A controversy erupted in the world of climate modeling last month when two scientists published a commentary in Nature magazine calling out their fellows for repeatedly exaggerating the likelihood of catastrophe, through the device of exaggerating likely future greenhouse gas emissions. For years they have used an improbable worse-than-worst case as if it were the most likely business-as-usual scenario. As a result, hundreds of studies have warned the public of things likely to happen if We Don't Act Now that the modelers know are all but impossible. Such criticism from insiders stung and led to considerable pushback against letting the cat out of the bag. But the alarmists told their fellow alarmists bluntly “The ‘business as usual’ story is misleading: Stop using the worst-case scenario for climate warming as the most likely outcome.” Here's another worst-case scenario: their colleagues ignore the warning.
The story began over a decade ago when the IPCC developed four scenarios for future greenhouse gas concentrations for modelers to use to project 21st century climate change. They were called Representative Concentration Pathways (RCPs) and had numbers attached to them to indicate severity, from RCP2.6 (smallest change, least warming), RCP4.5, RCP6.0 and RCP8.5. The final RCP8.5 scenario was added as a speculative worst-case scenario, but to get emissions that high would require changes no one believes would or even could happen, such as a five-fold increase in global coal consumption in the next few decades. The RCP4.5 and RCP6.0 scenarios are more likely descriptions of business-as-usual (i.e. nobody bothers with any policy but historical trends in improved energy efficiency and clean technology continue). RCP2.6 assumes those trends and also that the Paris Accord is implemented. So these are the more likely future outcomes, and the climate models indicate that they result in far less warming and far lower costs due to climate change.
Well, we can't have that, can we? As Roger Pielke Jr. noted, in 2013 the IPCC decided, against its own experts’ guidance, to declare the RCP8.5 scenario the most likely “Business-as-usual” outcome, which unleashed a tidal wave of studies presented to the public as the “likely” result of not cutting emissions, when in fact they were preposterously unlikely. In another column on the subject Pielke adds: “the deeper problem with RCP8.5 is not that it is simply implausible. It is that this scenario has been placed at the center of climate research.”
When actual CO2 levels over recent decades are compared to the RCP scenarios, even the most committed alarmists realize RCP8.5 ain't ever gonna happen. In fact global greenhouse gas concentrations have long been tracking the bottom end of the scenario range. The most likely outcome even if we do nothing will be in the middle or bottom half of that range. Which is good news, unless your goal in life is to scare the public into adopting your Utopian green agenda.
The authors of the Nature commentary shore up their alarmist credentials by insisting that “this admission [i.e. that they've been exaggerating the problem] does not make climate action less urgent.” But it does, which is why so few climate scientists will be dropping the RCP8.5 scenario any time soon.