Confirming the substantial contribution of ozone-depleting halocarbon emissions to global warming during the second half of the 20th century

Why this hidden climate story matters

Many people have heard that the Montreal Protocol saved the ozone layer, protecting us from harmful ultraviolet radiation. Less widely known is that the same chemicals that ate away at ozone—ozone‑depleting halocarbons—are also powerful heat‑trapping gases. This study asks a deceptively simple question with big implications: did these chemicals significantly warm the planet in the second half of the 20th century, and did their phase‑out under the Montreal Protocol really help slow global warming?

Powerful chemicals with a double edge

Ozone‑depleting halocarbons, such as older refrigerants and spray propellants, have two very different effects on the climate system. On the one hand, they are extremely strong greenhouse gases, molecule for molecule far more potent than carbon dioxide. On the other, they destroy ozone high in the stratosphere. Because ozone absorbs sunlight and warms that part of the atmosphere, losing it tends to cool the stratosphere and slightly reduce the amount of heat reaching the lower atmosphere and surface. For decades, scientists have debated how these opposing influences balance out: did the warming from the gases themselves outweigh the cooling from ozone loss, or did they nearly cancel?

Using past ozone changes as a natural test

To answer this, the authors analyzed a suite of sophisticated climate–chemistry models that simulate how these gases altered ozone, temperatures, and the flow of energy through the Earth system. They focused on two key periods: a longer "historical" window from the late 1950s to the early 2000s, and a "satellite era" from the mid‑1980s to early 2000s, when ozone observations are most reliable. Each model was run twice: once with real‑world halocarbon emissions, and once with halocarbons held fixed at their early‑1950s levels. By comparing these runs, the researchers could separate the specific impact of halocarbons from all other human and natural influences.

A central insight emerged from examining how much ozone was actually lost in the stratosphere. Models that produced stronger halocarbon‑driven ozone depletion also showed stronger cooling in the lower stratosphere and a different overall energy imbalance at the top of the atmosphere. Across models, there was a tight, nearly linear link between the amount of ozone lost and the net warming influence of halocarbons. That relationship allowed the authors to use real‑world ozone records—from satellite data sets and advanced atmospheric reanalyses—as an anchor to identify which model behaviors were physically plausible and which were outliers.

Confirming a net warming punch

Using this ozone‑based constraint, the study finds that the net effect of ozone‑depleting halocarbons on the planet’s energy balance in 2014 is strongly positive. The best estimate is about 0.2 watts per square meter of extra heat retained globally, and even the low end of the uncertainty range still implies warming rather than cooling. Put differently, ozone loss cancels at most about half of the direct greenhouse effect from these gases, not nearly all of it. When the authors translate this energy imbalance into surface temperature change using the same models, they find that halocarbons were responsible for roughly 0.1 °C of global warming between about 1960 and 2000—around 20% of the total human‑driven warming in that period.

Why some earlier estimates were smaller

Previous influential studies, which suggested that the net heating from these chemicals might be close to zero, relied mainly on changes in total ozone from the surface to the top of the atmosphere. That measure mixes together ozone loss in the stratosphere—driven strongly by halocarbons—with ozone increases in the lower atmosphere caused by other pollutants. The new work shows that when you focus specifically on stratospheric ozone, where halocarbon chemistry dominates, the picture becomes clearer and the inferred warming effect grows. The authors also demonstrate that models with unrealistic patterns of ozone depletion can strongly bias multi‑model averages if they are not checked against observations.

What this means for climate and policy

For a layperson, the bottom line is straightforward: the chemicals that damaged the ozone layer also made the planet significantly warmer, and cutting their emissions through the Montreal Protocol has already avoided additional global warming. According to this study, if those ozone‑depleting halocarbons had never been released, global warming over the latter half of the 20th century would have been about one‑fifth smaller. While uncertainties remain—especially in long‑term ozone records and some finer‑scale atmospheric responses—the research offers robust, observation‑backed confirmation that the Montreal Protocol has been not only an environmental success story for the ozone layer, but also one of the most effective climate actions taken to date.

Citation: Friedel, M., Chiodo, G., Weber, K. et al. Confirming the substantial contribution of ozone-depleting halocarbon emissions to global warming during the second half of the 20th century. npj Clim Atmos Sci 9, 106 (2026). https://doi.org/10.1038/s41612-026-01398-5

Keywords: ozone-depleting substances, halocarbons, Montreal Protocol, stratospheric ozone, global warming