Climate scientists are retiring a worst-case global warming scenario that has guided research and policy for more than a decade, saying modest gains in the fight against climate change have made the most catastrophic outcome implausible — but experts warn the picture is still far from rosy.
A new report shows that increasing use of renewable energy has lowered the ceiling on future carbon pollution projections, making the most extreme warming scenario no longer plausible. However, scientists say the gains have not come fast enough to meet the international climate goal set in 2015.
Dr. Max Boykoff, a professor in the Environmental Studies department at the University of Colorado Boulder and a Fellow in the Cooperative Institute for Research in Environmental Sciences, said the pledges countries made in the 2015 Paris Agreement appear to be working — with some notable exceptions.
"It is potentially seen as a good news story. That the commitments in the Paris agreement, barring the United States and a few other countries, put us on a path where it's more likely that we're actually going to be reducing admissions more rapidly, more capably than that scenario suggests," Boykoff said.
But Boykoff was clear that the news is not entirely positive.
"As things continue it's not an entirely good news story. It's just not quite as bad as we once thought it could be," Boykoff said.
The Paris Agreement set a goal of limiting global warming to 1.5 degrees Celsius since pre-industrial times, or the mid-1800s. Scientists now say that even their best-case scenario shoots past that benchmark.
Researchers have proposed a new list of seven plausible carbon pollution scenarios for the future, pushing aside the extreme projections on either end that have long anchored climate policy. The updated worst-case scenario projects end-of-the-century warming of about 3.5 degrees Celsius (6.3 degrees Fahrenheit) — a full degree Celsius (1.8 degrees Fahrenheit) less than the old worst-case scenario, which projected 4.5 degrees Celsius (8.1 degrees Fahrenheit) of warming by 2100. That older scenario, known as RCP8.5, was based on a coal-heavy energy future that scientists now say is out of date.
"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. This is very different than if you would ask the question, what is now the most likely scenario," said Keywan Riahi, director of the Energy, Climate and Environment Program at the International Institute for Applied Systems Analysis in Austria and lead author of the 2011 study that introduced the scenario.
Riahi called the shift a success story, noting that "in the last 10 years or the last 15 years, the cost of renewables, particularly solar and wind, have fallen by almost 90%."
The updated best-case scenario is a couple tenths of a degree Celsius (0.36 degrees Fahrenheit) warmer than previously projected, squeezing past the Paris goal, said climate scientist Detlef Van Vuuren of Utrecht University, lead author of a recent study laying out the new scenarios.
"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," said Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany.
A "middle" scenario projects warming of 3 degrees Celsius (5.4 degrees Fahrenheit) above pre-industrial times by the end of the century — roughly the path the world is currently on, scientists said. The world is now about 1.3 degrees Celsius (2.3 degrees Fahrenheit) above pre-industrial times. Even tenths of a degree of warming cause problems for Earth's ecosystems, as species die off, fresh water becomes more scarce and extreme weather events such as flooding and heat waves intensify.
Because carbon pollution keeps rising globally and stays in the atmosphere for about a century, the best-case scenario is for warming to shoot past the 1.5-degree mark, peak at 1.7 degrees Celsius (3.1 degrees Fahrenheit) for as long as 70 years, and eventually come back down below 1.5 degrees if technology can be developed to remove massive amounts of carbon from the air, said nine of the 10 scientists interviewed for the study. The world is warming at a pace of a tenth of a degree Celsius (nearly 0.2 degrees Fahrenheit) every five years.
"This is just physics," said climate scientist Bill Hare, CEO of Climate Analytics, a policy institute. "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."
Cornell University climate scientist Natalie Mahowald, co-author of a U.N. science report detailing the harms of exceeding 1.5 degrees of warming, said the stakes are high for vulnerable communities.
"There's a lot of implications for, you know, not being able to meet the 1.5. And, of course, the people who will suffer the most are on the small island developing states," Mahowald said. "Some of them will go underwater."
American Enterprise Institute's Roger Pielke Jr. said changes to the highest-end scenario matter because it was long presented as a likely future. Thousands of scientific studies have been based on RCP8.5, even though research had already shown it to be improbable.
"It was always presented as where we were headed absent explicit climate policy," even though it was based on out-of-date and incorrect coal-heavy energy theories, Pielke said in an email.
President Donald Trump responded to the news on social media, writing: "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 pushed back on that characterization.
"The risks of climate change have not disappeared," Van Vuuren said. "The good news is that we did not follow the most dramatic emission pathway. However, we are still heading towards a future with significant climate impacts; a future we should avoid."
Scientists also cautioned that the new scenarios only account for emissions from burning fossil fuels — the factor humans can control. Natural climate feedbacks, which humans cannot control, could add another half a degree Celsius (nearly a degree Fahrenheit) of warming on top of emissions-driven projections. Those feedbacks include the release of heat-trapping carbon stored in the world's oceans, forested areas and the Amazon, along with changes to ocean currents and cloud reflectivity.
President Trump withdrew the U.S. from the Paris Agreement when he took office for the second time. The U.S., Iran, Libya and Yemen are now the only countries not abiding by the climate pledge.
This story was reported on-air by a journalist and has been converted to this platform with the assistance of AI. Our editorial team verifies all reporting on all platforms for fairness and accuracy.
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