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Geoengineering won't curb sea-level rise

August 23, 2010 By Richard A. Lovett This article courtesy of Nature News.

Space mirrors and 'volcanic' blasts are not an easy fix for the rise in sea levels.

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Unless they involve extreme measures, geoengineering approaches to offset the effects of human-driven climate changes won't do much to combat rising sea levels, an international team of scientists reports in the Proceedings of the National Academy of Sciences1.

That is because sea levels respond slowly to changes in Earth's temperature, says John Moore, a palaeoclimatologist at Beijing Normal University and lead author of the study.

"We've got this 150-year legacy of fossil-fuel [burning], land-use changes, et cetera," he says. "You can't just slam on the brakes instantaneously."

Moore and his team examined two proposed geoengineering schemes: mirrors orbiting in space to reduce incoming sunlight, and sulphates being shot into the upper atmosphere to create a bright, sunlight-reflecting haze — similar to the one produced naturally by the 1991 eruption of Mount Pinatubo in the Philippines. Either scheme could reduce incoming solar energy by about 1–4 watts per square metre, enough to offset the atmospheric warming caused by carbon dioxide build-up until at least 2070.

To determine how this might affect sea levels, Moore and his group used a computer model that could track historical sea-level changes over the past 300 years. They found that under a business-as-usual fossil-fuel scenario, even the 4-watt space mirror reduced this century's sea-level rise by only some 39 centimetres out of a projected 'no-intervention' rise of about 1 metre.

Only in combination with fairly aggressive carbon dioxide emissions reductions, Moore and his team calculated, could these geoengineering schemes have a larger effect. Even then, they would not completely stop sea-level rise, with the oceans likely to be about 30 centimetres higher by 2100, depending on the emissions scenario.

Two and a half Pinatubos

To nip sea-level rise in the bud, the scientists calculated, would require either injections of sulphur dioxide aerosol into the stratosphere equivalent to more than 2.5 Pinatubo eruptions every four years, or a commitment to constructing an ever-expanding space mirror.

Another problem, Moore says, is that once started, geoengineering must be continued or temperatures will quickly rebound to what they would have been without intervention. An attendant surge in sea-level rise wouldn't occur quite as quickly, but it would follow soon enough, at a rate of up to 1–2 centimetres per year, he says.

"Those are speeds that were observed during the last deglaciation," says Moore, "so we're not forecasting anything that is out of the geological record."

The economic effects of that would be so extreme, he adds, that a cost-benefit analysis indicates that geoengineering isn't even worth considering if there is more than about a one-in-ten chance that it will be suddenly discontinued. "It's playing economic roulette."

Richard Alley, a glaciologist and climate researcher at Pennsylvania State University in Philadelphia, calls the findings a "nice advance". He notes, however, that it is only the beginning of trying to determine how glaciers might react to geoengineering.

"We don't really have an ice-sheet model that we trust," says Alley, noting that, in addition to global warming, glaciers react to local and regional changes in winds, ocean temperatures and ocean circulation. "In many ways," he says, "this large advance serves to show how far we have to go before climate modelling of geoengineering is really good enough that useful regional projections could be made to guide decision-makers."

Alan Robock, a geophysicist from Rutgers University in New Jersey, agrees, but says that one finding that does come through strongly is that geoengineering has only a relatively minor effect on sea-level rise. "Reducing emissions of greenhouse gases will have a much larger impact," he says.

Moore concurs. "Anything that isn't reducing the amount of carbon dioxide in the atmosphere is like putting [on] a bandage rather than actually solving the problem," he says.

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