The 1987 Montreal Protocol is often described as the most successful environmental treaty ever created. It has led to a steady global phaseout of chemicals that once damaged the ozone layer and increased risks such as skin cancer.
Research led by MIT has shown that cutting these ozone-depleting substances is allowing the ozone layer to slowly heal. (It could return to 1980 levels by as early as 2040, according to some estimates.) However, the agreement included an exception. Certain ozone-depleting chemicals can still be used as feedstocks to produce other materials, based on the belief that only a tiny fraction, about 0.5 percent, would escape into the atmosphere.
That assumption is now being challenged.
In recent years, scientists have detected higher-than-expected levels of ozone-depleting substances in the atmosphere. This has led to updated estimates showing that leakage from feedstock use is significantly greater than previously thought.
An international team of researchers, including scientists from MIT, has now examined how these higher leakage rates affect the ozone layer's recovery. Their findings show that if the issue is not addressed, ozone recovery could be delayed by about seven years.
"We've realized in the last few years that these feedstock chemicals are a bug in the system," says author Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry, who helped identify the original cause of the ozone hole. "Production of ozone-depleting substances has pretty much ceased around the world except for this one use, which is when you have a chemical you convert into something else."
The study, which will appear in Nature Communications, is the first to fully measure the impact of emissions from feedstocks. These chemicals are widely used to manufacture plastics, nonstick coatings, and replacement chemicals for substances already restricted under the Montreal Protocol.
The researchers emphasize that reducing both the use of these feedstocks and their leakage is becoming increasingly important, especially as global demand for products like plastics continues to grow.
"We've gotten to the point where, if we want the protocol to be as successful in the future as it has been in the past, the parties really need to think about how to tighten up the emissions of these industrial processes," says first author Stefan Reimann of the Swiss Federal Laboratories for Materials Science and Technology.
"To me, it's only fair, because so many other things have already been completely discontinued. So why should this exemption exist if it's going to be damaging?" says Solomon.
The research team includes scientists from institutions across the United States, Europe, and Asia, including MIT, NASA, NOAA, and several universities and research centers.
The concern over ozone depletion dates back to 1985, when scientists identified a growing hole in the ozone layer above Antarctica. This thinning allowed more harmful ultraviolet radiation from the sun to reach Earth's surface.
A year later, Solomon and other researchers traveled to Antarctica and confirmed the cause. The damage was driven by chlorofluorocarbons, or CFCs, which were widely used in refrigeration, air conditioning, and aerosol products.
These findings led to the creation of the Montreal Protocol, an international agreement involving 197 countries and the European Union to limit the use of these chemicals. The decision to allow feedstock use was partly based on industry estimates that leakage would remain very low.
"It was thought that the emissions of these substances as a feedstock were minor compared to things like refrigerants and foams," Western says. "It was also believed that leakage from these sources was minor, around half a percent of what went in, because people would essentially be leaking their profits if their feedstocks were released into the atmosphere."
Those assumptions no longer appear to hold. Western and Reimann are part of the Advanced Global Atmospheric Gases Experiment (AGAGE), a global monitoring network that tracks emissions of ozone-depleting substances.
Recent measurements suggest that feedstock leakage rates are closer to 3.6 percent, with some chemicals showing even higher losses.
In their analysis, the researchers used 3.6 percent as a baseline scenario and compared it with lower leakage rates of 0.5 percent and a scenario with no feedstock emissions. They also analyzed production trends from 2014 to 2024 to estimate future use of these chemicals through 2100.
The results show that total emissions of ozone-depleting substances continue to fall across all scenarios until about 2050, thanks to restrictions already in place. However, if higher leakage rates persist, emissions level off around 2045 and only drop by about 50 percent by 2100.
The team then evaluated how these emissions would affect the ozone layer itself. With leakage reduced to 0.5 percent, the ozone layer would recover to its 1980 condition by 2066. If emissions were eliminated entirely, recovery would occur by 2065.
But under current leakage estimates, recovery would be pushed back to 2073, roughly seven years later.
"This paper sends an important message that these emissions are too high and we have to find a way to reduce them," Reimann says. "Either that means no longer using these substances as feedstocks, swapping out chemicals, or reducing the leakage emissions when they are used."
Can Industry and Policy Adapt?
Despite the concern, researchers are optimistic that solutions are within reach. Solomon points out that the chemical industry has a long history of innovation and adaptation.
"There are a lot of innovators in the chemical industry," Solomon says. "They make new chemicals and improve chemicals for a living. It's true they can perhaps get too entrenched with certain chemicals, but it doesn't happen that often. Actually, they're usually quite willing to consider alternatives. There are thousands of other chemicals that could be used instead, so why not switch? That's been the attitude."
She also notes that the ability of monitoring networks like AGAGE to detect these emissions highlights how much progress has already been made in reducing other sources of ozone damage.
"This isn't the first time that the AGAGE Network has made measurements that have allowed the world to see we need to do a little better here or there," Western says. "Often, it's just a mistake. Sometimes all it takes is making people more aware of these things to tighten up some processes."
Countries that are part of the Montreal Protocol meet annually to review emerging issues. Feedstock emissions are already under discussion, and future meetings will likely focus on how to reduce or eliminate these leaks.
"We wanted to raise the warning flag that something is wrong here," Reimann says. "We could reduce the period of ozone depletion by years. It might not sound like a long time, but if you could count the skin cancer cases you'd avoid in that time, it would seem quite significant."
The research was supported in part by the National Science Foundation, NASA, the Swiss Federal Office for the Environment, the VoLo Foundation, the United Kingdom Natural Environment Research Council, and the Korea Meteorological Administration Research and Development Program.
