In October, British researchers supported by the U.K. government will attempt to pump water a kilometer into the air using little more than a helium balloon and a rubber hose. The experiment, which will take place at a military airfield along England’s east coast, is meant as a test of a proposed geoengineering technique for offsetting the warming effects of greenhouse gases. If the balloon and hose can handle the water’s weight and pressure, similar pipes rising 20 kilometers could pump tons of reflective aerosols into the stratosphere.
The scheme, called SPICE (stratospheric particle injection for climate engineering), is one of several proposed geoengineering methods under study. In this case, the idea is that particles injected into the stratosphere would reflect a small percentage of the sun’s energy back into space, thereby cooling the planet. The concept seeks to mimic the cooling effect of volcanoes that inject sulfide particles into the stratosphere in large quantity. A 2009 study by the U.K. meteorological office estimated that 10 million metric tons of sulfide particles injected annually into the stratosphere would cool the planet by approximately 2 °C within a few years.
Other methods of geoengineering have also been tested, including fertilizing oceans to encourage algae blooms and pulling carbon dioxide out of the air. But a 2009 report by the U.K.’s Royal Society concluded that reflective aerosol injected into the stratosphere would be the least expensive and most effective way to rapidly cool the planet.
In addition to the pipe tethered to the balloon, airplanes and rockets could be used to deploy the particles. But Hugh Hunt, a senior lecturer in engineering at the University of Cambridge and a member of the SPICE project, says the balloon-and-pipe approach that his group is testing would be significantly less expensive. “Trying to use airplanes or rockets ends up costing 100 or 1,000 times more than a pipe and balloon,” Hunt says. “At an altitude of 20 kilometers, an airplane can only carry one, maybe two, tons of payload. That means five to 10 million flights per year, burning roughly 1 percent of global oil production. It seems unlikely to me that that would be economically viable when a few dozen pipes would do just as good a job.”
The current pilot program will pump 100 kilograms of water per hour to an altitude of one kilometer. Full-scale designs call for as many as 64 pipes spread around the world, each lifting five kilograms of sulfur dioxide or other reflective particles per second—approximately 160,000 metric tons per year. Each pipe alone would weigh 30 tons and would be held aloft by a balloon 100 meters in diameter, slightly larger than the largest balloons ever built. The biggest challenge of all, however, would be developing a flexible pipe that can withstand ultrahigh pressures. To raise the particles to a height of 20 kilometers, the pipe would have to withstand 4,000 to 6,000 bar, or atmospheres of pressure.
“How do you make a flexible pipe that can carry 6,000 bar of pressure that sways in the wind of the jet stream and guarantee that it will last?” asks Justin McClellan, an engineer with Aurora Flight Sciences, which builds advanced aerospace vehicles for scientific and military applications. “A typical oil and gas rig might see 2,000 bar of pressure, and that is with a roughly quarter-inch-thick steel pipe. Solving the one-kilometer problem is probably not very hard, but when you add up all the requirements for a 20-kilometer pipe, this starts to look pretty unrealistic.”
Hunt acknowledges that multiple challenges put the project “on the edge of what is possible,” but he says that all engineering issues can be overcome within five years. Full-scale deployment could be achieved for approximately 5 billion pounds per year, he maintains.
David Keith, a professor of applied physics at Harvard University’s School of Engineering and Applied Sciences and professor of public policy at Harvard’s Kennedy School, is not impressed. He says the cost of a solution is not an issue, since the costs of climate change are so high. “The impacts of climate change are on the order of a trillion dollars a year, as are the costs of cutting emissions,” Keith says. But he says research should focus on developing the most effective and lowest-risk option for injecting sulfate particles into the atmosphere. Dispersing them by airplane allows for greater and more even distribution of particles, he says, thereby reducing the odds that they will clump together and fall back toward Earth.
“I think SPICE will be the most visible geoengineering project to date, and it may polarize public opinion. But in regards to scientific or engineering interest, I don’t see much,” Keith says.