The 'Montreal Protocol' Designed To Heal the Ozone Layer May Have Also Fended Off Several Degrees of Warming

James Temple writes via MIT Technology Review: In 1987, dozens of nations adopted the Montreal Protocol, agreeing to phase out the use of chlorofluorocarbons and other chemicals used in refrigerants, solvents, and other industrial products that were breaking down Earth's protective ozone layer. It was a landmark achievement, the most successful example of nations pulling together in the face of a complex, collective threat to the environment. Three decades later, the atmospheric ozone layer is slowly recovering, preventing additional levels of ultraviolet radiation that cause cancer, eye damage, and other health problems. But the virtues of the agreement, ultimately ratified by every country, are more widespread than its impact on the ozone hole. Many of those chemicals are also powerful greenhouse gases. So as a major side benefit, their reduction over the last three decades has already eased warming and could cut as much as 1C off worldwide average temperatures by 2050. Now, a new study in Nature highlights yet another crucial, if inadvertent, bonus: reducing the strain that ultraviolet radiation from the sun puts on plants, inhibiting photosynthesis and slowing growth. The Montreal Protocol avoided "a catastrophic collapse of forests and croplands" that would have added hundreds of billions of tons of carbon to the atmosphere, Anna Harper, a senior lecturer in climate science at the University of Exeter and a coauthor of the paper, said in an email. The Nature paper, published August 18, found that if production of ozone-depleting substances had continued ticking up 3% each year, the additional UV radiation would have curtailed the growth of trees, grasses, ferns, flowers, and crops across the globe. The world's plants would absorb less carbon dioxide, releasing as much as 645 billion tons of carbon from the land to the atmosphere this century. That could drive global warming up to 1C higher over the same period. It would also have devastating effects on agricultural yields and food supplies around the globe. The impact of rising CFCs levels on plants, plus their direct warming effect in the atmosphere, could have pushed temperatures around 2.5C higher this century, the researchers found. That would all come on top of the already dire warming projections for 2100.

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