Scientists Ditched a Scary Climate Scenario. What Now?

It’s rare for technical papers about climate modeling to kick off a heated public debate, or attract attention from the White House.

But that’s what happened recently after an international team of researchers published a major revision of the emissions scenarios used to study global warming.

When scientists try to model how hot Earth could get this century, they typically look at a range of possibilities for how much planet-warming pollution humans might pump into the atmosphere. These scenarios get updated every seven years or so.

In this latest update, the researchers abandoned a dire — and often criticized — high-emissions scenario known as RCP8.5 that has been prominently cited in thousands of climate studies over the past decade. The authors said the scenario was now “implausible” given recent energy trends.

That provoked online arguments among scientists. For years, critics of the high-emissions scenario had argued that it was always unrealistic, in part because it envisioned that countries would burn coal at absurdly high rates. They argued that any studies or news reports relying on that scenario exaggerated the risks of climate change. Why, those critics now asked, did the course correction take so long?

Other researchers, however, noted that scientists still can’t rule out extreme warming, even if the odds might be low, and that there are good scientific reasons for studying high-emissions scenarios.

Last weekend, President Trump weighed in, suggesting that the revision showed that global warming wasn’t a threat and that “the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!”

The majority of climate scientists still say global warming is a serious problem, and that even more plausible, medium-emissions scenarios can carry grave dangers. But the new paper has raised questions about whether some of the risks of climate change have been poorly communicated or overstated in years past and how best to think about those risks going forward.

The origins of a dire scenario

How much hotter the world will get depends on two big factors: how many greenhouse gases humans will emit by burning fossil fuels and cutting down forests, and how the Earth’s climate will physically respond.

Predicting emissions over the next century is extremely difficult, since so much depends on future economic growth and technological changes. So scientists try to consider a wide range of possibilities.

In the early 2010s, climate scientists created a suite of standardized low-, medium- and high-emissions pathways. The goal, they said, was to capture every plausible outcome, even unlikely ones. The highest one, RCP8.5, envisioned annual carbon dioxide emissions roughly tripling this century.

All the scenarios, once fed into complex computer models of Earth’s climate, project significant future warming, leading to increased heat waves, sea-level rise, wildfires, droughts and other changes. Yet RCP8.5, the high-emissions scenario, leads to the most dire outcomes by far.

It envisions Earth’s average temperature rising somewhere around 4.4 degrees Celsius, or 8 degrees Fahrenheit, by 2100, compared to preindustrial levels. By contrast, under RCP4.5, an intermediate scenario, models suggest that Earth would warm roughly 2.7 degree Celsius. That’s a big difference, since every degree brings substantially more severe effects.

Some studies have found that under RCP8.5, large swaths of the planet might regularly experience days hotter than the human body can tolerate. The risks of catastrophic “tipping points” such as widespread rainforest die-offs or the disintegration of polar ice sheets, are also far greater.

But there has long been confusion about what RCP8.5 represented and how it should be used.

The high-emissions pathway wasn’t meant to be a prediction, but more of a “worst case,” said Detlef van Vuuren, a climate scientist at Utrecht University and a leading figure in scenario development. “It’s simply exploring what happens if we end up with much higher emissions than we expect.”

Many scientists find it useful to probe such hypotheticals. Climate models often struggle to capture dynamics like whether ice sheets might collapse with moderate warming, even though this is an extraordinarily important question for the real world. So scientists will often use a high-emissions scenario in models to establish an upper bound for, say, how much sea levels might rise.

“A lot of climate research is about understanding how Earth’s climate system behaves under certain conditions rather than a statement about what’s most likely to happen in the future,” said Robert Kopp, a climate scientist at Rutgers University. “They’re both important, but those are two different questions.”

That distinction, however, has often been lost.

Many scientific studies have wrongly referred to RCP8.5 as a “business-as-usual” scenario, suggesting that this was the pathway humanity is currently headed for. News stories about climate research often emphasized results based on RCP8.5 as a picture of what the world can expect unless countries slash their emissions, which isn’t right, either.

One study in Science, for example, looked at how rising temperatures could cause lower agriculture yields or heat deaths in the United States, hurting the economy. While that study explored numerous harmful scenarios, the highest estimated damages based on RCP8.5 were a big focus and got more attention, including in The New York Times. (Many other climate stories have been more careful, highlighting how more plausible medium-emissions scenarios could affect, say, extreme heat or flooding.)

Some scientists soon began questioning what they saw as a disproportionate focus on the high-emissions scenario.

