My AEI colleague Roger Pielke Jr. posted some useful observations recently on the politicized use of extreme greenhouse gas (GHG) scenarios for purposes of the usual climate scaremongering and support of policies that I would describe as ideological opposition to fossil fuels. Roger takes particular aim at an extreme scenario called “Representative Concentration Pathway 8.5,” which Roger describes as not merely “highly unlikely” but instead “falsified” and “impossible.” (Independently, I have used identical language.)
Roger argues that “… the climate community has never made a systematic, scientific effort to assess either plausibility or … whether there might be even more extreme scenarios that might be plausibly ‘worser.’” And he goes on to argue that:
It turns out that defining a ‘worst case scenario’ is … a deeply value-laden process that must recognize that different people will value different outcomes differently. That makes the characterization of a ‘worst case outcome’ inevitably political…
I think that we can narrow down a useful definition of “extreme GHG scenario.” For the four scenarios RCP2.6, RCP4.5, RCP6, and RCP8.5, atmospheric concentrations of GHG in 2100 are, respectively, 490 parts per million, 650ppm, 850ppm, and 1370ppm.
Important elements of the data on atmospheric GHG concentrations for Mauna Loa published by the National Oceanographic and Atmospheric Administration are as follows.
GHG Concentration Data, Mauna Loa, Hawaii (PPM)
| Current (2024) | 424.6 |
| Average annual GHG concentration increase 1959-2024 | 1.673 |
| Average annual GHG concentration increase 1985-2024 | 2.017 |
| Average annual GHG concentration increase 2000-2024 | 2.290 |
| Single largest GHG increase | 3.53 (2024) |
| Second largest GHG increase | 3.36 (2023) |
| Year 2100 RCP2.6, 4.5, 6, 8.5 (respectively) | 490, 650, 850, 1370 |
| Average annual GHG concentration increase (respectively) | 0.9, 3.0, 5.7, 12.6 |
| Average temperature anomaly (°C), 2100 (respectively) | 2.0, 2.9, 3.9, 4.8 |
Historical GHG data: https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_mlo.txt.
Year 2100 GHG concentrations by RCP: https://skepticalscience.com/docs/RCP_Guide.pdf.
Average temperature anomalies, 2100 (relative to 1850-1900): https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter04.pdf (Table 4.2)
Accordingly, what is a sensible “extreme scenario?” As Roger points out, that is largely a subjective question, but it seems reasonable to assume that, say, an average annual GHG concentration increase double the single largest such increase since 1959 (3.53 ppm in 2024) would be “extreme.” That would be about 7 ppm annually, substantially lower than the annual increase under RCP8.5 (12.6 ppm), and somewhat greater than the average annual increase under RCP6 (5.7 ppm).
Obviously, “extreme” is in the eye of the beholder, but an average annual increase of 7 ppm yielding a GHG concentration of 950 ppm ([7*75]+425)—substantially more than the current concentration of 425 ppm—is an “extreme” assumption far more useful than RCP8.5. As an aside, Roger informs me that he (and colleagues) “are working on a paper that shows that the current emissions trajectory is undershooting RCP4.5.”