Scientists are discarding both their “worst” and “best” case scenarios for the future warming of the planet, according to a new set of carbon-pollution pathways described in reporting by the Associated Press. The change, climate researchers said, reflects how real-world shifts in energy have made some of the most extreme heating outcomes less probable—while still leaving no plausible route to meet the international goal set in 2015.
The updated projections, which replace two long-standing bookends of climate-policy modeling, narrow the envelope of what could happen by the end of the century. The researchers described the revision as evidence that the “most catastrophic” outcomes are less likely than they once were, but also as confirmation that limiting warming to 1.5 degrees Celsius remains out of reach.
Researchers tied the narrowing of the upper-end futures to changes in how the world powers itself. They said that as the use of green energy such as solar, wind and geothermal has increased, the top end of carbon pollution projections has fallen because those sources do not emit carbon dioxide the way fossil fuels do.
At the same time, the scientists said the changes have not been fast enough to prevent higher warming outcomes from becoming more likely at the lower end of earlier scenario ranges. In the scenario update, the “worst” and “best” futures are reshaped in opposite directions: less probability for some extremes paired with a higher baseline for what the more optimistic case would still allow.
The Paris climate agreement, adopted in 2015, set the goal of limiting warming to 1.5 degrees Celsius since pre-industrial times, and the widely used framing “1.5 to stay alive” has depended on keeping that threshold within reach. The new analysis, as presented through scientists quoted in the Associated Press story, says that even the updated best-case scenario now goes past 1.5°C.
Climate scientist Detlef Van Vuuren of Utrecht University, identified as lead author of the study that lays out future scenarios, said the new proposed worst case scenario has end-of-the-century warming of about 3.5 degrees Celsius. He also said the updated best-case future would be a couple tenths of a degree Celsius warmer than previously theorized, meaning it squeezes past the Paris goal rather than meeting it.
“The 1.5 goal is not just a number,” Cornell University climate scientist Natalie Mahowald said, in remarks included in the Associated Press report. Mahowald described implications for the people who would suffer most from higher warming, including small island developing states facing inundation risk.
Other scientists said the revised futures show a “narrowing” of possibilities rather than reassurance that the highest-risk outcomes are entirely averted. Johan Rockström, director of the Potsdam Institute for Climate Impact Research, said: “There is kind of a narrowing of the futures. It cannot be as bad as we thought, but it cannot be as good as we hoped.”
The report also describes why the best-case scenario still fails to align with the Paris target: carbon pollution persists in the atmosphere for about a century, so even the most optimistic pathways are expected to overshoot 1.5°C. Bill Hare, CEO of Climate Analytics, said the physical basis is that the world is losing the ability to limit warming even to two degrees Celsius without strong action and that it reflects a political failure rather than inevitability “from the weather” itself. “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. It’s not an act of God or anything. It is just because politicians in many places are not acting fast enough.”
The analysis also points to a “middle” pathway that has the world warming to around 3 degrees Celsius by the end of the century, described as roughly the path society is currently on. With the world already about 1.3 degrees Celsius warmer than pre-industrial levels, the researchers said even tenths of a degree matter for ecosystems and intensify hazards including floods and heat waves.
While some debate has focused on earlier “high-end” scenarios used in climate projections—particularly the coal-heavy pathway researchers have long referred to as RCP8.5—scientists in the new work said that narrowing the extremes changes what should be treated as plausible. American Enterprise Institute’s Roger Pielke Jr. said changes to the highest end scenario matter because it was presented as a likely future that could happen if nothing changed, while Keywan Riahi, the lead author of the 2011 study that introduced RCP8.5, said the scenario “was never a likely case,” describing it as a plausible higher bound rather than the most likely outcome.
The Associated Press report said President Donald Trump also weighed in on social media, calling the United Nations “TOP Climate Committee” admission about RCP8.5 projections “WRONG! WRONG! WRONG!” Van Vuuren and other scientists responded in the report that the risks of climate change have not disappeared even if some of the most dramatic emissions pathways have become less likely.
A remaining uncertainty in the projections, the story said, involves climate “feedbacks”—processes in the climate system that humans cannot directly control. Scientists said feedbacks can add additional warming beyond what emissions alone would cause, including heat-trapping carbon released from oceans, forests and the Amazon as well as changes to ocean currents and cloud reflectivity. Rockström said these feedbacks could add around another half a degree Celsius of warming on top of emissions-driven estimates, keeping an “asterisk” on how close the older high-end temperature estimates could still come to reality.