In September 1987, in response to evidence that the chemicals known as chlorofluorocarbons (CFCs) were damaging the atmosphere’s ozone layer, the countries of the world drafted a landmark agreement known as the Montreal Protocol, which limited the use of those chemicals. Since then, the protocol has been successful not only at protecting the ozone layer, but, since CFCs are also heat-trapping greenhouse gases, at helping limit the risk of climate change as well. In fact, the signatories amended the protocol last year to focus entirely on climate protection.

The story began in 1974, with the discovery by Mario Molina and Sherwood Rowland that CFCs, which were being used in increasing quantities as refrigerants (in refrigerators, freezers and air conditioners), in aerosol spray cans, as industrial solvents, and as blowing agents plastic foam and other products, could deplete the ozone layer by catalytic chemical reactions. CFCs are stable, man-made chemicals that are only broken down when they reach the stratosphere, between 15 and 50 kilometers above Earth’s surface. The chlorine atoms liberated in that process would, Molina and Rowland argued, initiate a series of chemical reactions resulting in the destruction of ozone. This was important because the so-called ozone layer in the stratosphere absorbs harmful solar ultraviolet radiation, thereby protecting living things on earth. An increase in UV radiation can, among other things, cause both skin cancers and cataracts.

This danger was largely theoretical at first, but in 1985, Joe Farman and his colleagues with the British Antarctic Survey discovered that ozone over the Antarctic continent was dropping by about a third every austral spring. They linked this to the increase in chlorine in the stratosphere. In 1995, Molina and Rowland, along with Paul Crutzen, shared the Nobel Prize in Chemistry for their groundbreaking work.

Alarmed by these discoveries, the countries of the world rapidly agreed to limit the use of CFCs under the Montreal Protocol, which was put in place just two years later. Successive amendments have strengthened and broadened the protocol. The result is that the use of CFCs is now phased out globally and emissions have decreased by 90 percent or more from their peak values. Measurements show that the atmospheric abundance of CFCs is also decreasing. Due to the 50- to 100-year atmospheric residence time of most CFCs, the ozone layer has responded slowly. But observations from satellites and ground based stations show that it has begun to recover. Without the Montreal protocol, CFCs would have continued to increase, and scientist have estimated that by 2100 the annual number of skin cancer cases would have quadrupled (Slaper et al. 1996).

In 2007, my U.S. colleagues and I (Velders et al., 2007) showed that the Montreal Protocol was not only important for the ozone layer, but also contributed to climate protection. CFCs are potent greenhouse gases; per kilogram emission the major CFCs are 5,000 to 11,000 times more powerful at climate forcing than the main greenhouse gas CO2. In fact, by 2010, the Montreal Protocol had achieved five to six times larger climate benefits than the Kyoto Protocol, which was designed for climate protection.

The story is not over, however. The Montreal Protocol banned CFCs and other chemicals, but we still have refrigerators and air conditioners. This was possible because we were able to substitute other gases, especially hydrofluorocarbons (HFCs), in their place. As a result, the use of HFCs has grown significantly over the past two decade. HFCs do not affect the ozone layer; unfortunately, they, too, are potent greenhouse gases. Projections I made with colleagues in 2009 and 2015 (Velders et al., 2009, 2012, 2015) showed that if this growth continues, their contribution to climate change will significantly offset the climate benefits of the Montreal Protocol.

Based on the scientific evidence, rooted in observations, theory and modeling studies, the politicians decided to act again. They agreed to use the Montreal Protocol, designed to protect the ozone layer, to fight climate change. In October 2016 in Kigali, Rwanda, they agreed to amend the protocol to strongly limit the future use of HFCs globally. Between 2036 and 2046 the use of HFCs must now be reduced by 85 percent compared to an agreed baseline. In the final hours of the meeting in Kigali, I calculated that with this agreement, the contribution from HFCs to the global average surface warming would be limited to less than 0.1 degree Celsius by 2100, compared with 0.3 to 0.5 degree C without the agreement. This is a significant step towards the goals of the Paris climate accord of December 2015: to keep global temperatures below 2 degrees C by the end of the century, relative to pre-industrial times. I hope that a sufficient number of countries will have ratified the Kigali Amendment for it to enter into force on the agreed-upon date of January 1, 2019.

The Montreal Protocol is widely considered to be the most successful international environmental agreement. In my view, the success, for both ozone and climate protection is the result of sound scientific research, clear communication, fruitful interactions between scientists, policymakers, industry, and environmental groups, and the willingness of all these parties to reach an agreement, while respecting each other’s position. A great example to follow.

The Falling Walls Conference is supported by the German Federal Ministry of Education and Research, the Helmholtz Association, the Robert Bosch Stiftung and the Berlin Senate. It receives support and advice from a wide variety of international top-class universities and research institutions as well as foundations