Launching sun-blocking aerosols into space or growing vast new forests could help cool the planet, but such "geoengineering" schemes will have a tough time stopping sea-level rise by the end of the century, a new study suggests. More extreme schemes could help, however, as might combining geoengineering with drastic cuts in greenhouse gas emissions.
Scientists hope that geoengineering could, in addition to curbing global warming, stop sea-level rise by cooling the planet enough to slow glacier melting and flow. Rising waters are a threat to coastal cities around the world—and even with strict cuts in emissions, scientists believe sea levels will rise significantly by 2100.
The new study is the first to estimate how much geoengineering might reduce sea-level rise, says study leader John Moore of Beijing Normal University in China. According to a model developed by Moore and colleagues, without geoengineering, sea levels would rise between 50 to 100 centimeters over the next century. Relatively modest geoengineering schemes would help only a bit. One of the these schemes would involve mimicking large volcanic eruptions, like the 1991 eruption of Mount Pinatubo—the second largest of the past 100 years—which shot sunlight-blocking sulfate aerosols high into the atmosphere. Moore's team found that pumping a Pinatubo-worth of sulfates into the stratosphere every 4 years would limit sea-level rise to about 30 centimeters. A similar effect would be achieved by combining biomass power plants that capture their CO2 with vast new forests and enormous quantities of biochar—a form of charcoal that locks away carbon from the air.
To cut sea-level rise to zero, Moore says researchers would need to combine these approaches with drastic cuts in greenhouse gas emissions—or employ "extreme geoengineering" strategies. These could include injecting aerosols at Pinatubo levels into the atmosphere every 18 months instead of every 4 years, or launching trillions of tiny lenses into space, the team reports online this week in the Proceedings of the National Academy of Sciences.
These extreme geoengineering approaches are potentially hazardous, however, as blocking sunlight can alter rainfall patterns and even damage the ozone layer, Moore says. "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.
Geochemist Ken Caldeira of the Carnegie Institution for Science in Stanford, California, says the findings are "worthy" but that it's too early to decide which approach is best. "Significantly more study is needed before we are justified in making general claims." Every geoengineering method has pros and cons, Caldeira says, and involves a tradeoff of costs, risks, and effectiveness.