Science News

April 25th, 2008

Web edition

Climate Fix Could Deplete Polar Ozone

http://sciencenews.org/view/generic/id/31472/

By Sid Perkins

Effect would be especially large after extremely cold winters

Scientists seeking to cool Earth's climate by injecting millions of tons of sulfuric acid

droplets high in the atmosphere might trim rising temperatures but could also destroy

much of the ozone in polar regions, a new study suggests.

Major volcanic eruptions spew large amounts of tiny particles, or aerosols, high into the

atmosphere, where they scatter light back to space and significantly cool Earth for

months to years (SN: 2/18/06, p. 110). Some researchers have proposed lofting tons of

aerosols into the stratosphere to achieve the same result, but that process - often

dubbed geoengineering - could have a number of detrimental side effects. Last year,

for example, scientists noted that average precipitation worldwide dropped significantly

in the 16 months immediately following the 1991 eruption of Mount Pinatubo (SN:

8/25/07, p. 125).

Now, count ozone destruction among the drawbacks of geoengineering. High-altitude

ozone helps block damaging ultraviolet radiation from reaching Earth's surface.

Ozone-destroying chemical reactions occur most readily on the surfaces of high-altitude

ice crystals and droplets of sulfuric acid spewed by volcanoes, says Simone Tilmes,

an atmospheric scientist at the National Center for Atmospheric Research in Boulder,

Colo.

So, Tilmes and her colleagues estimated the ozone loss that would be triggered by two

geoengineering scenarios, each designed to counteract the warming effect caused by

doubling the pre-industrial atmospheric levels of carbon dioxide, as expected to occur

late this century.

In one scenario, scientists inject about 2 million metric tons of sulfur-bearing aerosols

into the stratosphere each year, each droplet approximately 0.46 micrometers in

diameter. The other scenario lofts only 1.5 million metric tons of sulfur each year but in

the form of smaller aerosols, which are more effective at scattering sunlight back into

space.

Ozone destruction estimates are based on observations gathered during the last couple

of decades, says Ross Salawitch, an atmospheric chemist at the University of

Maryland, College Park. Results indicate that over the next few decades, ozone loss

high above the Arctic after a particularly cold winter - one that produced large numbers

of high-altitude ice crystals - could approach 75 percent, Tilmes, Salawitch and their

colleagues report in an upcoming Science.

The effects of sulfate-aerosol geoengineering would be smaller later this century than

today, primarily because atmospheric levels of ozone-destroying chemicals such as

chlorofluorocarbons are now declining. Nevertheless, injecting sulfates into the

atmosphere could delay the recovery of the ozone hole over Antarctica by 30 to 70

years.

Ozone loss due to geoengineering "is a real concern, but I don't see it as a

showstopper," says Ken Caldeira, a climate modeler at the Carnegie Institution of

Washington in Stanford, Calif. Even in the worst case cited by Tilmes and her

colleagues, polar residents would experience levels of ultraviolet radiation no higher

than those routinely seen in San Diego today, he contends. There are several ways to

address detrimental side effects of geoengineering, he suggests. For example,

scientists could design the aerosols to drop out of the atmosphere before they reach

polar regions, where they wreak most of their havoc.

Other researchers aren't so sanguine. The new research is "a valuable first step that

shows both the limits and the strengths of such analyses," says Michael J. Mills, an

atmospheric scientist at the University of Colorado, Boulder. "Climate is a complex

system, and before we do something like this, a lot more modeling needs to be done."

"It's always been clear that geoengineering would have some detrimental effect, but this

paper quantifies it," says Bill Chameides, an atmospheric chemist at Duke University

in Durham, N.C. Also, he notes, masking the planet-warming effects of carbon dioxide

emissions rather than reducing them doesn't do anything to reduce ocean acidification,

another harmful side effect of burgeoning atmospheric concentrations of carbon dioxide

(SN: 3/15/08, p. 170).

Adrian Tuck, formerly an atmospheric scientist at the Earth System Research Laboratory in Boulder, Colo. "Most of us share worries about geoengineering, which is seen as a cure-all to avoid having to bite the bullet about carbon emissions," he adds.

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