Air conditioning and heat pumps are spreading around the world as people try to manage rising heat using technology.
Cooling is increasingly vital to safeguard human health – especially in urban environments in the Global South – and heat pumps can also aid the fight against climate change, often by replacing fossil-fuelled boilers.
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This story is part of Dialogue Earth’s work on the Community Adaptations to City Heat (CATCH) project, in partnership with Boston University. The project is funded by Wellcome. All Dialogue Earth content is editorially independent.
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No expert Dialogue Earth spoke to is opposed to their roll out, but some researchers worry there may be a hidden cost to the advance. These technologies rely on refrigerant chemicals, which, if they end up in the atmosphere, can become greenhouse gases with warming effects thousands of times greater than carbon dioxide.
Researchers say more attention needs to be paid to what exactly is being used, how it is working and how it is disposed of. In some places, simple measures like painting roofs white can reduce the need for cooling in the first place, they say.
“The human enterprise is industrialised to the extent that we can actually change large infrastructure surprisingly quickly,” says Eric Kort, director of the atmospheric chemistry department at Germany’s Max Planck Institute for Chemistry. “That can be a great benefit for humanity, but it also can have impacts if it’s not done with proper considerations,” he adds.
“What might the consequences be? And how can we be in front of that, so we don’t inadvertently cause a bunch of environmental problems we were not aware of in advance?”
The global picture on cooling
Air conditioning units and heat pumps use compounds called refrigerants to move heat from one part of the system to another. An AC unit might shift heat from inside a home to outside in summer, while a heat pump does the opposite in winter.
Heat pumps can produce heat and cooling five times more efficiently than traditional gas boilers. They can be powered with renewable energy, and are often more cost-effective. This is driving global adoption, often fuelled by government grants. Annual sales in the EU could rise to 7 million each year by 2030, up from 2 million in 2021 according to a report from the International Energy Agency.
Overall, the global stock of cooling equipment, including ACs and heat pumps, could triple by 2050, driven by rising incomes in low- and middle-income countries and intensifying heat, according to a 2025 report from UNEP’s Cool Coalition network of experts.
“I see an uptick in the use of ACs and heat pumps as broadly a good thing. Heat pumps almost always replace the combustion of fossil fuels, which is going to be better for climate and health,” says Yannai Kashtan, an air quality scientist at the California-based research and policy institute PSE Healthy Energy.
“But as we see increased adoption globally, especially in the Global South, we should make sure that the refrigerants powering them are environmentally friendly, inexpensive, standardised and safe. Which is not the case right now.”
Chemical choice and atmospheric impact matters because refrigerants leak. One 2014 report on heat pumps in the UK found an annual leakage rate of about 3.5%.
Chemicals of concern
There may be a lesson from history to be learned: when chlorofluorocarbons (CFCs) were introduced as refrigerants in the 20th century, they ended up accidentally destroying significant amounts of Earth’s protective ozone layer.
The hundreds of refrigerants in use today are the result of decades of environmental regulation. This systematically replaced ozone-destroying chlorofluorocarbons (CFCs) with somewhat safer hydrochlorofluorocarbons (HCFCs), and then with the hydrofluorocarbons (HFC) commonly used in heat pumps and aircon units today.
HFCs do not deplete atmospheric ozone and they linger for less time in the atmosphere. “But they’re very high global warming potential gasses,” says Genevieve Plant, a researcher at the University of Michigan who specialises in greenhouse gas observations.
Common HFC refrigerant difluoromethane, also called R-32, has a global warming potential of 675, meaning that over a 100-year timescale one kilogram causes 675 times the warming of a kilo of CO2. “Methane has 28. So HFCs are very, very strong. [R-32 is] one of the lower ones, too,” says Plant. Over the same timescale, the HFC blend R-410A has a warming potential of over 2,000.
