IPCC uses worst-case scenario to exaggerate emission forecasts

You might never have heard of RCP8.5, but you have definitely heard of climate forecasts based on it. Whenever you read a media story about that this or that frightening catastrophe looming over our future if we don’t slash greenhouse gases, in other words under the “business-as-usual” future scenario, it’s almost certain they’re referring to RCP8.5.

It’s also almost certain that nowhere in the story do they point out that RCP8.5 is an implausible worst-case scenario that was never meant to represent a likely base case outcome, and scientists have long known that using it as such is misleading. They do it anyway.

RCP stands for “Representative Concentration Pathways,” which yield projections of how much carbon dioxide (CO2) will accumulate in the atmosphere due to fossil fuel use over the coming century. The Intergovernmental Panel on Climate Change (IPCC) generated a set of four RCP scenarios a decade ago, attaching to each a number indicating how much “radiative forcing” (a measure of global warming potential) each one generates. RCP2.6 refers to a benign, low-end emission scenario with correspondingly minimal radiative forcing. In the middle are RCP4.5 and RCP6.0, and at the top end is RCP8.5, a scorcher which predicts historically-unprecedented increases in global CO2 emissions.

To appreciate how implausible RCP8.5 is, consider its coal use trajectory. From the 1920s to the year 2000, global coal consumption stayed between 15 and 20 gigajoules per capita, peaking at 20 in 1960, falling back to 15 by 2000, then rising to about 23 earlier this decade with the sudden industrialization of China and India. Groups like the International Energy Agency expect it will gradually return to the 15-20 gigajoule per capita range by 2040. RCP8.5 instead says it will rise to about 30 gigajoules by 2040, 45 gigajoules by 2060, and 70 gigajoules by 2100. No one seriously believes this is even possible, including people who use RCP8.5 in their climate simulations.

It gets worse. A recent study by Matthew Burgess of the University of Colorado and co-authors pointed out that RCP8.5 doesn’t even make sense in its own modeled reality. It projects so much economic growth that today’s poor countries will be richer in 2100 than the wealthiest countries are today (which would be nice if it happened), but they will also experience so much warming they become uninhabitable wastelands. How can both be true?

RCP8.5 was conceived as an outlier; an improbable worst-case scenario, not a likely business-as-usual forecast. Yet countless scientists and economists present it as one. You know how the game works. Feed RCP8.5 into a climate model, observe the catastrophe, then call it the “likely” no-policy base case if we don’t cut emissions. Last fall, in a commentary in Nature magazine, climate experts Zeke Hausfather and Glen Peters scolded their colleagues for misleading the public this way, and distorting the policy debate in the process.

In truth exaggerated emission forecasts are nothing new. Another analysis last fall by Zeke Hausfather compiled CO2 concentration forecasts from the 1970s to the 2000s. The figure shown on this page is based on the data in their own Figure S4 (from their online appendix; sadly not shown in the paper itself). In the 1970s scientists made CO2 projections through 2000. The range encompassed reality, though was somewhat too high. But from the 1980s on, ranges of projection were systematically too high and reality has hugged the bottom end. (The only exception is James Hansen’s so-called 1988C scenario, which assumed all global CO2 emissions cease in the year 2000; once again an improbable outlier never meant as a realistic forecast.)

The IPCC likewise has form on this. Prior to commissioning the RCP group the IPCC issued the Special Report on Emission Scenarios (SRES), which ranged from low and slow CO2 growth to the infamous A1FI top-end scorcher, which climate modelers considered unrealistic but which allowed the IPCC to issue scary-sounding “up to six degrees” warming forecasts. In a column I published on this page on April 4, 2002, I noted that by the time it was shown to experts in early 2000 A1FI had already overestimated global coal consumption growth in the 1990s by 40 per cent.

The SRES scenarios came in for more controversy in 2002 when Ian Castles, the former Chief Statistician of Australia, and the former OECD Chief Economist David Henderson, wrote to the IPCC to inform them that the SRES team had used an incorrect formula for computing foreign exchange rates, which systematically exaggerated the growth and emission forecasts in low-income countries.

I got interested in the issue around that time in part because the IPCC would not attach probabilities to their scenarios. In two papers later published in 2012 and 2013 I and my co-authors showed that the likelihood of SRES emission scenarios could be assessed using historical evidence and indicators about how global energy markets constrain fossil fuel use, and each time the lower half of the range of scenarios was more probable than the upper half.

Thus for at least 30 years, when the IPCC and others have issued emission scenario ranges, the bottom end has always been the realistic path and the rest has been exaggerated, yet the upper end gets all the media and academic attention. This has had the rhetorical effect of allowing them to issue warnings about what “might” happen, as in: warming of “up to” six degrees and “as many as” half the world’s species dying off, or whatever, unless we take drastic measures to cut emissions.

The purpose of global climate policy is to get us from the dangerous upper end of the forecast range down to the safe bottom end. But what users of climate projections need to understand is we are already there. We don’t need to kill the global economy to get onto an emissions path we never left in the first place. If we want to avoid the RCP8.5 future scenario all we have to do is stop feeding it into climate models, because that’s the only place it exists.