We are losing the capacity to limit warming to even two degrees
Worst-case warming of 4.5°C by 2100 is now considered implausible; new scenarios cap maximum warming at 3.5°C due to renewable energy expansion and policy progress. Even best-case scenarios exceed Paris Agreement targets; current trajectory points to 3°C warming by century's end, with world already 1.3°C above preindustrial levels.
- Worst-case warming scenario reduced from 4.5°C to 3.5°C by 2100
- World already 1.3°C above preindustrial levels; best case peaks at 1.7°C
- Middle trajectory points to 3°C warming by century's end
- Solar and wind costs fell approximately 90% over the past decade
Scientists have eliminated extreme climate scenarios as implausible due to renewable energy advances and slowing emissions growth, but confirm the 1.5°C Paris target is now unattainable, with warming likely reaching 1.7°C at best.
The worst-case climate scenario that haunted scientific models for over a decade has been retired. The catastrophic 4.5-degree warming projection—the one that assumed humanity would double down on coal, ignore renewable energy, and let emissions spiral unchecked—is no longer considered plausible. Scientists have quietly shelved it, along with the most optimistic projections, leaving a narrower band of futures that are neither as dire as feared nor as manageable as hoped.
This shift reflects genuine progress. Solar and wind costs have collapsed by roughly 90 percent over the past decade. Renewable energy deployment has accelerated. Global emissions growth has slowed. These are not trivial achievements. They have pushed the upper bound of warming down by a full degree Celsius, from 4.5 to 3.5 degrees by the end of the century. But the progress has come too late and too slowly to change the fundamental trajectory. The world is already 1.3 degrees Celsius warmer than it was before industrialization. Even in the most optimistic scenario scientists now consider realistic, warming will peak at around 1.7 degrees—overshooting the Paris Agreement's 1.5-degree target by a margin that once seemed impossible to breach.
The 1.5-degree limit was never just a number. When nations agreed to it in Paris in 2015, they were drawing a line between manageable climate change and something far more dangerous. Natalie Mahowald, a climate scientist at Cornell University and contributor to a United Nations assessment on the consequences of exceeding that threshold, put it plainly: the nations that will suffer most are small island developing states. Some of them will be submerged. Tenth-of-a-degree increments matter when your country is measured in meters above sea level.
The retirement of the extreme scenario has become politically fraught. The old worst-case projection, known as RCP8.5, was based on outdated energy assumptions and assumed sustained, intensive coal use alongside high population growth and minimal technological innovation. It was always presented as a ceiling—a plausible upper bound for exploring maximum impacts, not a likely outcome. Yet thousands of scientific studies built their projections around it. When Donald Trump saw the scenario being phased out, he celebrated on social media, claiming the most alarming climate warnings had been wrong all along. The science, he suggested, had been exaggerated.
But the scientists involved in the shift tell a different story. Keywan Riahi, who led the 2011 study that introduced RCP8.5, emphasized that it was never meant to be probable. It was a boundary condition—a way to ask what the absolute worst impacts might look like if humanity made the worst possible choices. As conditions changed, as renewable costs plummeted and policies began to take hold, the scenario became increasingly implausible. By 2017, academic research was already calling it "exceptionally unlikely." The scientific community eventually caught up to that reality.
Yet the narrowing of possibilities is not cause for relief. The middle-ground scenario—the one that roughly tracks where the world is actually headed—points to three degrees of warming by 2100. That is a world of intensified heat waves, more severe flooding, accelerating species extinction, and freshwater scarcity spreading across regions already stressed by drought. Bill Hare, director of Climate Analytics, was blunt about what the shift means: "This is pure physics. We are losing the capacity to limit global warming to even two degrees without drastic action, and people need to understand that this is a political failure, not a miracle or anything else. It is simply because politicians in many places are not acting fast enough."
The uncertainty cuts both ways. Climate feedback mechanisms—the ways the natural world responds to warming in ways humans cannot control—remain poorly understood. The release of carbon trapped in oceans, forests, and the Amazon; shifts in ocean currents; changes in cloud reflectivity: these could add another half degree of warming on top of what emissions alone would produce. The world is warming at roughly one-tenth of a degree Celsius every five years. Even if emissions stopped tomorrow, the carbon already in the atmosphere would continue warming the planet for a century. The best realistic scenario requires not just stopping emissions but removing vast quantities of carbon from the air—a technology that does not yet exist at scale. Johan Rockström, director of the Potsdam Institute for Climate Impact Research, captured the strange new reality: "There is a kind of narrowing of future possibilities. It cannot be as bad as we thought, but it also cannot be as good as we hoped."
Notable Quotes
We are losing the capacity to limit global warming to even two degrees without drastic action, and people need to understand that this is a political failure.— Bill Hare, director of Climate Analytics
There is a kind of narrowing of future possibilities. It cannot be as bad as we thought, but it also cannot be as good as we hoped.— Johan Rockström, director of the Potsdam Institute for Climate Impact Research
The Hearth Conversation Another angle on the story
So scientists are saying the worst-case scenario won't happen. That sounds like good news.
It is, in a narrow sense. We're not headed toward 4.5 degrees of warming. But the reason is not that we solved the problem—it's that we made just enough progress to make the absolute worst outcome implausible. We're still headed toward something genuinely dangerous.
What's the actual trajectory now?
Best case, we peak at about 1.7 degrees. Worst case, 3.5 degrees. The middle path, where we're actually headed if current policies hold, is around 3 degrees by the end of the century. We're already at 1.3 degrees.
And 1.5 degrees was the target everyone agreed to in Paris.
Right. It was supposed to be the line between manageable and catastrophic. Now it's clear we're going to cross it. For small island nations, that's not an abstract policy failure—it's existential. Some of them will disappear.
Why did scientists use that extreme 4.5-degree scenario for so long if it was always unlikely?
It was useful as a boundary condition. You model the worst plausible case to understand the maximum impacts. But over time, the assumptions it was built on—sustained coal dependence, minimal renewable adoption—stopped matching reality. By the time scientists formally retired it, it had already become implausible.
And now politicians are saying the warnings were exaggerated?
That's the trap. The scenario was never meant to be probable. But because it was used in thousands of studies, it shaped how people understood climate risk. Retiring it doesn't mean the problem is smaller—it means we've narrowed the range of futures, and almost all of them are still very difficult.