This story has been updated.

When the world moved to phase out ozone-destroying chlorofluorcarbons, or CFCs, it solved one enormous and urgent environmental problem — but it left behind another. CFCs were bad for the ozone layer and also caused a great deal of global warming to boot. But a key substitute — hydrofluorocarbons, or HFCs — spare the ozone layer but are still powerful greenhouse warming agents.

That’s why diplomats and leading national ministers have assembled in Vienna this week for negotiations under the Montreal Protocol, the treaty that led to the phaseout of CFCs and is now aiming its sights at HFCs. If an amendment to the treaty can be adopted this year, advocates say, it could represent the single largest tangible piece of climate progress in all of 2016.

HFCs are used in refrigerants in car and home air conditioners, as well as in foams, solvents and other products. They are being used more and more — in large part because they are the heirs to the CFC phaseout — and when they get into the atmosphere, they are far more powerful than carbon dioxide at warming the planet.

According to the Institute for Governance and Sustainable Development, which focuses on the issue, the “most abundant and fastest growing” of these gases, HFC-134a, remains in the atmosphere for 13.4 years (not nearly as long as carbon dioxide) but causes 1,300 times as much warming as carbon dioxide does over a span of 100 years. One recent study noted that by 2050, if nothing is done, HFC-134a could add 9 to 19 percent to the warming caused by carbon dioxide.

For the broader group of HFCs, one recent study found that HFC emissions as a whole grew from 198 million tons (as measured in carbon-dioxide equivalents) in 2007, to 275 million tons by 2012.

“The HFCs effect now is very small. The problem with the HFCs is it’s the fastest-growing greenhouse gas,” said Veerabhadran Ramanathan, a climate scientist at the Scripps Institution of Oceanography. “So by banning HFCs, you prevent another disaster downstream. It could be as high as half to one degree [Celsius] by the end of the century.”

Data like these explain why diplomats and leading national ministers have assembled in Vienna this week for negotiations under the Montreal Protocol, the treaty that led to the phaseout of CFCs and is now aiming its sights at HFCs. And signs look positive that a phase-down amendment could happen this year, giving a key boost to climate-change momentum, said Durwood Zaelke, head of the Institute for Governance and Sustainable Development.

“This year we have a tailwind because the parties to the Paris agreement understand that they need the Montreal Protocol success to keep their own ambition going and their own momentum going,” said Zaelke, who spoke by phone from Vienna on Friday.

His institute’s research suggests that phasing out HFCs before they can become more prevalent in the atmosphere can avoid about 100 to 200 billion tons of carbon-dioxide-equivalent emissions by 2050. Moreover, Zaelke notes, there’s a bonus — phasing out HFCs will also require air-conditioner manufacturers to go back to the drawing board and will lead to increased energy efficiency gains in air conditioners. That, in turn, will also take another major chunk out of greenhouse-gas emissions at a time when the world is expected to install enormous numbers of new air conditioners in coming years.

For instance, a study by Lawrence Berkeley National Laboratory last year found that adding 30 percent more efficiency to the world’s future air conditioners, while also switching away from HFCs, could avoid 98 billion tons of carbon-dioxide-equivalent emissions into the atmosphere by 2050.

For all of these reasons, the stars seem to be aligning for major action on HFCs this year. “The Montreal Protocol HFC amendment is now perceived universally in the climate context as the piece that you need to do this year,” Zaelke said. “There’s no disagreement about the value of this issue.”

“The phase out of HFCs will achieve the largest temperature reduction in this century — 0.9 degrees Fahrenheit — of any available policy action,” Paul Bledsoe, a former Clinton administration adviser on climate change, said by email. “It will also eliminate one of the six major greenhouse gases” and reduce “near-term climate impacts.”

Some alternatives to HFCs already exist. Honeywell has already announced that it will use far fewer HFCs in coming years in favor of alternatives with less global warming potential. The list of potential alternative substances is long and varied,and includes hydrocarbons, carbon dioxide, and numerous other chemicals. There are also some HFCs with considerably less global warming potential than the currently used varieties.

The unresolved questions at the center of the current HFC phaseout negotiations turn on the different roles of developed and developing countries, and how to fund a transition, explains David Doniger, who heads the climate and clean-air program at the Natural Resources Defense Council and is also in Vienna for the meeting.

“You have to have commitments for a schedule of reductions of these chemicals from developed countries, another schedule with a little bit of delay for developing countries, and then an agreement on money through which the developed countries help the developing ones with some of the transition costs,” Doniger said. “Those are the main issues.”

But progress looks good, observers say. It helps that President Obama and Indian Prime Minister Narendra Modi recently pledged to pursue a key agreement on HFCs. India is expected to see a dramatic air-conditioning boom in coming decades, which will be accompanied by major HFC growth unless there is a corresponding technological change.

The White House announced Monday that Gina McCarthy, administrator of the Environmental Protection Agency, would be leading a U.S. delegation to Vienna for the high level talks that will commence Friday.

Granted, an amendment to phase out HFCs is not expected to be formally adopted this month in Vienna. Rather, that is more likely to occur at a second meeting, in October, in Kigali, Rwanda, meeting observers say.

If it is successful, then when the parties to the U.N. Framework Convention on Climate Change meet in Marrakesh, Morocco, in November to start the process of putting the Paris agreement into action, they will be riding a wave of accomplishment and be able to think rather optimistically about the work before them. Doniger wrote recently that achieving an HFC phaseout would represent “the biggest climate protection achievement of 2016.”

“The ozone treaty has been effectively a climate treaty also,” he said in an interview. “So it can be another win for the climate from the treaty that saved the ozone layer.”

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