Of all the ideas circulating for blocking solar heat, however, sulfur-aerosol injection—the
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.”