Over the last few weeks, the United States right loudly claimed victory in a battle that few people on earth knew was happening. The National Review’s Editorial Board gloated, “Science Has Spoken Against Climate Alarmism.” Several papers owned by Rupert Murdoch ran similarly hyperventilating headlines that scientists had reversed “doomsday predictions” and “quietly scrapped the apocalyptic forecasts that have terrified policymakers and the public.” Donald Trump wrote on social media that the United Nations’ “TOP Climate change committee,” the Intergovernmental Panel on Climate Change, or IPCC, “admitted that its own projections (RCP8.5) were WRONG WRONG WRONG!”
That isn’t what happened. And it’s hard to overstate the gulf between the scale of the right’s triumphalism and the size of the thing they are ostensibly talking about. The alleged victory in question is in reality an academic paper published last month by a team of Earth system modeling experts convened by the Coupled Model Intercomparison Project, or CMIP, an initiative of the UN’s World Climate Research Program. The paper describes several new forward-looking climate scenarios created to help researchers understand how and why the earth warms, and what might happen as it does. Such scenarios have been a mainstay of climate science since the 1980s, and are updated frequently to account for new research and observations. “These scenarios are not prediction machines,” said Detlef van Vuuren of the University of Utrecht, a veteran of emissions scenario development and the lead author on the CMIP paper. “They are simply ways to explore possible futures.” As the present changes, so too do researchers’ models of possible futures.
So what exactly was Trump celebrating? The CMIP paper—which was published in April—notes that its researchers now consider one older scenario “implausible.” First created in the early 2010s, RCP8.5 outlines a world that is between 4.2 and 5.4 degrees Celsius warmer by 2100, thanks to extremely high levels of coal burning. Somewhat confusingly, the “8.5” in RCP8.5 doesn’t refer to temperature degrees but to specific levels of radiative forcing—a measure of change in the earth’s energy balance caused by human-caused greenhouse gas emissions. It was also developed at a time of rapidly rising fossil fuel use and relatively expensive renewables. “RCP” itself stands for “representative concentration pathway,” meaning that it was chosen to represent a range of similarly dire preexisting scenarios.
Importantly, RCP8.5 was always intended as a “low-probability, high-risk” scenario among several others that show much lower radiative forcing, emissions, and warming. RCPs 2.6, 4.5, and 6.0 appeared prominently alongside RCP8.5 in the fifth assessment report of the UN Intergovernmental Panel on Climate Change, released in 2013. In 2017, modelers replaced RCPs with something called “shared socioeconomic pathways,” and used RCP8.5 to inform the high-end SSP that appeared in the IPCC’s sixth assessment report in 2023. (The IPCC does not create its own scenarios or conduct original research, but relies on largely volunteer experts to synthesize the latest-available climate science across a range of topics every few years.)
Some 15 additional years of observable data about climate change, real-world climate policy, and the relatively recent, rapid proliferation of cheap renewables have now made the coal-heavy world of RCP8.5 look much less likely than it did in 2013. “We’re now in a very different position than RCP8.5 would have taken us, which is good,” van Vuuren said. “That doesn’t mean that RCP8.5 was wrong.” Continually updating scenarios to account for new data and understandings, he says, “is just the regular way we do our climate research.”
The reasons why RCP8.5 seems to have struck such a nerve on the right stems back to a series of wonky debates among academics that started nearly a decade ago. Two researchers at the University of British Columbia—Justin Ritchie and Hadi Dowlatabadi—published a pair of papers back in 2017 arguing that the modelers relied too heavily on coal, and that extremely high coal usage in RCP8.5 entailed burning through more coal than exists in the world’s recoverable reserves of that resource. Subsequent entries into this debate cautioned against treating RCP8.5 as a “business as usual” outcome when the world looked increasingly on track to warm by roughly 3 rather than 4 or 5 degrees Celsius by 2100.
Right-wing think tanks and climate skeptics glommed onto this debate to make a handful of bad-faith arguments that have relatively little to do with the generally good-faith academic discussions about the merits of RCP8.5. The National Review synthesized these takes last week, castigating Greta Thunberg and “a vast horde of activists, rent-seekers, academics, journalists, and politicians who seized on an out-of-date and flawed hypothetical to pursue an extreme climate agenda.” This is a remarkably dense version of a long-standing attack from the right—and the corporate polluters who often fund them—that climate advocates are deranged and/or opportunistic catastrophists trying to whip the public and policymakers up into an expensive and unnecessary frenzy. The right’s coverage of the CMIP paper accordingly mirrored an ad taken out by a coal and utility industry front group in 1991: “Doomsday is cancelled. Again.”
