The IPCC’s Climate Math Doesn’t Add Up. Will Anyone Notice?
The high and rising costs of climate policy — now including the inability of jurisdictions that bet big on renewables to guarantee enough energy for their citizens to survive the coming winter — don’t just entitle us to question the basis for it: they demand we do so.
Ultimately, the justification for renewables is the view that carbon dioxide emissions have a big effect on the climate that will cause devastating harm at some point in the future. [bold, links added]
Scientists measure the effect using a concept called “Equilibrium Climate Sensitivity” or ECS, which estimates how much long-run average warming will occur as a result of doubling the amount of carbon dioxide in the atmosphere.
Some important new evidence pointing to a low ECS value just emerged in the scientific literature.
ECS has long been uncertain. In 1979 the U.S. National Academy of Sciences estimated it to be between 1.5 and 4.5 degrees Celsius, with a best estimate of 3.0 degrees C.
That range, which runs from “no big deal” to “very bad outcomes,” was accepted by the UN Intergovernmental Panel on Climate Change (IPCC) in its first report in 1990 and thereafter until 2007 when, citing greater warming projections in newer models, it raised the bottom end to 2.0 degrees C.
But over the next few years, literature developed using, not model simulations, but observed warming rates since the late-1800s to estimate ECS.
Its results typically centered around 2.0 C or less. So in 2013, the IPCC reduced the bottom end of the range back to 1.5 C and declined to offer a best estimate. In other words, after three decades climate science hadn’t narrowed the uncertainty at all.
The economic implications of ECS being 2 C rather than 3 C are enormous.
Economic models used by the U.S. Environmental Protection Agency and others assume ECS is 3 C when computing the social cost of carbon.
Some co-authors and I have shown that if the ECS parameter is instead centered around 2 C, the estimated social cost of carbon plummets and becomes very small at least through the middle of this century. The justification for costly climate policy essentially disappears.
Given the discrepancy between models and observations, the IPCC changed the way it handled the ECS issue in its latest (2021) report.
It no longer relied on model estimates, but neither did it go with the existing estimates in the empirical literature. Instead, it turned to a 2020 paper by Australian climate scientist Steven Sherwood and 10 co-authors, who used a new technique to combine data from modern climate change with that from the end of the last Ice Age and even further back.
They concluded the likely sensitivity range was from 2.6 to 3.9 C. On this basis, the IPCC revised its estimate of the likely ECS range to between 2.5 and 4.0 C with a best estimate of 3.1 C. And it specifically ruled out ECS being less than 2.0 C.
But as so often happens when a new paper appears in the literature that solves a political problem for the IPCC, they pounce on it before experts in the field have had a chance to check the numbers — which is what a new paper in the peer-reviewed journal Climate Dynamics by U.K. mathematician Nicholas Lewis does.
Lewis shows that the Sherwood paper made some mathematical errors and also relied on outdated data.
Interestingly, many of the data updates were done by the IPCC in other parts of its 2021 report but weren’t applied in the Sherwood study. Other data updates were done in the wider peer-reviewed literature by specialists in the field.
Lewis shows that correcting the math actually increases the sensitivity estimate slightly.
But updating the data does the opposite: the ECS best estimate drops to 2.2 C with a likely range from 1.8 to 2.7 C.
And if the analysis focuses only on the period after 1870 (recognizing that most of the world has little or no reliable temperature data prior to that) the best estimate drops even more: to 1.8 C.
In other words, with updated data, the Sherwood paper would largely confirm the empirical literature the IPCC had passed over.
This is a big deal. But amid the nonstop stream of climate news, you won’t hear about it: the world’s climate journalists aren’t trained to follow important topics like this — though that never stops them from lecturing their readers, viewers, and listeners about what to think about climate science.
It will be five years or more before the IPCC issues a new report. If history is any guide, months before that a group of authors heavily involved in the report-writing process will rush a paper into print that drags ECS back up to the 3 C range just long enough for the new IPCC Summary for Policymakers to declare the same old best estimate.
But reality keeps pointing to lower values.
Don’t let anyone tell you “the science” demands we simply accept the increasingly lethal climate policy agenda. It would fail a cost-benefit test even if ECS were 3 degrees C.
But it’s even less justified with an ECS of 2 degrees C, which is the level the evidence seems to insist on.
Read more at Financial Post