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Over the Thanksgiving weekend the Trump administration released Volume 2 of the Fourth National Climate Assessment, eliciting the usual array of apocalyptic predictions from the media about the fate of mankind should we fail to impose sharp limits on the emissions of greenhouse gases (GHG). Herewith, a few observations on this report.
The report assumes one particular scenario for atmospheric concentrations of greenhouse gases through 2100: “representative concentration pathway 8.5.” This is the most extreme of the four scenarios used by the Intergovernmental Panel on Climate Change (IPCC) in its Fifth Assessment Report. (The others are RCP 2.6, 4.5, and 6.) (The numbers are not temperature effects; they are theoretical calculations of “radiative forcing” impacts in watts per square meter caused by a given scenario by the year 2100.) Under RCP 8.5, GHG concentrations through 2100 rise at an annual average rate of 11.9 parts per million. The average was 1.9 ppm for 1985–2017 and 1.6 ppm for 1959 (the earliest year reported by the National Oceanic and Atmospheric Administration) through 2017. The single biggest increase was 3.4 ppm in 2016.
There is nothing wrong with the use of extreme scenarios for analytic purposes and sensitivity comparisons, but using the most extreme scenario as the sole basis for climate analysis is not entirely honest. Moreover, the new report predicts 8°C of warming by 2100, a figure that even the IPCC finds dubious (p. 12-3): “Warming above 4°C by 2081–2100 is unlikely in all RCPs (high confidence) except for RCP 8.5 where it is as likely as not (medium confidence).”
Despite the fact that the climate alarmists routinely use RCP 8.5 to drive their predictions, it is a scenario difficult to take seriously. For example, under RCP 8.5, global coal consumption rises by about 900 percent by 2100. That assumption is combined with “high energy use and emissions” and “slow GDP growth.” The report is silent on precisely how the latter two conditions can be made consistent. (They cannot.) I am willing to believe RCP 6 (average GHG increase through 2100: 5.5 ppm) for purposes of discussion — coal use in Asia is likely to rise — but under that scenario temperatures in the standard general circulation models rise by about 3 degrees by 2100. Note, however, that these models (the 102 CMIP-5 models tracked by the Lawrence Livermore National Laboratory) as a crude generalization have overpredicted warming since 1975 (roughly the beginning of the satellite record) by a factor of about 2. I discuss the policy implications of the four RCPs in this essay.
Again, the dire predictions in the latest National Climate Assessment (NCA) are driven by the climate models, which as a group cannot predict the past or the present. Accordingly, they provide a dubious basis for policy formulation. Patrick J. Michaels of the Cato Institute points out (here and here), correctly, that an honest analysis would use the model(s) that have done the best job of predicting what is actually happening. That turns out to be the Russian INM-CM4 climate model, which predicts the least amount of future warming among all the CMIP-5 models. Obviously, that is why the authors of the Fourth NCA did not use it.
Moreover — amazingly — Volume 2 of the Fourth Assessment simply ignores the systematic evidence on climate phenomena in the context of increasing GHG concentrations. In brief:
The climate alarmists implicitly have conceded all of this by moving the goalposts: Instead of the absolute necessity of limiting warming (the “carbon budget”) to 2 degrees, the safe limit now purportedly is 1.5 degrees, a tacit admission that the 2 degree goal will be met by 2100 without any GHG policies at all. I discuss this shift here.
Volume 2 of the Fourth NCA lays out a series of policy admonitions; fine, but notwithstanding the curious view of many journalists, policy prescriptions are not “science,” and scientists have no special claim to deference on their policy arguments, with all their implicit tradeoffs and subjective value judgments.
The Obama Climate Action Plan, if implemented, would have reduced temperatures by 2100 by 0.015 degrees (using the Environmental Protection Agency climate model). Paris: 0.17 degrees. An aggressive policy (Table 3): 0.525 degrees. Such policies would impose costs of at least 1 percent of global gross domestic product annually, while yielding virtually no environmental benefits at all even under the alarmists’ assumptions, with the costs borne disproportionately by the poor.
When the report was released over the Thanksgiving weekend, I believed that the Trump administration had made a serious mistake, indeed, a blunder. Doing so allowed the journalists and others to argue that the administration was trying to hide something. Upon further reflection, I am not so sure: Because each report takes four years to complete, this latest one is likely to reflect the views of the Obama administration much more closely than those of the Trump administration. I wonder if the various agencies releasing the report over Thanksgiving did so as a means to embarrass the White House and as a tool to increase their political support and budget approvals in the new Congress. Perhaps I am too cynical.
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