May 2008

A close look at one ­geoengineering scheme

Researchers find that plan to artificially cool Earth could damage ozone layer

As society grapples with how to address climate change, some scientists have turned their attention toward a bold, direct route for cooling Earth: geoengineering.

Geoengineering is a broad term for rearranging Earth’s environment on a large scale to suit human needs and enhance habitability. For climate, this includes futuristic-sounding schemes to launch mirrors into orbit to shield the planet from the Sun, as well as more down-to-Earth plans such as reforesting the globe on a massive scale to absorb carbon dioxide. Some scientists and policymakers believe that such plans could function as an insurance policy for society if efforts to reduce fossil fuel consumption are not sufficient to slow climate change.

One geoengineering proposal that has received considerable attention is to regularly inject sulfate particles into the stratosphere to block sunlight. The goal would be to cool Earth’s surface much as sulfur particles from major volcanic eruptions have cooled temperatures in the past.

According to new research by Simone Tilmes (ESSL/ACD and ASP), however, such injections could have a drastic impact on Earth’s protective ozone layer. In a study published in Science Express in April, Simone and colleagues describe how the particles would delay the recovery of the Antarctic ozone hole by decades and cause significant ozone loss over the Arctic during very cold Arctic winters.

“Our research indicates that trying to artificially cool off the planet could have perilous side effects,” Simone says. “We knew that sulfate injections would impact the ozone layer, but the extent was never quantified before.”

The ozone layer is critical for life on Earth because it blocks dangerous ultraviolet radiation from the Sun. The international community took action to protect it with the Montreal Protocol of 1987, which restricted the production of ozone-destroying chemical compounds known as chlorofluorocarbons (CFCs). Scientists have closely monitored the Antarctic ozone hole since then, and expect it to recover by around 2068.

Simone expects the new research to encourage scientists and decision makers to proceed with extra caution. “While climate change is a major threat, more research is required before society might attempt global geoengineering solutions,” she says. “Scientists need to understand the consequences for the entire atmosphere and biosphere that could result from such an approach, so as not to worsen the situation.”

Sulfates and ozone

Simone collaborated with Rolf Müller (Jülich Research Center) and Ross Salawitch (University of Maryland). To assess the potential impact of the geoengineering proposal, the researchers focused on ozone over the poles. Airborne sulfates from volcanic eruptions have a negative effect on this ­atmospheric region, because as the particles drift into the lower stratosphere above the poles they provide a surface on which chlorine gases can become activated, causing chemical reactions that intensify destruction of ozone molecules.

The researchers found that if sulfates were injected into the atmosphere at the magnitude under discussion, they would likely destroy from about one-fourth to three-fourths of the ozone layer above the Arctic over the next few decades, depending on the size of aerosols used and the severity of Arctic winters. Because chlorine activation occurs under very cold temperature conditions within the polar vortex, very cold Arctic winters are estimated to deplete more ozone than warmer winters.
The sulfates would also delay the expected recovery of the ozone hole over Antarctica by about 30 to 70 years.

Carrying out the study

To determine the relationship between sulfates and ozone loss, the researchers used a combination of measurements and computer simulations. They estimated future ozone loss by looking at two hypothetical geoengineering schemes, one that would use sulfates the same size as those from volcanoes and another that would use much smaller ones.

The study found that injections of the smaller particles would reduce the ozone layer over the Arctic by 100–230 Dobson Units over the next 20 years. As the average thickness of the ozone layer in the Northern Hemisphere is 300–450 Dobson Units, this represents a significant loss of ozone. Injections of the larger particles would result in a loss of 70–150 Dobson Units. The ozone loss would drop in the later part of the century to about 60–150 Dobson Units, depending on the size of the sulfates and the severity of winters.

A Dobson Unit is equivalent to the number of ozone molecules that would create a layer 0.01 millimeters thick under conditions at Earth’s surface.

Above Antarctica, most of the ozone layer is already depleted. Sulfate injections would not significantly reduce its thickness. Instead, they would significantly delay the recovery of the ozone hole.

The impacts of sulfate injections in both regions would likely be somewhat less during the second half of the century as the ozone layer recovers in response to the Montreal Protocol’s restrictions on CFCs.

The researchers caution that the actual impacts on ozone could be different than estimated if atmospheric changes led to unusually warm or cold polar winters, and they warn that a geoengineering project could lead to even more severe ozone loss if a major volcanic eruption took place at the same time. They also emphasize that more research is needed on how climate change and geoengineering impact the dynamical and chemical conditions in the stratosphere above low and mid latitudes.

In this issue...

A close look at one geoengineering scheme

Talks and treats for National Library Week

People, planet, and productivity: Sustainable UCAR

Multimedia Services helps staff collaborate across time and space

“The Stories Clouds Tell” gets a facelift

Delphi questions

Warren M. Washington Digital Collection

Just One Look