I f we were transported forward in time, to an Earth ravaged by catastrophic climate change, we might see long, delicate strands of fire hose stretching into the sky, like spaghetti, attached to zeppelins hovering 65,000 feet in the air. Factories on the ground would pump 10 kilos of sulfur dioxide up through those hoses every second. And at the top, the hoses would cough a sulfurous pall into the sky. At sunset on some parts of the planet, these puffs of aerosolized pollutant would glow a dramatic red, like the skies in Blade Runner. During the day, they would shield the planet from the sun’s full force, keeping temperatures cool—as long as the puffing never ceased.
Technology that could redden the skies and chill the planet is available right now. Within a few years we could cool the Earth to temperatures not regularly seen since James Watt’s steam engine belched its first smoky plume in the late 18th century. And we could do it cheaply: $100 billion could reverse anthropogenic climate change entirely, and some experts suspect that a hundredth of that sum could suffice. To stop global warming the old-fashioned way, by cutting carbon emissions, would cost on the order of $1 trillion yearly. If this idea sounds unlikely, consider that President Obama’s science adviser, John Holdren, said in April that he thought the administration would consider it, “if we get desperate enough.” And if it sounds dystopian or futuristic, consider that Blade Runner was set in 2019, not long after Obama would complete a second term.
Humans have been aggressively transforming the planet for more than 200 years. The Nobel Prize–winning atmospheric scientist Paul Crutzen—one of the first cheerleaders for investigating the gas-the-planet strategy—recently argued that geologists should refer to the past two centuries as the “anthropocene” period. In that time, humans have reshaped about half of the Earth’s surface. We have dictated what plants grow and where. We’ve pocked and deformed the Earth’s crust with mines and wells, and we’ve commandeered a huge fraction of its freshwater supply for our own purposes. What is new is the idea that we might want to deform the Earth intentionally, as a way to engineer the planet either back into its pre-industrial state, or into some improved third state. Large-scale projects that aim to accomplish this go by the name “geo-engineering,” and they constitute some of the most innovative and dangerous ideas being considered today to combat climate change. Some scientists see geo-engineering as a last-ditch option to prevent us from cooking the planet to death. Others fear that it could have unforeseen—and possibly catastrophic—consequences. What many agree on, however, is that the technology necessary to reshape the climate is so powerful, and so easily implemented, that the world must decide how to govern its use before the wrong nation—or even the wrong individual—starts to change the climate all on its own.
I f geo-engineers have a natural enemy, it is the sun. Their first impulse is to try to block it out. Stephen Salter, a Scottish engineer, has mocked up a strategy that would cool the planet by painting the skies above the oceans white. Salter’s designs—based on an idea developed by John Latham at the National Center for Atmospheric Research—call for a permanent fleet of up to 1,500 ships dragging propellers that churn up seawater and spray it high enough for the wind to carry it into the clouds. The spray would add moisture to the clouds and make them whiter and fluffier, and therefore better at bouncing sunlight back harmlessly into space. Salter, who has investigated the technical feasibility of this idea minutely (down to the question of whether ship owners would mind affixing spray nozzles to their hulls with magnets), estimates the cost to build the first 300 ships—enough to turn back the climatological clock to James Watt’s era—to be $600 million, plus another $100 million per year to keep the project going.
Roger Angel, an astronomy and optics professor at the University of Arizona, would block the sun by building a giant visor in space. He proposes constructing 20 electromagnetic guns, each more than a mile long and positioned at high altitudes, that would shoot Frisbee-size ceramic disks. Each gun would launch 800,000 disks every five minutes—day and night, weekends and holidays—for 10 years. The guns would aim at the gravitational midpoint between the Earth and the sun, so that the disks would hang in space, providing a huge array of sunshades that would block and scatter sunlight and put the Earth in a permanent state of annular eclipse. Angel’s scheme relies on launch technology that doesn’t yet exist (no one has ever wanted to shoot Frisbees at the sun before), and would cost several trillion dollars. “I know it sounds like mad science,” he says. “But unfortunately we have a mad planet.”
Of all the ideas circulating for blocking solar heat, however, sulfur-aerosol injection—the Blade Runner scenario—may actually be the least mad. And it provides an illustrative example of the trade-offs that all geo-engineering projects of its scale must confront. The approach is already known to work. When Mount Tambora erupted in Indonesia in 1815 and spewed sulfur dioxide into the stratosphere, farmers in New England recorded a summer so chilly that their fields frosted over in July. The Mount Pinatubo eruption in the Philippines in 1991 cooled global temperatures by about half a degree Celsius for the next few years. A sulfur-aerosol project could produce a Pinatubo of sulfur dioxide every four years.
The aerosol plan is also cheap—so cheap that it completely overturns conventional analysis of how to mitigate climate change. Thomas C. Schelling, who won the 2005 Nobel Prize in economics, has pointed out how difficult it is to get vast international agreements—such as the Kyoto Protocol—to stick. But a geo-engineering strategy like sulfur aerosol “changes everything,” he says. Suddenly, instead of a situation where any one country can foil efforts to curb global warming, any one country can curb global warming all on its own. Pumping sulfur into the atmosphere is a lot easier than trying to orchestrate the actions of 200 countries—or, for that matter, 7 billion individuals—each of whom has strong incentives to cheat.
But, as with nearly every geo-engineering plan, there are substantial drawbacks to the gas-the-planet strategy. Opponents say it might produce acid rain and decimate plant and fish life. Perhaps more disturbing, it’s likely to trigger radical shifts in the climate that would hit the globe unevenly. “Plausibly, 6 billion people would benefit and 1 billion would be hurt,” says Martin Bunzl, a Rutgers climate-change policy expert. The billion negatively affected would include many in Africa, who would, perversely, live in a climate even hotter and drier than before. In India, rainfall levels might severely decline; the monsoons rely on temperature differences between the Asian landmass and the ocean, and sulfur aerosols could diminish those differences substantially.
Worst of all is what Raymond Pierrehumbert, a geophysicist at the University of Chicago, calls the “Sword of Damocles” scenario. In Greek legend, Dionysius II, the ruler of Syracuse, used a single hair to suspend a sword over Damocles’ head, ostensibly to show him how precarious the life of a powerful ruler can be. According to Pierrehumbert, sulfur aerosols would cool the planet, but we’d risk calamity the moment we stopped pumping: the aerosols would rain down and years’ worth of accumulated carbon would make temperatures surge. Everything would be fine, in other words, until the hair snapped, and then the world would experience the full force of postponed warming in just a couple of catastrophic years. Pierrehumbert imagines another possibility in which sun-blocking technology works but has unforeseen consequences, such as rapid ozone destruction. If a future generation discovered that a geo-engineering program had such a disastrous side effect, it couldn’t easily shut things down. He notes that sulfur-aerosol injection, like many geo-engineering ideas, would be easy to implement. But if it failed, he says, it would fail horribly. “It’s scary because it actually could be done,” he says. “And it’s like taking aspirin for cancer.”