they would form microscopic particles to block sunlight. The idea is based on a natural phenomenon. Every few decades a volcano erupts so violently that it sends several millions of tons of sulfur–in the form of sulfur dioxide–more than 10 kilometers into the upper reaches of the atmosphere, a region called the stratosphere. The resulting sulfate particles spread out quickly and stay suspended for years. They reflect and diffuse sunlight, creating a haze that whitens blue skies and causes dramatic sunsets. By decreasing the amount of sunlight that reaches the surface, the haze also lowers its temperature. This is what happened after the 1991 eruption of Mount Pinatubo in the Philippines, which released about 15 million tons of sulfur dioxide into the stratosphere. Over the next 15 months, average temperatures dropped by half a degree Celsius. (Within a few years, the sulfates settled out of the stratosphere, and the cooling effect was gone.)
Scientists estimate that compensating for the increase in carbon dioxide levels expected over this century would require pumping between one million and five million tons of sulfur into the stratosphere every year. Diverse strategies for getting all that sulfur up there have been proposed. Billionaire investor Nathan Myhrvold, the former chief technology officer at Microsoft and the founder and CEO of Intellectual Ventures, based in Bellevue, WA, has thought of several, one of which takes advantage of the fact that coal-fired power plants already emit vast amounts of sulfur dioxide. These emissions stay close to the ground, and rain washes them out of the atmosphere within a couple of weeks. But if the pollution could reach the stratosphere, it would circulate for years, vastly multiplying its impact in reflecting sunlight. To get the sulfur into the stratosphere, Myhrvold suggests, why not use a “flexible, inflatable hot-air-balloon smokestack” 25 kilometers tall? The emissions from just two coal-fired plants might solve the problem, he says. He estimates that his solution would cost less than $100 million a year, including the cost of replacing balloons damaged by storms.
Cloud Brightening: Tiny droplets made by spraying an extremely fine mist of seawater into low-lying clouds could make them reflect more sunlight than ordinary clouds. Pros: Shading could be targeted—to stop the melting of Arctic Sea ice, for example. Cons: Scientists don’t know how it would affect precipitation and temperatures over land, where it would matter most.
Not surprisingly, climate scientists are not ready to sign off on such a scheme. Some problems are obvious. No one has ever tried to build a 25-kilometer smokestack, for one thing. Moreover, scientists don’t understand atmospheric chemistry well enough to be sure what would happen; far from alleviating climate change, shooting tons of sulfates into the stratosphere could have disastrous consequences. The chemistry is too complex for us to be certain, and climatemodels aren’t powerful enough to tell the whole story.
“We know Pinatubo cooled the earth, but that’s not the question,” Schrag says. “Average temperature is not the only issue.” You’ve also got to account for regional variations in temperature and effects on precipitation, he explains–the very things that climate models are notoriously bad at accounting for. Prinn concurs: “If we lower levels of sunlight, we are unsure of the exact response of the climate system to doing that, for the same reason that we don’t know exactly how the climate will respond to a particular level of greenhouse gases.” He adds, “That’s the big issue. How can you engineer a system you don’t fully understand?”
The actual effects of Mount Pinatubo were, in fact, complex. Climate models at the time predicted that by decreasing the amount of sunlight hitting the surface of the earth, the haze of sulfates produced in such an eruption would reduce evaporation, which in turn would lower the amount of precipitation worldwide. Rainfall did decrease–but by much more than scientists had expected. “The year following Mount Pinatubo had by far the lowest amount of rainfall on record,” says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, CO. “In fact, it was 50 percent lower than the previous low of any year.” The effects, however, weren’t uniform; in some places, precipitation actually increased. A human-engineered sulfate haze could have similarly unpredictable results, scientists warn.
Ocean Fertilization: Adding iron or other nutrients to the ocean could promote algae blooms, which would capture carbon dioxide and store some of it deep in the ocean. Pros: It would directly address the root of climate change: carbon dioxide in the atmosphere. Cons: At best, it could offset an eighth of the greenhouse-gas emissions attributed to humans, and it could harm ecosystems.
Even in a best-case scenario, where side effects