Scientists are reshaping long-term projections for a warming world, saying both the most disastrous and the most optimistic futures that often anchored climate planning have slipped out of the range of what they consider plausible. The shift comes as researchers update a new set of seven carbon pollution scenarios meant to reflect changes in how the world produces energy, while also confronting the reality that governments have not yet achieved the Paris climate treaty’s central target.
The new analysis, outlined in research interviews reported by the Associated Press, discards two familiar bookends of climate policy: the extreme ends of earlier projections. Scientists say modest progress in cutting the highest-end carbon pollution has made the worst futures less likely than previously thought, but they also say that progress has not been fast enough to keep the Paris temperature limit within reach—especially given the behavior of carbon dioxide in the atmosphere over the coming century.
Researchers said their updated scenario set pushes aside an older pattern in which high-carbon pathways could lead to much higher end-of-century warming. In the updated framework, they said the proposed worst case for warming by the end of the century is about 3.5 degrees Celsius, a full degree lower than the previous estimate. Detlef Van Vuuren, a climate scientist at Utrecht University and lead author of the scenario study, said the Paris 1.5 degrees Celsius goal is still overshot even in the best case scenario.
Van Vuuren said the best case future is only marginally higher than previously theorized, describing it as a couple tenths of a degree Celsius warmer than earlier calculations. But even that updated “best” pathway, scientists said, runs past 1.5 degrees Celsius, meaning the policy goal is no longer achievable without relying on a future technology capable of removing massive amounts of carbon from the air after temperatures peak. Scientists said the Paris goal effectively assumed a pathway where warming could rise and then be brought back down through artificial cooling.
The updated scenarios also attempt to capture how the extremes of future emissions have changed as renewable power spreads. The researchers said expanding use of solar, wind and geothermal—sources that do not emit carbon dioxide—has helped reduce the top-end carbon pollution projections. At the same time, they said those changes have not proceeded quickly enough to pull up the bottom-end outcomes, leaving lower-end emissions still on a path that yields warming beyond 1.5 degrees Celsius.
Johan Rockström, director of the Potsdam Institute for Climate Impact Research, said the update reflects “kind of a narrowing of the futures,” while warning that the range is bounded by physics. “It cannot be as bad as we thought, but it cannot be as good as we hoped,” Rockström said, describing a scenario landscape that is still too hot for the international temperature goal set in 2015.
In the United States, the updated scenarios helped spark public debate over what has really changed and what older modeling assumptions remain relevant. American Enterprise Institute climate scholar Roger Pielke Jr. said that changes to the high end of earlier scenario framing matter because that scenario had been presented as a likely future absent policy action. Pielke said in an email that the earlier high-end case, known as RCP8.5, was based on out-of-date and incorrect coal-heavy energy theories, adding that thousands of studies built on the earlier projection.
Keywan Riahi, lead author of the 2011 study that introduced RCP8.5, disputed that characterization, saying the scenario was not intended as a forecast. “It was never a likely case. It was basically, given the underlying studies in the literature at that time, a plausible higher bound of what possible emissions could look like,” Riahi said. He also described what he called a success story, saying the cost of renewables—particularly solar and wind—had fallen by almost 90% over the previous decade and a half.
The debate also spilled into politics through a social media post by President Donald Trump, who wrote, “GOOD RIDDANCE! After 15 years of Dumocrats promising that ‘Climate Change’ is going to destroy the Planet, the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!” Van Vuuren responded to that framing by saying, “The risks of climate change have not disappeared,” while describing the update as evidence that the world did not follow the most dramatic emission pathway even as society remains on course for significant climate impacts.
Scientists also cautioned that even if emissions pathways improve, climate feedbacks could still add additional warming beyond what the emissions-driven “control knob” alone would suggest. Natalie Mahowald of Cornell University said the 1.5 degrees Celsius goal is not just a number and pointed to the international consequences of missing it, including impacts on small island developing states. “Some of them will go underwater,” Mahowald said, adding that those countries would likely suffer most from the higher warming.
Bill Hare, CEO of Climate Analytics, said the updated scenarios reinforce a central constraint: politics has not matched the needed pace of emissions cuts. “This is just physics,” Hare said. “We’re losing the ability to limit warming even by two degrees without strong action and people need to be aware of that and be aware that it’s a political failure.” He said it is not an act of God, but rather a result of governments in many places not moving quickly enough.
Finally, scientists said the newest scenario set still carries an “asterisk” because it focuses on emissions from burning fossil fuels—the part that humans can control—while leaving open the size of additional warming that could come from climate feedbacks. They said scientists have struggled to project those feedbacks, which can include releases of heat-trapping carbon stored in oceans and forests, as well as changes to ocean currents and cloud reflectivity. Rockström said those feedbacks could amount to another half a degree Celsius of warming on top of the warming directly caused by emissions, underscoring why the Paris goal remains out of reach under current conditions even as the worst-case outcomes are considered less probable than before.