p>Mimicking volcanoes by throwing particles high into the sky. Maintaining a floating armada of mirrors in space. Burning plant and other organic waste to make charcoal and burying it—or burning it as fuel and burying the CO2 emissions. Even replanting trees. All have been mooted as potential methods of "geoengineering"—"deliberate large-scale manipulation of the planetary environment," as the U.K.'s Royal Society puts it.

The goal, of course, is to cool the planet by remove heat-trapping gases in the atmosphere or reflecting sunlight away. But rising temperature is just one impact of our seemingly limitless emission of greenhouse gases, largely carbon dioxide, into the atmosphere. Arguably a more devastating consequence would be the rise of the seas as warmer waters expand and melting icecaps fill ocean basins higher, potentially swamping nations and the estimated 150 million people living within one meter of high tide. Can geoengineering hold back that tide?

That's what scientists attempted to assess with computer models in a paper published online August 23 in Proceedings of the National Academy of Sciences. In their words, "sea level rise by 2100 will likely be 30 centimeters higher than 2000 levels despite all but the most aggressive geoengineeering." In large part, that's because the ocean has a lot of thermal inertia: it only slowly warms as a result of increasing greenhouse gas levels—and it will only slowly cool down again.

To appreciably change that lag time, the researchers estimate that humanity would require the equivalent of the eruption of Mount Pinatubo—which put 20 million metric tons of particles into the air—at least every other year. That's not as impossible as it sounds, given that commercial aircraft each year bring that much mass to the tropopause (the layer between the troposphere and the stratosphere) but it only buys time, delaying sea level rise by roughly 60 years, and if only if done continuously without pause—a daunting prospect.

Nor would drawing down the carbon dioxide in the atmosphere yield better results. Replanting trees on all the lands that have been cleared of forests during the past 200 years only ends up lowering atmospheric concentrations of greenhouse gases by 45 parts per million (current levels are roughly 390 ppm, 110 ppm above pre-industrial levels). Biochar nets even less: 35 ppm, though it has other benefits.

Perhaps the only way to reduce warming enough to minimize the rise of the oceans is an all-out effort that also includes burning biomass as fuel (either to replace coal or gasoline or both) and pairing it with CO2 capture and storage. Together, they could suck down greenhouse gas levels by 180 ppm—more than enough to bring us below pre-industrial levels. As a result, sea level rise is held to just 10 centimers by 2100, according to the author's modeling.

Such extensive geoengineering seems impractical given its economic (and environmental) cost. But interfering with the planet's carbon cycle—something we're already doing by adding so much CO2 to the atmosphere—appears to be the better bet, even if only by curbing current CO2 emissions. Otherwise, we're leaving our descendants one heck of a mess or, as the authors put it, "substituting geoengineering for greenhouse gas emission abatement or removal constitutes a conscious risk transfer to future generations."

Image: iStockphoto.com / Gregory Albertini