Keywan Riahi led the 2011 study that introduced RCP8.5. Now he says the scenario “was never a likely case.”
Riahi, director of the Energy, Climate and Environment Program at the International Institute for Applied Systems Analysis in Austria, described it as a plausible higher bound on emissions given the research of the time. That, he said, is “very different” from asking what the most likely scenario is today. Climate scientists have now retired the scenario. Denver7 reported they did so because modest progress against climate change made the most catastrophic outcome implausible, even as experts warn the picture is far from rosy.
What replaced it
A new report shows that expanding renewable energy has lowered the ceiling on future carbon pollution. Researchers put forward seven plausible scenarios in place of the extremes at either end. The worst now reaches about 3.5 degrees Celsius (6.3 degrees Fahrenheit) of warming by century’s end, a full degree Celsius (1.8 degrees Fahrenheit) below the 4.5 degrees Celsius (8.1 degrees Fahrenheit) in the old version.
Riahi called the shift a success story. Solar and wind costs, he said, have fallen by almost 90% in the last 10 or 15 years.
At the other end, the best case is a bit hotter than before. Detlef Van Vuuren of Utrecht University, lead author of a recent study laying out the scenarios, said it now squeezes past the Paris goal of 1.5 degrees Celsius. The “middle” scenario of 3 degrees Celsius (5.4 degrees Fahrenheit) is roughly the course the world is on, scientists said, and the planet already sits about 1.3 degrees Celsius (2.3 degrees Fahrenheit) above pre-industrial temperatures.
Good news, with limits
Max Boykoff of the University of Colorado Boulder said the Paris pledges appear to be working, with exceptions. “It’s not an entirely good news story,” he said. “It’s just not quite as bad as we once thought it could be.”
Johan Rockström, director of the Potsdam Institute for Climate Impact Research in Germany, summed up the revised range: “It cannot be as bad as we thought, but it cannot be as good as we hoped.”
Nine of the 10 scientists interviewed for the study said carbon pollution is still rising and lingers in the atmosphere for about a century. So the best route would push warming past 1.5 degrees, hold it near 1.7 degrees Celsius (3.1 degrees Fahrenheit) for as long as 70 years, and bring it back down only if technology can remove huge amounts of carbon from the air.
The scenarios track only emissions from burning fossil fuels. Natural feedbacks could add about half a degree Celsius on top. Those include carbon released from oceans, forests and the Amazon, plus shifts in ocean currents and cloud reflectivity.
Roger Pielke Jr. of the American Enterprise Institute argued the old scenario mattered because it was “always presented as where we were headed absent explicit climate policy.” Thousands of studies drew on it.
Trump’s reaction
President Donald Trump wrote 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 disagreed. “The risks of climate change have not disappeared,” he said, adding that the world is still heading toward significant impacts.
Fox News also reported that researchers writing in Geoscientific Model Development called the SSP5-8.5 emission levels implausible. Trump withdrew the U.S. from the Paris Agreement when he began his second term. The U.S., Iran, Libya and Yemen are now the only countries not abiding by the pledge.
Sources
- Climate scientists drop worst-case warming scenario, but say it's not entirely good news
- Trump celebrates after UN climate committee moves away from its most extreme global warming scenario
