Research & Developments is a blog for brief updates that provide context for the flurry of news that impacts science and scientists today.
The Intergovernmental Panel on Climate Change (IPCC), the United Nations body whose mission is to “provide governments at all levels with scientific information that they can use to develop climate policies” will likely update the emissions and land use scenarios used in the models it considers in its bellwether assessment reports.
The IPCC has used these scenarios, known as Shared Socioeconomic Pathways (SSPs) or Representative Concentration Pathways (RCPs), in its two most recent assessment reports (AR), AR5 released in 2014 and AR6 released in 2023. The upcoming AR7 will be informed by a new set of scenarios, as described in a paper published last month in Geoscientific Model Development.
The paper is drawing widespread attention—both within the scientific community and in wider discourse—for its statement regarding one current scenario that has become familiar to anyone following climate science and policy. The scientists said the emissions levels associated with the most extreme, worst-case scenario, SSP5-8.5 (and its predecessor, RCP8.5), “have become implausible.”
Even President Donald Trump weighed in with a post on Truth Social on 17 May, where he wrote “GOOD RIDDANCE,” and “the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!”
But as scientists have pointed out for years, RCP8.5 was never meant to represent a likely emissions scenario or a forecast of humanity’s future. Some scientists questioned whether it’s even possible for RCP8.5 to play out in real life.
RCP8.5 is one of four hypothetical emissions scenarios developed in 2011 for climate modeling experiments. When RCP8.5 was created, it was meant to represent a “very high baseline emission scenario” that would warm the world nearly 5°C (9°F) compared with preindustrial temperatures by 2100. Parallel scenarios (SSPs) were presented in 2017. SSP5-8.5 is the worst-case scenario in that framework, representing a world in which fossil fuels are widely exploited and more of the world adopts energy-intensive lifestyles alongside the warming projected by RCP8.5.
“The scenarios we create today are different than the scenarios we created 15 years ago, because the world is different today than 15 years ago.”
The authors of the new paper wrote that “trends in the costs of renewables, the emergence of climate policy and recent emissions trends” justify the implausibility of the highest-emissions scenarios such as RCP8.5 and SSP5-8.5.
For scientists, the idea of dropping these scenarios is neither new nor controversial. As three climate scientists (Zeke Hausfather of Berkeley Earth, Glen Peters of the CICERO Center for International Climate Research, and Piers Forster at the University of Leeds) wrote in a blog post: “[RCP8.5] was never a likely outcome even in a world that did not address climate change; rather it was always intended to represent a worst case scenario that pushed fossil fuel expansion to the max.”
The new scenarios presented in Geoscientific Model Development include a high-emissions scenario in which clean energy policy is rolled back, and the world warms about 3.5°C (6.3°F) by 2100—still a level at which humanity can expect very severe impacts, from worsening weather extremes to rapidly rising sea levels.
The IPCC’s likely elimination of RCP8.5, even if it was never a plausible scenario, is a small sign of improvement in global climate change mitigation efforts, Hausfather, Peters, and Forster wrote: “Rapid declines in clean energy costs have bent the curve of future emissions downward, with new scenarios designed to reflect current policies notably lower than most baseline scenarios in the literature.”
“Of course, we still have a long way to go to get emissions down to (net) zero and stabilize global temperatures,” they noted.
The new paper captures the difficult road ahead for climate action: The new scenarios are based on a reduced projection for the increase in emissions, not for the overall amount of emissions—those are still increasing. Unlike before, none of the new emissions scenarios keep the world below 1.5°C (2.7°F) of warming, the limit originally set by the Paris Agreement in 2016. That’s no surprise to scientists, who suggest Earth is already in the 20-year period in which warming will formally surpass this benchmark.
“The scenarios we create today are different than the scenarios we created 15 years ago, because the world is different today than 15 years ago,” Hausfather told the Washington Post.
—Grace van Deelen (@gvd.bsky.social), Staff Writer
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