Ozone-protection treaty had climate benefits

The largest ozone hole over Antarctica (in purple) was recorded in September 2006. 

Thanks to the Montreal Protocol, the amount of ozone-depleting chemicals in the atmosphere peaked in the late 1990s and Antarctica's ozone hole is expected to recover by 2060. 

Credit: NASA.

The global treaty that headed off destruction of earth's protective ozone layer has also prevented major disruption of global rainfall patterns, even though that was not a motivation for the treaty, according to a new study in the Journal of Climate.

The 1987 Montreal Protocol phased out the use of chloroflourocarbons, or CFCs, a class of chemicals that destroy ozone in the stratosphere, allowing more ultraviolet radiation to reach earth's surface.

Though the treaty aimed to reverse ozone losses, the new research shows that it also protected the hydroclimate.

The study says the treaty prevented ozone loss from disrupting atmospheric circulation, and kept CFCs, which are greenhouse gases, from warming the atmosphere and also disrupting atmospheric circulation.

Had these effects taken hold, they would have combined to shift rainfall patterns in ways beyond those that may already be happening due to rising carbon dioxide in the air.

At the time the Montreal Protocol was drafted, the warming potential of CFCs was poorly understood, and the impact of ozone depletion on surface climate and the hydrological cycle was not recognized at all.

"We dodged a bullet we did not know had been fired," said study coauthor Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory.

Today, rising carbon dioxide levels are already disturbing earth's hydrological cycle, making dry areas drier and wet areas wetter. But in computer models simulating a world of continued CFC use, the researchers found that the hydrological changes in the decade ahead, 2020-2029, would have been twice as severe as they are now expected to be.

Subtropical deserts, for example in North America and the Mediterranean region, would have grown even drier and wider, the study says, and wet regions in the tropics, and mid-to-high latitudes would have grown even wetter.

The ozone layer protects life on earth by absorbing harmful ultraviolet radiation. As the layer thins, the upper atmosphere grows colder, causing winds in the stratosphere and in the troposphere below to shift, displacing jet streams and storm tracks.

The researchers' model shows that if ozone destruction had continued unabated, and increasing CFCs further heated the planet, the jet stream in the mid-latitudes would have shifted toward the poles, expanding the subtropical dry zones and shifting the mid-latitude rain belts poleward.

The warming due to added CFCs in the air would have also intensified cycles of evaporation and precipitation, causing the wet climates of the deep tropics and mid to high latitudes to get wetter, and the subtropical dry climates to get drier.

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