“Some of them will go underwater,” said Natalie Mahowald, a Cornell University climate scientist, of the small island developing states. She co-authored a U.N. science report on the harms of passing 1.5 degrees of warming, and she was speaking about what the latest climate scenarios mean for the most vulnerable places.
Those scenarios arrived with a headline change. Scientists have retired the worst-case warming path that has guided research and policy for more than a decade, concluding that modest gains against climate change have made the most catastrophic outcome implausible. Denver7 reported that a new report ties the shift to the rising use of renewable energy, which has lowered the ceiling on projected carbon pollution.
Seven new paths
Researchers proposed a list of seven plausible carbon pollution scenarios, setting aside the extreme ends that long anchored climate policy. The new worst case projects about 3.5 degrees Celsius (6.3 degrees Fahrenheit) of warming by the end of the century. The old one, known as RCP8.5, projected 4.5 degrees Celsius (8.1 degrees Fahrenheit) by 2100 and rested on a coal-heavy energy future that scientists now call out of date.
A “middle” scenario shows 3 degrees Celsius (5.4 degrees Fahrenheit) above pre-industrial levels, roughly where the world is heading today. The planet is now about 1.3 degrees Celsius (2.3 degrees Fahrenheit) above that baseline and warms by a tenth of a degree Celsius every five years.
The best case got worse. Detlef Van Vuuren of Utrecht University, lead author of a recent study on the new scenarios, said it runs a couple of tenths of a degree Celsius warmer than before, slipping past the Paris goal of 1.5 degrees. Nine of the 10 scientists interviewed for the study said the best outcome would see warming peak at 1.7 degrees Celsius (3.1 degrees Fahrenheit) for as long as 70 years, then fall back below 1.5 if technology can pull large amounts of carbon from the air.
Reactions from scientists
Keywan Riahi, lead author of the 2011 study that introduced RCP8.5, said the scenario “was never a likely case.” He called the change a success story, pointing out that the cost of renewables, particularly solar and wind, has fallen by almost 90% over the last 10 to 15 years.
Max Boykoff of the University of Colorado Boulder said it could be seen as good news that the Paris commitments, “barring the United States and a few other countries,” seem to be working. His caveat: “It’s just not quite as bad as we once thought it could be.” Johan Rockström of the Potsdam Institute for Climate Impact Research put it as a narrowing of futures. “It cannot be as bad as we thought, but it cannot be as good as we hoped.”
Bill Hare, CEO of Climate Analytics, was harsher. “We’re losing the ability to limit warming even by two degrees without strong action,” he said, calling it a political failure.
Roger Pielke Jr. of the American Enterprise Institute said the change matters because RCP8.5 was long presented as where the world was headed without explicit climate policy. Thousands of studies used it, he noted, even though research had already shown it was improbable.
Trump and other views
President Donald Trump responded on social media: “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. “The risks of climate change have not disappeared,” he said.
The Center Square, writing about the IPCC’s decision, found a split among analysts. Travis Fisher of the Cato Institute called the assumptions “implausibly, absurdly ridiculous” from the start. Writers at the Center for Progressive Reform argued RCP 8.5 stays a useful tool for understanding impacts ahead, even as the emissions path it represents becomes less likely.
The scientists also cautioned that the new scenarios count only emissions from burning fossil fuels. Natural feedbacks, such as carbon release from oceans, forests and the Amazon, could add another half a degree Celsius.
Sources
- Climate scientists drop worst-case warming scenario, but say it's not entirely good news
- Climate science without a notorious worst-case scenario – The Daily Bo Snerdley
- Trump celebrates after UN climate committee moves away from its most extreme global warming scenario
