You could plant huge new forests where none have been before. You could blast particles into the sky to block the sun's radiation. You could put mirrors in space. These planetary hacks could slow global warming, but one thing that none of them could do, most likely, is to stop the rising sea levels that a warming planet will bring. That's the contention of John Moore, lead author of a study out in the Proceedings of the National Academy of Sciences. Moore's team examined five different means that scientists have proposed to hack the planet and save ourselves from anthropogenic global warming. The geoengineering schemes—forestation, atmospheric aerosols, space mirrors, biochar, and the use of biofuels plus carbon sequestration—are focused either on reducing the amount of energy the Earth absorbs or pulling carbon out of the atmosphere. So Moore wanted to see what they could do about a side effect of the extra heat: melting ice raising the global average sea level. The results weren't terribly encouraging. Sea levels respond slowly to changes in the planet's temperature, Moore told Nature News, so "you can't just slam on the brakes."
Injecting sulphate aerosols into the stratosphere – which reduces the amount of sunlight reaching the surface of the Earth – had little effect. If emissions are allowed to grow at current rates, the model showed sea levels rising by 1.1 metres by 2100. Aerosols could reduce that to 0.8 metres by 2100, but with the rate of rise showing no sign of slowing down at the end of the century, this would simply delay greater rises, not prevent them. [New Scientist].
Space mirrors began to reverse the rising sea level trend, but only at about the end of the 21st century. If people quickly developed biofuels and became adept at carbon sequestration, things were even a tad better—but in Moore's model the sea level still rose by 30 centimeters, or about a foot, mostly because of effects that are already locked into the system. Says Moore:
"I think that sucking CO2 out of the atmosphere is the best way to stop sea-level rise before 2100." That could be accomplished with the biomass power plants and new forests considered in the study, or by massively scaling up CO2 removal techniques currently deployed in spacecraft and submarines" [ScienceNOW].
Given the unintended consequences that could come with tinkering with the planet on such a massive scale, keeping intervention to a minimum would seem like the ideal choice. But Moore's study reiterates that fear that it might be too late for little steps. It might be time to consider the "extreme geoengineering"—say, atmospheric aerosol injections every year and a half instead of every four years—that potentially could slow down rising temperatures and sea levels... at an unknown cost. But once you start, you can't stop.
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" [Nature].
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