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How a key number in that new deal to curb refrigerating chemicals was born

When world leaders reached a deal last month in Kigali to curb the use of hydrofluorocarbons (HFCs)—planet-warming chemicals widely used in air conditioners and refrig­erators—many boasted the move would prevent nearly 0.5°C in warming by 2100. That is a big number, given that the Paris climate agreement aims to keep total warming to less than 2°C. If the HFC number is correct, it will make it easier for nations to achieve the Paris goal.

But there’s a bit more scientific uncer­tainty surrounding that half-degree claim than the politicians let on. The figure has its origins in a 2006 dinner held by five scien­tists in a village in the Swiss Alps. The U.S. and European researchers, who work for gov­ernment and industry, were part of a group that advises policymakers on the Montreal Protocol, the 1987 pact that curbed the use of chemicals that harm the ozone layer. The re­searchers found that the protocol also helped reduce global warming, because some of the regulated chemicals were potent greenhouse gases. But they realized the pact had a warm­ing downside, too, says David Fahey, a physi­cist at the National Oceanic and Atmospheric Administration’s Earth System Research Lab­oratory in Boulder, Colorado. That’s because some of the newer, ozone-friendlier chemi­cals that the protocol thrust into use, such as HFCs, trap heat thousands of times more effectively than carbon dioxide. Soon, the researchers were trying to figure out what that meant for the planet.

The half-degree estimate was first floated in a 2013 paper co-authored by one of the dinner guests, physicist Guus Velders of the National Institute for Public Health and the Environment in Bilthoven, the Netherlands. The study, published in Atmospheric Chemis­try and Physics, forecast that rising HFC use in the developing world would push global temperatures up by 0.35°C to 0.5°C by 2100.

Those numbers caused a stir, because they were substantially higher than HFC warm­ing forecasts made by other climate models, including those underpinning the massive reports of the Intergovernmental Panel on Climate Change (IPCC). Ultimately, however, they helped galvanize support for the Ki­gali agreement, which aims to cut HFC use by 80% to 85% by 2047. And advocates and negotiators tended to cite the higher, 0.5°C estimate in their public remarks.

That didn’t sit well with Andrew Jones, co-director of Climate Interactive, a promi­nent climate analysis group. A day after na­tions announced the 15 October Kigali deal, Jones wrote a blog post hailing it as “excel­lent news for the climate.” But he cautioned against counting on the full 0.5°C benefit. One reason, he wrote, is that he considers the 2013 paper to be an outlier, because it proj­ects HFC warming that is roughly four times greater than that projected by a model cited by IPCC. “I’m not really buying it,” says Jones, who is in Asheville, North Carolina.

Velders says his team came up with higher warming estimates than IPCC because their model accounts for trends that others don’t, such as the faster-than expected adop­tion of HFCs driven by the Montreal Protocol, and an air-conditioning boom in the devel­oping world. Still, he concedes that forecast­ing HFC use is difficult. If warming prompts greater demand for air conditioners in India, for instance, future HFC impacts could be even greater. His team was careful to clarify the uncertainty, he notes, by presenting a range of forecasts, with 0.5°C at the high end.

More sophisticated models that offer a range of possible futures, such as different patterns of economic growth, could improve such estimates, says climate modeling specialist Steven Smith of the Department of Energy’s Pacific Northwest National Laboratory, who is based in College Park, Maryland. “No slight to their work,” he says of the Velders group’s HFC projection. “They’ve clearly done the best work on this to date.”

But although Velders and other scientists routinely acknowl­edge the uncertainty in their forecasts, “that’s not what poli­ticians do,” says Durwood Zaelke, president of the Institute for Governance & Sustain­able Development in Washington, D.C., which backed aggressive HFC reductions.

For Velders, the Kigali agreement, and the role of the work done by his team, is a point of pride. “The half-degree estimate was the offshoot of a series of papers over a decade by the scientists, research that proved pivotal in the evolving understanding of the ties [Kigali agreement], of course it feels great that people are using your work,” he says.

Now, Velders is offering a single new number: 0.06°C. That is his new estimate of how much warming HFCs will cause by 2100 if the Kigali deal hits its targets.