When nearly 200 countries agreed last October to dramatically reduce their reliance on climate change-causing chemicals called hydrofluorocarbons (HFCs) in air conditioning and refrigeration, the hunt for alternatives was already underway. Now, a new study in the journal Nature Communications that comprehensively explored pure liquid options found only 27 candidates, and problems with all of them.

For this study, researchers spent years reviewing millions of pure liquid candidates to replace the refrigerants widely used in small-scale air conditioning, a major source of greenhouse gas emissions. After accounting for various factors, including their global warming potential (GWP), toxicity, energy efficiency and flammability, the pool of options had narrowed to 27. The most promising of these options were at least slightly flammable, the researchers found. They also suggested additional replacements could come from blending chemicals, a more expensive alternative that some companies were already pursuing.

"There are no perfect options for low GWP refrigerants," said Mark McLinden, the study's primary author and a researcher at the Colorado-based National Institute of Standards and Technology. "There are tradeoffs—and the biggest tradeoff is [between] GWP and flammability."

The refrigerant industry has confronted a similar task before. In the late 1980s, countries signed the international treaty called the Montreal Protocol to end the use of chlorofluorocarbons (CFCs) as refrigerants because they depleted the ozone layer that shields the Earth from harmful ultraviolet radiation from the sun.

That's how hydrofluorocarbons (HFCS) came to largely replace CFCs. But subsequent research revealed they had their own flaw—the short-lived climate pollutants have a large greenhouse gas footprint. Countries signed the Kigali Amendment last fall as an update to the Montreal Protocol, agreeing to sharply reduce their reliance on HFCs by mid-century. Meeting this goal could avoid about half a degree of warming by 2100. The refrigeration industry has supported the Montreal Protocol and its updates, and many companies are doing their own research into alternative technologies.

The Kigali Amendment calls for the phase down of 19 chemicals. This includes the two chemicals in a blend called R-410A that's widely used in small air conditioners. This blended refrigerant has a global warming potential 1,924 times greater than carbon dioxide over a 100-year period. A different refrigerant, R-22, used in some developing countries is both bad for the ozone layer and has a global warming potential 1,760 times greater than carbon dioxide. R-22 is targeted in the Montreal Protocol.

The study's goal was to identify climate-friendly options. The researchers started with a database of 60 million molecules and a search for small-sized ones narrowed the pool down to 184,000. An additional review based on whether the molecules could work efficiently as refrigerants yielded 138 options. The remaining ones were assessed for their toxicity, flammability, global warming potential and other factors.

Of the 27 that made the final cut, the best were still flammable to some degree. Examples include propane, a type of hydrocarbon, and ammonia, an inorganic compound.

McLinden and his colleagues only evaluated pure fluids in this study, but noted the potential for substitutes that could result from blending. The refrigerant industry is already experimenting with some blends for this purpose. This new research confirms their efforts and may offer new clues to guide further research.

"This should give confidence to industry and other stakeholders that the right compounds have been identified and will be part of the solution in pure form, or in blends," Helen Walter-Terrinoni, head of global regulatory affairs at the company Chemours Fluorochemicals, wrote in an email to InsideClimate News.

Compared to pure fluids, however, blends "are more complicated molecules," McLinden said. "They are inevitably going to be more expensive to produce than the simple HFCs they are replacing."

Linden added that this isn't "good news for the U.S. and Europe," which are leading this transition, and "it's even less good news for countries that can't afford the expensive refrigerants."

Under the Kigali Amendment, the United States and Europe are the first to start significantly cutting back their use of HFCs; these countries also agreed to help fund the research and commercialization of alternative technologies. The goal is that by the time developing countries start making big cuts, alternatives will have flooded the market and their prices will have dropped enough to be affordable. It remains an open question whether this will happen.

Moreover, the United States agreed to these commitments under the Obama administration and it's unknown whether President Donald Trump will change course. Although Trump has pledged to roll back domestic climate policies, and has spoken of pulling the U.S. out of the Paris climate agreement, he has not yet indicated his plans for the HFCs agreement.