Matt Rigby, University of Bristol and Lucy Carpenter, University of York

As temperatures broke records this summer, sales of air conditioning units soared. But this technology can compound climate change.

Air conditioners pump a gas, known as a refrigerant, around a sealed loop to carry heat out of (and sometimes into) the room. Some of that gas inevitably leaks out over the unit’s working life and when it is scrapped. Unfortunately, many refrigerants are potent greenhouse gases. Worldwide, leaked refrigerants account for roughly one-third of the climate impact of cooling, with electricity used to run the equipment responsible for the rest.

Air conditioning units have different labels. One that has recently been installed in a house or office probably says “R-32”, a type of hydrofluorocarbon (HFC). Older units often say “R-410A”, which is a blend of HFCs. Kilogram for kilogram, R-410A traps around 2,000 times more heat than carbon dioxide. R-32 is still a potent greenhouse gas, but only about a third as warming as R410A.

Scientists have been concerned about these refrigerants for decades. But if you wind the clock back to the 1970s and 1980s, they were more worried about the fact that they harmed the ozone layer than about their impact on the climate. Around this time, researchers discovered that the most widely used refrigerants back then, chlorofluorocarbons (CFCs) were damaging the stratospheric ozone that shields us from the ultraviolet radiation that causes skin cancer.

Following the discovery of a “hole” in the ozone layer over Antarctica, nations agreed to sign a landmark environmental treaty known as the Montreal protocol in 1987.

This protocol and its later amendments phased out the CFCs, but allowed, as an interim measure, compounds called hydrochlorofluorocarbons (HCFCs) that were less harmful for the ozone layer. During the 1990s, HCFCs began to be replaced by HFCs, which don’t deplete ozone at all.

Without the Montreal protocol, studies suggest that ozone holes could have developed over the Arctic as well as Antarctica. Instead, the ozone layer is starting to recover.

Problem solved? Unfortunately not. HFCs, like CFCs and HCFCs, are incredibly potent greenhouse gases. Projections suggest that, without controls, they could add 0.3–0.5°C to global temperatures by the end of this century, a substantial fraction of the roughly 1.2°C of warming caused by human activities.

On October 15 2016, the parties to the Montreal protocol came together in Kigali, Rwanda. They agreed to phase down HFC production in favour of gases with lower global warming potentials (a measure of how potent the gas is relative to carbon dioxide). For the first time, this protocol was used to control gases purely because of their effect on the climate.

In 2014, two years before the Kigali amendment to the Montreal protocol was agreed, the EU adopted rules to phase down HFCs. These rules enable Europe and the UK to meet their Kigali commitments. Since January 2025, new small split air conditioners (units that heat or cool a single room) sold in the UK can no longer use R-410A, an HFC with high global warming potential. Portable units have been barred from using gases with high warming potential since 2020. The EU now aims to phase out HFCs almost entirely by 2050.

Beyond looking at labels on air conditioners, how do scientists like us know whether the Kigali amendment is working? The most direct test is to measure the atmosphere.

At Mace Head, an atmospheric research station on the windy west coast of Ireland, instruments have been measuring these gases continuously since 1987. Mace Head is one of a handful of stations around the world measuring these gases at concentrations of just a few parts per trillion.

In 2019, these measurements helped pinpoint the source of renewed global production of CFCs, years after the Montreal protocol’s ban. After a crackdown on illegal CFC production for use in insulating foams, emissions dropped.

The atmospheric data shows that the abundance of HFC compounds has increased substantially in the last decade. But overall growth in their emissions has slowed, and HFC emissions are substantially lower than those projected before the Kigali amendment was agreed.

A 2025 study projected that the climate impact of air conditioning could be similar to, or lower than, its impact today, even with a major increase in demand. But this depends on us meeting or exceeding our Kigali commitments, making air conditioners more efficient, expanding the use of zero-energy cooling measures (such as shading) and switching to low-carbon electricity.

Keeping a close eye on concentrations

Despite the apparent early success of the Kigali amendment, the atmospheric data show there can be no room for complacency.

Last year, scientists showed that emissions of a waste gas called HFC-23, the most potent climate warmer of all the HFCs, are far higher than the emissions of the gas officially reported by signatories to the Kigali amendment. Because HFC-23 is such a strong greenhouse gas, the discrepancy equates to just over half of the UK’s annual greenhouse gas emissions. Researchers and governments are now trying to figure out the reasons for the unexpected emissions.

The contents of air conditioners and fridges have been shaped by the Montreal protocol for almost 40 years. We know where it has worked, and where it needs closer scrutiny, only because researchers keep measuring the atmosphere. The challenge now is to maintain and expand that monitoring, so that the Kigali amendment can continue this remarkable legacy.

Matt Rigby, Reader in Atmospheric Chemistry, University of Bristol and Lucy Carpenter, Professor of Atmospheric Chemistry, University of York

This article is republished from The Conversation under a Creative Commons license. Read the original article.