An ozone hole in the Arctic is expected to grow larger as a result of greenhouse gas accumulation and should worsen through the year 2020 before recovering, according to a new study by a team of climate scientists at Columbia University and NASA's Goddard Institute for Space Studies.
The loss of ozone in the Arctic by 2020 will be about double what would occur without greenhouse gases, the scientists report. Ozone absorbs harmful ultraviolet radiation from the sun, and its depletion over the poles is thought to be a cause of increased global levels of skin cancer.
Researchers used computer models to project over time future emissions of greenhouse gases and ozone-depleting halogens, the first time such interactions have been studied. The work appears in the April 9 issue of the British journal Nature and is reported by Drew T. Shindell, associate research scientist at Columbia's Center for Climate Systems Research; David H. Rind, senior scientist at the Goddard Institute, adjunct professor of earth and environmental sciences at Columbia and adjunct senior research scientist at Columbia's Lamont-Doherty Earth Observatory, and Patrick Lonergan of Science Systems and Affiliations Inc.
Ozone losses had increased greatly in the 1990s in the Arctic and in late 1997 were the greatest ever observed, according to measurements by NASA satellites. Ozone, found in a thin layer of the upper atmosphere, absorbs harmful ultraviolet radiation from the sun. But when polar stratospheric clouds form, reactions take place between ozone gas and chlorine, bromine and other halogen gases on the surface of ice or water droplets in these clouds, depleting the ozone. Most of those halogens are from chlorofluorocarbons emitted into the atmosphere by industrial processes.
Because the number of particles that form in polar stratospheric clouds is extremely sensitive to changes in temperature, the reaction that results in ozone depletion is also very sensitive, occurring only below about minus 108 degrees Fahrenheit. Though greenhouse gases cause atmospheric warming at the Earth's surface, they cool the stratosphere, where ozone resides, and thus are a likely cause of the increased ozone depletion, the researchers said.
"Since ozone chemistry is very sensitive to temperature, this cooling results in more ozone depletion in the polar regions," Dr. Shindell said. "Even very small amounts of stratospheric cooling can greatly increase ozone depletion."
Temperatures are slightly warmer in the Arctic than the Antarctic during their respective winter and spring seasons, with the result that ozone losses in the Northern Hemisphere had been lower than in the Southern. But the Arctic stratosphere has gradually cooled over the last decade, resulting in the increased ozone loss, the scientists believe.
One of the reasons for the warmer Arctic is that large-scale planetary atmospheric waves, similar to solitons in the oceans, deposit heat energy in the North, breaking up an atmospheric vortex of cold air that sits over the Arctic. In the simulations performed by the NASA-Columbia team, temperature and wind changes induced by greenhouse gases alter the propagation of planetary waves, which no longer disturb the Arctic vortex as often. The combination of greenhouse-induced stratospheric cooling and the increased stability of the Arctic polar vortex dramatically increase ozone depletion.
Because of international controls on the emission of ozone-depleting halogens, those gases are expected to peak about the year 2000. In the Columbia-NASA model, Arctic ozone depletion will be worst in the decade 2010 to 2019, with two-thirds of atmospheric ozone lost in the most severely affected areas.
The work was supported by the NASA Atmospheric Chemistry Modeling and Analysis Program and the NASA Climate Modeling Program. Lamont- Doherty Earth Observatory and the Center for Climate Systems Research are part of the Columbia Earth Institute, launched to develop innovations for wise stewardship of the Earth.
For more information on the research, see also: http://www.giss.nasa.gov/research/Intro/shindell.02/
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