The global treaty that headed off destruction of earth's protective ozone¹ 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 Protocol² 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（水文气候）. The study says the treaty prevented ozone loss from disrupting atmospheric³ 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 depletion⁴ on surface climate and the hydrological cycle was not recognized at all. "We dodged⁵ a bullet we did not know had been fired," said study coauthor Richard Seager, a climate scientist at Columbia University's Lamont-Doherty Earth Observatory⁶.

 

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 Mediterranean⁷ region, would have grown even drier and wider, the study says, and wet regions in the tropics, and mid⁸-to-high latitudes⁹ 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（同温层） 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（向极的）. The warming due to added CFCs in the air would have also intensified¹⁰ cycles of evaporation¹¹ 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.

 

The study builds on earlier work by coauthor Lorenzo Polvani, a climate scientist with joint¹² appointments at Lamont-Doherty and Columbia's Fu Foundation School of Engineering and Applied¹³ Science. Polvani and others have found that two human influences on climate --ozone loss and industrial greenhouse gases -- have together pushed the jet stream in the southern hemisphere south over recent decades. As the ozone hole over Antarctica closes in the coming decades, the jet stream will stop its poleward migration¹⁴, Polvani found in a 2011 study in the journal Geophysical Research Letters. The projected stopping of the poleward jet migration is a result of the ozone hole closing, canceling the effect of increasing greenhouse gases.

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