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Ozone-hole recovery may spur Antarctic warming

A full recovery of the stratospheric ozone hole could strongly
modify climate change in the Southern Hemisphere and possibly amplify warming of the Antarctic continent, a new study finds.
"If the successful control of ozone-depleting substances allows for a full recovery of the ozone hole over Antarctica, we may finally see the interior of Antarctica begin to warm with the rest of the world," says Judith Perlwitz of the University of Colorado at Boulder and the National Oceanic and Atmospheric

Administration (NOAA). Perlwitz is lead author of the study.

While average surface temperatures have been increasing globally, the interior of Antarctica has exhibited a unique cooling trend during the austral (Southern
Hemisphere) summer and fall, Perlwitz notes. The cooling is attributed to ozone
depletion. She and her colleagues conclude that, as stratospheric ozone levels
return to near pre-1969 levels by the end of the 21st century, large-scale
atmospheric circulation patterns currently shielding the Antarctic interior from
warmer air masses to the north will begin to break down during the austral summer.

These circulation patterns are collectively known as a positive phase of the Southern Annular Mode, or SAM.

The scientists find that, as ozone levels recover, the lower stratosphere over the polar region will absorb more ultraviolet radiation from the sun. This will cause air temperatures roughly 10-20 kilometers (6-12 miles) above Earth's surface to rise by as much as 9 degrees Celsius, reducing the strong north-south temperature gradient that currently favors the positive phase of SAM.

The new study also indicates that ozone-hole recovery would weaken the intense
westerly winds that currently whiz around Antarctica and block air masses from
crossing into the continent's interior. As a result, Antarctica would no longer be isolated from the warming patterns affecting the rest of the world.
Ozone recovery will essentially reverse summertime climate and atmospheric
circulation changes that have been caused by the presence of the ozone hole, says co-author Steven Pawson of NASA Goddard Space Flight Center in Greenbelt, Md.
To examine how changes in the ozone hole might influence climate and weather
near Earth's surface, the scientists used a NASA computer model that includes
interactions between the climate and stratospheric ozone chemistry. The team will publish its findings on 26 April 2008 in Geophysical Research Letters, a journal of the American Geophysical Union, or AGU.
Besides affecting Antarctica, the anticipated seasonal shift in large-scale
circulation patterns would also have repercussions for Australia and South
America. Studies show that the positive phase of SAM is associated with cooler
temperatures over much of Australia and increased rainfall over Australia's
southeast coastline. The positive phase of SAM is also associated, during late
spring and early summer, with drier conditions in South America's productive
agricultural areas: Argentina, Brazil, Uruguay and Paraguay. If ozone recovery
induces a shift away from a positive SAM, Australia could experience warmer and drier conditions while South America could get wetter, according to Perlwitz.
But just how influential a full stratospheric ozone recovery will be on Southern Hemisphere climate largely depends on the future rate of greenhouse gas emissions, according to the study. Projected increases in human-emitted

greenhouse gases such as carbon dioxide will be the main driver for strengthening the positive phase of SAM.

"In running our model simulations, we assumed that greenhouse gases like carbon dioxide would double over the next 40 years and then slowly taper off. If human activities cause more rapid increases in greenhouse gases, or if we continue to produce these gases for a longer period of time, then the positive SAM may dominate year-round and dwarf any climatic effects caused by ozone recovery," says Perlwitz.

Perlwitz of the NOAA Cooperative Institute for Research in Environmental
Studies, in Boulder, and Pawson also collaborated on the study with other
scientists at NASA Goddard and at NOAA's Earth System Research Laboratory,
also in Boulder.
NASA provided major funding for the study.

Peter Weiss | American Geophysical Union
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