Another generation of refrigerants called hydrofluoroolefins (HFOs) has been introduced as having lower global warming potential. But many of these have the added problem of being per- and polyfluoroalkyl substances or ‘PFAS’, also known as ‘forever chemicals’ because they accumulate in animals, with possible cancer-causing effects in humans.
Some may also break down in the atmosphere into potent warming gases. Hydrocarbon-based refrigerants like propane, isobutane and even carbon dioxide could potentially be used. Propane, for example, has a 100-year global warming potential of just three, although there are some concerns about flammability.
Are the refrigerants out there?
Data on how much refrigerants are reaching the atmosphere is limited but growing. International monitoring networks suggest HFC emissions have been rising steadily since they were phased in during the mid-1990s.
Work published earlier this year shows HFC levels increasing in China between 2011 and 2021. Plant has published work showing halocarbons – a group of compounds that includes HFCs – in samples taken from the atmosphere above New York City.
The amounts detected were greater than official available estimates, likely because leakage was too, Plant explains. Her work concluded the detected halocarbons would produce a greenhouse gas effect comparable to all the methane detected in the same air samples.
It is often not possible to trace refrigerant pollution back to its origin. But Plant’s study provides some clues.
Kort, who worked with her on this study, said they found a “suggestive result” of higher emissions of gases used in AC when the weather was warmer.
That data is limited and not conclusive, but it does raise questions around the impact that refrigerant-based cooling appliances could have, especially where they are in more frequent use in hot locations.
Use may not even be the biggest emissions source, with leakages also occurring during production, transport and when equipment reaches the end of its life. Even where venting refrigerants into the air is illegal, it often happens anyway, as safe disposal can be difficult and expensive.
Kashtan says this should not dissuade people from switching from traditional ACs or gas furnaces to more efficient heat pumps. But additional load is the real concern, he says. As global cooling demand ramps up, there will be more refrigerant emissions.
Without better data on the problem, researchers are concerned that very significant climate impacts may be overlooked. This could even create a feedback loop where leakages fuel higher temperatures which then drive increased cooling demand that further increases leakages, Kort and Plant say in a paper on their work.
More than the emissions
“I think the heat pumps have been pushed as a universal, clean, sustainable, climate-friendly solution. And I don’t know that there’s been quite enough consideration of all of the dimensions,” says Lucy Hutyra, a researcher at Boston University who studies the terrestrial carbon cycle.
Hutyra notes that in places experiencing temperature extremes like icy weather, heat pumps have to work harder to warm houses, which uses more electricity at greater cost. When it comes to cooling, meanwhile, her own research has shown that in some places simple white-roofing measures can be very effective at reflecting heat and cooling interiors.
The Global Cooling Pledge, a 75-country strong UN initiative launched at the COP28 climate conference, aims to limit cooling-related emissions by 68% by 2050, stressing passive cooling approaches like white-roofing and improved building design. It also notes the importance of refrigerants with low global-warming potential.
For most researchers Dialogue Earth spoke with, the question is not whether we should use them, but how to manage their impacts and solutions, starting with more research on refrigerant leakage from ACs and heat pumps.
There are still many unanswered questions, from where the hotspots are to whether the idiosyncrasies of heat pumps result in higher emissions than ACs.
We are not paying anywhere near enough attention to something that is importantLucy Hutyra, researcher at Boston University
“My read on the literature is that we are not paying anywhere near enough attention to something that is important. We have a very, very poor handle on it,” says Hutyra.
“I’d like to see research across the whole life cycle, of leakage at the time of installation, through operation, and at disposal. I think we need to not just have manufacturer-provided numbers.”
More data on the origin and scale of refrigerant emissions could then guide solutions, whether those are tighter maintenance plans to ensure these systems emit less, or stricter regulations and incentives to encourage their proper disposal at end-of-life.
It is unlikely leakage can be stopped completely, says Kort.
“Past experience suggests that in the real world, it’s hard to make things as leak-tight as we would like. And so then it becomes a question of, what are the impacts of these different gasses, and what trade-offs are we considering there?”
