September 2019

As reported in the Washington Post and New York Times, Chapter 29 of Vol. 2 of the 4th U.S. National Climate Assessment claims that unchecked warming could raise global temperatures 8°C by century’s end, which in turn would reduce U.S. GDP by 10 percent.

Those estimates are based on this chart from Hsiang et al. 2017:

The chart shows the probability distribution of global warming projections when the IPCC Fifth Assessment Report’s climate model ensemble, known as CMIP5, is run with the RCP8.5 forcing trajectory.

On average, CMIP5 models hind-cast two to three times more warming in the tropical mid-troposphere during 1979-2017 than actually occurred.

Although often billed as a “business as usual” scenario, RCP8.5 is actually a high emissions scenario. It assumes the kind of forcing trajectory that would emerge if coal scaled up rapidly to provide almost half of global energy from all sources by 2100–a market share not seen since 1940.

In short, the National Assessment ran a group of overheated models with an inflated emissions baseline. Yet, as the chart from Hsiang et al. reveals, even with that biased combo, global warming hits 8°C in only 1 percent of model projections.

Curiously, that’s a detail the National Assessment did not mention. Nor did it point out that even if U.S. GDP in the 2090s is 10 percent lower than it might otherwise be, the economy is still projected to be much bigger than it is today.

Indeed, in 2100, global per capita GDP is projected to be greater and income inequality lower in the IPCC’s fossil-intensive shared socioeconomic pathway (SSP5) than in all other SSPs, including the “green road/sustainable” pathway (SSP1). Here’s the relevant chart from Riahi et al. (2017).

In SSP5, global per capita GDP in 2100 is 10-fold greater than it is in 2000, and wealth is more widely shared than at any time in human history. Predictably, that bit of IPCC-approved “science” is nowhere to be found in the 2018 National Assessment.

You’re government at work.

(Post updated 9/18/2020)