In 2017, a study by Justin Ritchie and Hadi Dowlatabadi pointed out that RCP8.5 envisioned a fivefold expansion in global coal use, including fueling cars with coal-based liquids, which they explained was “exceptionally unlikely.”

Another paper in Nature in 2020 argued that recent global energy trends, including the falling cost of renewable energy, made the high-emissions pathway “increasingly implausible” and that medium-emissions pathways seemed much likelier. The authors warned that RCP8.5 should not be cited as “business as usual.”

Yet many policymakers and researchers continued to emphasize the high-emissions scenario for years afterward, said Roger Pielke, Jr., a senior fellow at the American Enterprise Institute and a leading critic of the use of RCP8.5. Central banks used it to test whether financial institutions are prepared for the effects of climate change. Governments often use it for adaptation planning, and insurance companies use it to set rates.

“We’ve known for a decade that this scenario is implausible, and that the community got off on a bad track very early on when this scenario set was created,” Dr. Pielke said.

If the past 15 years of climate messaging had been based on a more realistic scenario instead, he added, “it wouldn’t make climate change go away, but it would certainly make climate change look far less apocalyptic, it would look more like a significant risk with serious consequences.”

Many on the political right, including Trump administration officials, have cited the misuse of RCP8.5 as a reason to dismiss climate science as overly alarmist. Matthew Burgess, an environmental economist at the University of Wyoming, said that scientists, in turn, were often overly defensive about the issue, which fed into polarization.

Some scientists now say the scenario was often badly communicated.

“RCP8.5 was never meant to be a business-as-usual scenario, but that point often got lost as it was used by more and more people,” said Joeri Rogelj, a climate scientist at Imperial College London.

A shift to new scenarios

The new emissions scenarios, published last month, are an attempt to create better inputs for the next generation of climate models.

The authors said they discarded the RCP8.5 scenario because it had “become implausible” thanks to actions countries have taken to tackle climate change. (Some experts dispute that rationale, saying that RCP8.5 was never plausible.) While the authors do propose new high-emissions scenarios that could lead to similarly high estimated levels of warming later in the 22nd century, they’ve added a warning that these are not business-as-usual pathways.

The researchers also discarded some very low-emissions scenarios because nations are unlikely to slash their fossil-fuel use as deeply as many world leaders had urged. In effect, the scenarios are becoming less pessimistic but also less optimistic.

In addition, the researchers included a medium-emissions outlook that is explicitly meant to reflect current government policies. That scenario leads to roughly 2.6 degrees Celsius of global warming by 2100, compared to preindustrial levels.

That’s lower than the temperature forecasts from the old RCP8.5 scenario. But experts warn that even moderate levels of warming can pose significant dangers. For more than a decade, world leaders have pledged to keep warming “well below” 2 degrees Celsius to avoid what they deemed unacceptable risks. (The planet has already warmed roughly 1.3 degrees since preindustrial times.)

“It’s good news that we can drop the highest-emissions scenario,” said Dr. Rogelj. “But the other side is that we’re also finding that the risks for lower levels of warming are often worse than we thought.”

For example, at 2 degrees Celsius of warming, the world could lose most of its coral reefs, and an additional 410 million people in urban areas could face water scarcity because of severe drought, according to a recent U.N. climate assessment. At 3 degrees, global flood damages could increased threefold without adaptation, while more than one-quarter of known plant and animal species on land could face a high risk of extinction.

There is also still plenty of uncertainty around exactly how much Earth will heat up from a given level of greenhouse gases in the atmosphere. That means the world could still get even hotter than models project, even if the particular assumptions about coal-burning in RCP8.5 don’t come to pass.

“There is how all the various feedbacks in the climate system, like clouds, snow, ice cover and water vapor in the atmosphere, respond to warming,” said Dr. Hausfather, a climate scientist at Berkeley Earth. “If you put all of those together, you really get a pretty wide range of possible outcomes this century, even under a medium-emissions scenario.”

Despite the new scenarios, it’s not feasible for scientists to quickly go back and redo older studies, experts said, since the complex computer climate models that simulate Earth’s atmosphere and oceans take months to run.

But researchers say it would be imprudent to dismiss all past studies that looked at RCP8.5. Many climate risks scale linearly with temperature, so that research can still provide insights into a lower-emissions world. Some recent work also suggests that even medium-emissions scenarios can lead to extreme impacts, such as severe droughts in food-producing regions, that were previously thought only to occur in high-emissions scenarios.

“If you are working on local adaptation and trying to figure out how high to build a sea wall, you certainly want to consider what the worst that can happen is,,” said Madison Condon, an associate professor of law at Boston University who works on climate adaptation.