Seemingly reflecting the purchase RCP8.5 critiques have among the right, Trump’s executive order from last May about “Restoring Gold Standard Science” mentioned it by name. Although the White House listed RCP8.5 among several examples of alleged “falsification of data by leading researchers,” its criticism of the scenario is relatively mild: “Scientists have warned that presenting RCP 8.5 as a likely outcome is misleading.”
As even the harshest critics of RCP8.5 tend to acknowledge, however, there are plenty of valid reasons for scientists to study it as a worst-case scenario. Chris Smith—a coauthor on the CMIP paper, and a senior research scholar at the International Institute for Applied Systems Analysis in Austria—notes that such high-risk models “can be useful because they can tell us what kinds of climate impacts we can expect for high levels of warming.” The high so-called “signal-to-noise” ratio in RCP8.5 allows modelers to track relationships that wouldn’t be obvious in more plausible, lower-emissions scenarios. Higher-impact scenarios can be especially useful in adaptation planning, and preparing for worst-case scenarios that could happen in particular places. Continued greenhouse gas emissions could make RCP8.5-level emissions more plausible by 2150 or 2200.
Although the experts I spoke to agreed that the specific emissions in RCP8.5 look implausible, the world could still experience RCP8.5-like climate impacts. “Just because we might have ruled out that particular emissions pathway doesn’t mean we’ve ruled out that climate outcome,” Smith told me. “There could be a surprise in the climate system. There could be unknown feed-backs and tipping points that get you there even with a lower-emissions scenario.” As climate scientist Gavin Schmidt has written, real-world observations of melting ice sheets have already shown melt rates higher than in ice sheet models using RCP8.5-level radiative forcing. “Thus for a situation where the (ice sheet) models are insufficiently sensitive,” Schmidt writes, “a higher than expected forcing might give you a more likely outcome.” To put that another way: at 1.4 degrees Celsius of warming, ice sheets are already melting faster than they do in RCP8.5.
It’s worth dwelling on this. The main scholarly criticisms of RCP8.5 have been that it was misrepresented as a “business as usual” scenario, and that its estimations for coal burning, specifically—as opposed to other sources of emissions—were unrealistically high. Yet the kind of climate impacts that happen when RCP8.5 is plugged into a global climate model are very much not off the table. Even in the lower “high” scenario which has now replaced RCP8.5 for CMIP, climate feedbacks—like the considerable carbon-dioxide emitted by wildfires that burn through natural carbon sinks—could lead to higher greenhouse gas concentrations in the atmosphere, and warming that is “similar to the old RCP8.5,” van Vuuren said.
The fact that RCP8.5 is not “WRONG WRONG WRONG!” matters, in part, because of how some experts fear the RCP8.5 debate could be weaponized. “The worry now is that they’re going to take this new scenarios update and really try to discredit any papers that have ever used RCP8.5 as an input scenario,” said Madison Condon, an associate law professor at Boston University who studies climate risk. A great many peer-reviewed articles that use RCP8.5 and other scenarios have been presented as part of the evidentiary basis for climate rule-making. If even well-meaning policymakers come to believe that RCP8.5 is misleading or even just controversial, they might be swayed to throw out the justifications that underly those rules. Should that happen, then what has been a largely incomprehensible debate about ethics in climate modeling “is going to have real world consequences,” Condon said.
As is often the case, it would be nice to believe in the climate skeptics’ vision of a world freed from RCP8.5: where concerns about climate change were simply the special interest of an unusually influential group of doomsday cultists. Unfortunately, there is still quite a lot to worry about. “In the current emissions scenario we end up with 2.5 to 3 degrees [of warming], which is very much not good for us,” van Vuuren told me; a world that is 2.5 or 3 degrees warmer is one that promises no shortage of catastrophes. “All in all we are now in a much more difficult situation than we were in 2010,” when RCP8.5 was first being developed.
The media circus around van Vuuren and his colleagues’ research has focused on their phasing out RCP8.5. But their new modeling framework has narrowed the range of possible futures on both ends. He told me that “it has become impossible to stay below 1.5 degrees” of warming without significant overshoot—i.e. emissions in excess of the levels needed to meet that goal, which technologies such as direct air capture might at some point be able to remove from the atmosphere, bringing temperatures back down to below 1.5 degrees Celsius above pre-industrial averages. It’s good that some worst case scenarios for fossil fuel emissions now seem implausible. But what had been considered best case scenarios—for emissions, warming, and the disasters they’ll strengthen—are implausible now, too.
