Ozone treaty holds down atmospheric CO₂ levels, scientists say

An international treaty to protect stratospheric ozone is slowing the growth of atmospheric levels of carbon dioxide, according to a team of scientists (Nature 2021, DOI: 10.1038/s41586-021-03737-3). The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer is reversing loss in the stratosphere of the triatomic molecule that screens the Earth’s surface from the sun’s ultraviolet rays, protecting both humans and ecosystems from damage. One consequence is that land plants are healthier and able to absorb more CO₂ for photosynthesis than they would have absent the pact, the researchers say. Led by Paul J. Young, an atmospheric scientist at Lancaster University in the UK, the team used computer models to estimate the benefits of avoided increases in UV radiation on terrestrial plants and their ability to act as a carbon sink. The researchers estimate that by 2080–99 the Montreal Protocol will lead to atmospheric CO₂ levels that are lower by 115–235 parts per million than if the treaty didn’t exist. This translates to about 0.5–1.0 °C of average global surface temperature, they say.