Scientists retired climate's old doomsday case, but the best-case future slipped away, too

Climate modelers are dropping the once-notorious RCP8.5 pathway from their standard set of future warming scenarios, reflecting the view that its assumptions no longer match the world's likely direction. But the same update carries a harsher message: Planners also can no longer treat the most hopeful climate future as a realistic benchmark.

As E&E News reported, the latest round of scenario work removed RCP8.5 while also recognizing that keeping warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels without first exceeding it is no longer a realistic assumption for planning.

What happened?

The change affects the scenarios scientists use to estimate how the climate may develop over time.

RCP8.5 had long filled the role of the far-end danger case, based on very large fossil fuel growth and pollution high enough to bring about roughly 5 C (9 F) of warming by 2100.

Researchers now say that the storyline is too improbable to carry forward into the next major modeling cycle. Renewable energy has grown, and governments have adopted more climate policies than those earlier assumptions included.

That does not mean scientists replaced it with a new best-case future.

Instead, modelers created a new upper-end pathway that reaches similar warming around 2150 rather than 2100, and they also removed the old lowest-warming pathway.

Detlef van Vuuren, who leads the international committee behind the climate scenarios used by modelers worldwide, told E&E News that the most optimistic remaining pathway now "overshoots 1.5 and goes first around 1.7, 1.8, and then comes back."

The update also drew attention after President Donald Trump praised the retirement of the old scenario on Truth Social, calling its projections "climate alarmism."

Why does it matter?

Removing one extreme pathway does not make the changing climate less dangerous.

What has changed, scientists say, is the range of futures still considered plausible. The bleakest outcome looks less likely than it did 15 years ago, but so does the safest one.

Governments, researchers, and communities use these scenarios to plan for climate impacts.

As E&E News noted, current policies still leave the world on track for as much as 3 C (5.4 F) of warming by century's end, a level that could reshape daily life across the globe.

Jennifer Morris of MIT's Center for Sustainability Science and Strategy told the outlet that worst-case scenarios are often misunderstood: "We buy insurance to protect ourselves not from the likely outcomes but from the worst-case outcomes. It's the same thinking."

What are people saying?

Van Vuuren said the update should not be seen as good news.

"As a result, we are in a much worse situation than we were in 15 years ago," he said. "Our ability to still reach the Paris goals is becoming more difficult and more difficult."

Morris said scientists should do more to explain how likely each scenario is, adding, "I'd love to see more of that."

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