How Earth-Scale Engineering Can Save the Planet

5. Enhance Clouds to Reflect Sunlight
Feasibility: 6
Cost: $$
RISK: 7
Some proposed solutions to global warming don´t involve capturing carbon dioxide. Instead they focus on turning down the heat by deflecting or filtering incoming sunlight.

On any given day, marine stratocumulus clouds blanket about one third of the world´s oceans, mostly around the tropics. Clouds form when water vapor clings to dust or other particles, creating droplets. Seeding clouds with tiny salt particles would enable more droplets to form-making the clouds whiter and therefore more reflective. According to physicist John Latham, a senior research associate at the National Center for Atmospheric Research in Boulder, Colorado, boosting reflectivity, or albedo, in just 3 percent of marine stratocumulus clouds would reflect enough sunlight to curb global warming. “It would be like a mirror for incoming solar radiation,” Latham explains.

Latham is collaborating with Stephen Salter, an emeritus professor of engineering design at the University of Edinburgh, who is making sketches for GPS-steered wind-
powered boats that would cruise the tropical latitudes,
churning up salt spray. “I am planning a flotilla of unmanned yachts sailing backward and forward across the wind,” Salter says. “They would drag propellers through the water to generate electricity, which we´d use to make the spray.”

Salter wants to outfit each boat with four 60-foot-tall
Flettner rotors, which look like smokestacks but act like sails. An electric motor starts each rotor spinning, which, along with the wind, creates a pressure differential (less pressure in front of the rotor, more in back), generating forward thrust. From the top of the rotor, an impeller would blast a fine saltwater mist into the air.

Until the concept is tested, Salter isn´t sure exactly how many ships would be needed to mitigate global warming. “Maybe between 5,000 and 30,000,” he says. That may sound like a lot, but Salter notes that for World War II, the U.S. built nearly 100,000 aircraft in 1944 alone.

The Timeline
Latham initially raised the notion in a 1990 paper. “The article went down like a lead balloon,” he says. But early last year in England, at a geoengineering conference hosted by MIT and the Tyndall Centre for Climate Change Research, he presented the concept again. “The consensus was that a number of ideas originally thought to be outlandish were deemed sufficiently plausible to be supported further. Our work fell into that category.” Latham needs a few million dollars to test his idea. “On the scale of the damage that will be caused by global warming, that is utterly peanuts.”

The Promise
What´s nice about this idea is that it can easily be fine-tuned. “If we tried it and there was some deleterious effect, we could switch it off, and within four or five days all evidence would have disappeared,” Latham says.

The Perils
One worry is that although the tiny salt particles released by evaporating sea mist are perfect for marine
stratocumulus-cloud formation, they are too small to create rain clouds. “You might make it harder for rain to form,” Salter says. “Therefore, you would not want to do this upwind of a place where there is a bad drought.”

6. Deflect Sunlight With A Mirror
Feasibility: 1
Cost: $$$$
RISK: 5
One of the most ambitious schemes is a giant space “mirror” positioned between the Earth and sun to intercept sunlight. To build the mirror, physicist Lowell Wood, a senior staff scientist at Lawrence Livermore, proposes using a mesh of aluminum threads that are only a millionth of an inch in diameter and a thousandth of an inch apart. “It would be like a window screen made of exceedingly fine metal wire,” he explains. The screen wouldn´t actually block the light but would simply filter it so that some of the incoming infrared radiation wouldn´t reach Earth´s atmosphere.

The Timeline
Wood, who has been researching the mirror idea for more than a decade, says it should be considered only as a safety net if all other means of reversing global warming “fail or fall grossly short over the next few decades.”

The Promise
Once in place, the mirror would cost almost nothing to operate. From Earth, it would look like a tiny black spot on the sun. “People really wouldn´t see it,” says Michael MacCracken. And plant photosynthesis isn´t expected to be affected by the slight reduction in sunlight.

The Perils
Wood calculates that deflecting 1 percent of incoming solar radiation would stabilize the climate, but doing so would require a mirror spanning roughly 600,000 square miles-or several smaller ones. Putting something that size in orbit would be a massive challenge, not to mention exorbitantly expensive.