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If fossil fuel technologies got us into the global warming crisis, can technology get us out? Scientists say massive, planet-saving interventions are possible, but they aren't for the faint of heart. Efforts to "geo-engineer" earth, they warn, will incur staggering costs and pose risks as unpredictable as global warming itself. "Cutting carbon must come first," says David Keith, an energy expert at the University of Calgary. "To think of geo-engineering as a solution is ludicrous." Still, even Keith agrees that, in a worst-case scenario, big fixes may be needed. Arctic temperatures in recent years have risen about three times faster than the world average, stoking fears that sea levels might climb rapidly. The looming costs of climate change in terms of lives, land, and infrastructure lost could eventually justify extraordinary acts. Here are some proposals--and risks:1 SOLAR SHADE
Scientists have long toyed with the idea of orbiting a huge mirror around the earth to block the sun's rays. Any such contraption would be too heavy to loft, so J. Roger P. Angel, a professor of optics at the University of Arizona, envisions shooting trillions of ultrathin, lightweight glass "fliers" into space instead. The discs would be aimed at a sweet spot where the gravitational pull of the sun is equal to that of the earth. They would refract just 2% of sunlight--too little to notice on the ground, but enough to offset a doubling of CO2 levels. Angel proposes using exotic magnetic launchers to shoot bins of fliers into orbit. Risks: Huge costs, plus it would be impossible to turn off the shading effect.2 ALGAE FARMING
In the same way that trees are planted to capture CO2, algae growth could be stimulated in the oceans. As the mass of plankton grew, it would mop up CO2; when the little plants died, they would sink to the ocean floor. The missing ingredient to trigger one of these blooms is iron, which is scarce in some otherwise mineral-rich waters around the globe. But in experiments researchers have shown that a spritz of iron can spawn algae blooms even in fairly barren waters. Risks: Unlike a solar shade, this approach would take a long time to lower global temperatures, and huge blooms might harm sea life by depleting waters of vital minerals.3 CLOUD FACTORIES
Some of the sun's rays are reflected back into space by clouds, ice, snow, and dust. The impact can be significant. A single, big volcanic eruption can spew enough fine matter into the air to lower the earth's surface temperatures. To replicate this effect, Stephen Salter at the University of Edinburgh ponders a fleet of floating cloud generators. Mounted on active or retired freighters and powered by wind or wave energy, the vessels would use vertical turbines to spin ocean water into a fine salty mist. Once airborne, the salt molecules would seed the formation of big banks of white, highly reflective clouds. Risks: This approach could cause unwanted increases in rainfall in many areas. The size and cost of deploying a mammoth cloud fleet is also unknown.4 MOVING OCEANS
If the seas rise too high, why not relocate the waters to low-lying deserts? Futurist Kim Stanley Robinson explores this idea in his upcoming novel Sixty Days and Counting. Engineers might construct sprawling nuclear-powered waterworks to pump the oceans into desert basins such as Death Valley or the Arabian Desert. Risks: In principle, this isn't so different from rerouting rivers to thirsty cities, but the energy expenditure would be enormous. And the impact of the new bodies of water on nearby regions is hard to predict.5 SALTING THE SEAS
Scientists worry that freshwater from melting Arctic ice and Greenland's glaciers will dilute and disrupt the Gulf Stream as it loops through the North Atlantic. In the past, when this conveyor belt of warm water has stalled, Northern Europe was sent into a mini-Ice Age. To keep the current flowing, Robinson suggests that tanker loads of mineral salt could be dumped into the sea at key points along the Gulf Stream. Since saltier, denser water sinks, staggered deliveries of salt could jolt the cycle and keep the current going. Risks: Altering ocean chemistry on this scale could have catastrophic effects on sea life. Will we have a choice, though? "I'm as dubious about geo-engineering as anyone else," says Robinson. "But the fact is, with climate change we're already doing it." By Adam Aston