Scientists made headlines when they recently retired a catastrophic climate change scenario, concluding that rampant fossil fuel expansion and extremely high planet-warming emissions “have become implausible.”
But the seemingly good news has a catch. Scientists also retired the best-case scenario, as the world’s most ambitious climate target has slipped out of reach.
“As a result, we are in a much worse situation than we were in 15 years ago,” said Detlef van Vuuren, head of the international committee that designs the climate scenarios used by modelers around the world. “Our ability to still reach the Paris goals is becoming more difficult and more difficult.”
Older modeling scenarios still assumed world leaders could meet their most ambitious climate goals, reducing emissions fast enough to cap global warming at 1.5 degrees Celsius. But emissions haven’t fallen fast enough, and scientists largely agree that the planet will at least temporarily exceed the 1.5 C target.
“We are today already almost at 1.5 degrees,” van Vuuren said. “And so the best possible pathway that we could think of is a pathway that overshoots 1.5 and goes first around 1.7, 1.8, and then comes back.”
Even that’s an optimistic scenario, he added. World leaders could potentially use carbon-capturing techniques or artificial cooling technologies to lower global temperatures in the future. But these strategies are largely unproven.
And while scientists got rid of their most extreme scenario — known as RCP8.5 — they replaced it with a new worst-case scenario.
RCP 8.5 assumed around 5 C (or 9 degrees Fahrenheit) of global warming by the year 2100. The new high-end climate trajectory puts the world on the same track by 2150.
The inside story makes all the excitement around the demise of RCP8.5 a bit of a red herring, van Vuuren said.
The worst possibilities scientists imagined 15 years ago have now receded, but the world’s most ambitious targets have also slipped out of reach. Global carbon emissions have generally tracked a middle-of-the-road scenario over the past few decades, with current global policies putting the planet on track for as much as 3 C of global warming by the end of the century. The entire range of possible planetary futures has narrowed as time has passed.
“I think overall we are simply in a situation that is bad news,” van Vuuren said.
Inside the process
RCP8.5 has been controversial for years. When it was designed in the early 2010s as a tool for gaming out a hypothetical worst-case climate trajectory, it assumed a dramatic expansion of fossil fuel use and extremely high greenhouse gas emissions through the rest of this century. That was never a likely outcome — even fossil fuel companies thought it would be difficult to achieve, modeling experts say.
But RCP8.5 wasn’t meant to be a prediction. It was largely intended as a thought exercise, useful to policymakers as a way to design adaptation plans that could withstand the worst possible impacts of climate change over the next century. For safety, those impacts would need to be nearly impossible.
“It’s really no different in other areas in life that we deal with,” said Jennifer Morris, an energy modeler at the Massachusetts Institute of Technology Center for Sustainability Science and Strategy. “We buy insurance to protect ourselves not from the likely outcomes but from the worst-case outcomes. It’s the same thinking. We need to be thinking about what are the worst-case scenarios in addition to what are the more likely scenarios.”
But the scenario has become steadily less likely over time as renewable energy use has expanded and world leaders have adopted more ambitious carbon-cutting policies.
Scientists have been transparent for years about RCP8.5’s low probability. But conservative lawmakers and activists have frequently seized on the scenario as evidence that scientists exaggerate the seriousness of climate change. Headlines about the scenario’s official retirement last month were accompanied by gleeful commentary from President Donald Trump, who posted on Truth Social that the defunct scenario’s projections amounted to “climate alarmism.”
Climate scientists say the whole scenario — along with its history and purpose — has been widely misunderstood by the public.
At the heart of the RCP8.5 saga is an international climate science collaboration known as the Coupled Model Intercomparison Project, or CMIP. The program, launched in the 1990s and coordinated by the U.N. World Climate Research Programme, helps research groups around the world improve the computer simulations they use to study climate change, coordinate their experiments and share their results.
The program starts a new project approximately every seven years, typically timed to coincide with major climate reports periodically published by the U.N. Intergovernmental Panel on Climate Change. Organizers have been planning the current cycle for several years, and the simulations it produces will help inform the IPCC’s next assessment.
Part of the planning process involves designing the climate scenarios that scientists use to run their models. Every cycle includes a handful of different trajectories, each with different assumptions about future greenhouse gas emissions. They’re all hypothetical scenarios, some of which are more likely to occur than others.
RCP8.5 has had a few iterations since it was first designed in the early 2010s alongside a suite of less dire alternative scenarios. These original RCPs, short for “representative concentration pathways,” assumed differing levels of future carbon dioxide concentrations in the atmosphere, corresponding with higher or lower levels of future warming.
Scientists upgraded the RCPs to a new set of scenarios in 2017, known as the shared socioeconomic pathways, or SSPs. These scenarios involved more complex assumptions about future population growth, energy use and economic conditions. But they also included a high-end trajectory — SSP8.5 — implying similarly extreme fossil fuel expansion and dramatic warming by the end of the century. The SSPs informed the IPCC’s last assessment report, which was completed in 2023.
Now the IPCC is planning its next report, and the latest CMIP cycle is under way. As part of the process, organizers have proposed a series of scenarios for modelers to use in their next wave of simulations. These include the new high-end and low-end climate trajectories.
Misunderstood?
Despite recent debate about the 8.5 scenario, it wasn’t a given that the former worst case would be retired, van Vuuren said. The committee hosted workshops to solicit input from the scientific community, consulted with IPCC leadership and eventually published an initial set of recommendations that they opened up for feedback from modeling teams around the world.
“We were asking ourselves, how high is the high-emissions scenario going to be?” van Vuuren said. “We also asked the emissions modeling teams to explore different options.”
In the end, the committee concluded that the 8.5 scenario was so extreme it was no longer useful to policymakers. It settled on a new high-end scenario — also designed to be unlikely, but technically possible — that delayed 8.5’s emissions outcomes by about 50 years.
The outcomes came as a surprise to some scientists participating in the process, van Vuuren said. Some researchers had expected the committee to land on a slightly more extreme scenario.
“There was quite a bit of discussion,” he said. “I think in the end this is a good outcome.”
Still, some modeling experts say the whole process is opaque to both policymakers and the public. That means the new scenarios could be misunderstood, just as RCP8.5 was for years.
“IPCC — and the scientific community in this space more broadly — has historically been reluctant to assign probabilities or likelihoods to climate scenarios,” said Morris, the MIT energy modeler. “They’ve left scenario users to make that judgment themselves, which has left scenarios like RCP8.5 to be really easily misunderstood.”
Morris added that there’s a growing appetite among scientists for better communication about the likelihood of any given scenario — especially the extremes at either end.
“I’d love to see more of that,” she said.
