there are hardly any reliable temperature records, and using some recent IPCC estimates of the aerosol cooling effect, the ECS best estimate fell even further, to 1.8 C. So the best estimate of ECS is below the level that the IPCC said it couldn’t possibly be.
This finding means we are in a world where even large increases in atmospheric carbon dioxide just won’t have much effect on the climate. Models haven’t yet caught up to the new science of ECS and are still projecting too much warming. Eventually reality will win out. We just hope that climate policy doesn’t kill too many people before that happens.