The stratospheric ozone layer, which protects the Earth from harmful solar ultra-violet radiation, has thinned dramatically over the past half-century. The depletion of ozone has shifted the Southern Hemisphere’s climate so that dry areas in the subtropics now see 10 percent more precipitation in summer than they used to, report the scientists from Columbia University's School of Engineering and Applied Science.
The researchers say the ozone hole has affected the entire circulation in the Southern Hemisphere all the way to the equator. While previous studies has shown that the ozone hole is changing the atmospheric flow in the high latitudes, the Columbia research report -- “Impact of Polar Ozone Depletion on Subtropical Precipitation” -- demonstrates that the ozone hole is able to influence the tropical circulation and increase rainfall at low latitudes in the Southern Hemisphere.
This is the first time that ozone depletion, confined solely to the Polar regions, has been linked to climate change from Pole to the equator.
“Ozone is now widely believed to be the dominant agent of climate change in the Southern Hemisphere, so this actually means that the international agreements regulating climate change cannot be confined to dealing with carbon dioxide,” Sarah Kang of Columbia University, lead author of the study, told Reuters by telephone.
“They also need to consider ozone,” said Kang.
Carbon dioxide emission from natural and human-made sources, largely the burning of fossil fuels, is the most frequently blamed target from policymakers seeking to cut climate change caused by humans.
However, the depletion of the ozone, due mostly to commercial and industrial use of chemicals containing chlorofluorocarbons (CFCs), has a powerful impact on large bands of the Southern Hemisphere, said researchers.
The ozone layer normally absorbs ultra-violet radiation, warming the air below. But with the opening of the ozone layer over the South Pole due to chlorofluorocarbon pollution, there was severe cooling rather than warming, which led to a southern shift in the winds that flow from west to east around Antarctica, the researchers found.
As the band of winds moved toward the pole, a corresponding dry belt in the subtropics also moved southward, said researchers. This left room nearer the equator for a band of increased summer precipitation.
The ozone hole drives most of this change, with some smaller contributions from greenhouse gases in the atmosphere, the study found.
Since the signing of the 1989 Montreal Protocol -- a treaty designed to protect the ozone layer by phasing out the use of CFCs -- most production of the harmful chemicals around the world has ceased and experts expect the hole to close back up by the middle of this century.
“While the ozone hole has been considered as a solved problem, we're now finding it has caused a great deal of the climate change that's been observed,” said co-author Lorenzo Polvani, senior research scientist at the Lamont-Doherty Earth Observatory, and co-author of the paper.
The study used two separate climate models -- Canadian Middle Atmosphere Model and the United States' National Center for Atmospheric Research (NCAR) Community Atmosphere Model -- in four experiments comparing data on sea ice, surface temperatures, precipitation and the ozone hole.
Analysis of the data showed that the hole was the prominent driver of heavy summer rains across eastern Australia, the southwestern Indian Ocean and the Southern Pacific Convergence Zone.
While the ozone hole drives about a 10 percent change for one particular zone, in “Australia, it’s about 35 percent,” Kang told BBC News.
The researcher’s modeling indicated that global warming due to greenhouse gas emissions was also a factor -- although natural climate cycles are also thought to be important, as Australia suffered severe droughts in the era before ozone depletion and before warming seen in the late 20th century.
Professor Myles Allen of Oxford University, a leading UK climate modeler, told BBC News: “This study does illustrate the important point that different mechanisms of global change are contributing to the climate impacts we're seeing around the world.”
“It's very important to unpack them all rather than assuming that any impact we see is down simply to greenhouse gas-mediated warming,” he said.
Earlier this month, The World Meteorological Organization reported record loss of the protective ozone layer over the Arctic, which unlike the Antarctic phenomenon, is not an unusual occurrence. The 40 percent loss of ozone over the Arctic came even after the “very successful” 1989 Montreal Protocol, said the WMO. The protocol called for the ban of CFCs which, in turn, has reversed the ozone depletion over Antarctica, and should repair the ozone there by 2045-60.
Kang cautions that this alone will not restore prior climate conditions to Australia or anywhere else in the Southern Hemisphere. “As the ozone hole repairs, it is going to work to reverse this trend; but there is also the rising trend in carbon dioxide, and that is acting in the same direction as the ozone hole,” she said.
Australia's persistently dry weather has caused severe impacts on communities, farms and nature. In recent years, the volume of water flowing into the reservoirs of Perth, the Western Australian capital, has been just a third of what it was during most of the 20th Century.
“The ozone hole is not even mentioned in the summary for policymakers issued with the last IPCC report,” noted Lorenzo M. Polvani, Professor of Applied Mathematics and of Earth & Environmental Sciences. “We show in this study that it has large and far-reaching impacts. The ozone hole is a big player in the climate system.”
Kang and Polvani also plan to study extreme precipitation events, which are associated with extreme floods, mudslides, and other disasters.
“We really want to know if and how the closing of the ozone hole will affect these,” added Kang.