Radical Ways to Cool the Planet

The sudden explosion of Mount Pinatubo on June 15, 1991, sent a vast column of ash into the sky, blotting out the sun, killing hundreds and demonstrating one way to save humanity from a potential climate disaster.

The mountain's 20 million tons of sulfur dioxide rose from the Philippines into the stratosphere, blanketing the planet in a haze that reflected part of the sun's heat back out into space. Over the next several years, meteorologists watched in amazement as the haze lowered the earth's temperature by a cumulative total of half a degree Celsius—setting the clock back on global warming. In the century before Pinatubo, greenhouse gases released by human industry had helped raise the earth's temperature by 1 degree.

The effect was temporary—temperatures started rising again after a year or so. But scientists began to wonder if the volcano hadn't revealed a possible weapon against climate change. It takes only a back-of-the-envelope calculation to see that it would be possible to do artificially what the mountain did naturally. A judicious application of sulfur dioxide to the upper atmosphere, which could be accomplished by launching the gas from rockets, spraying it from high-altitude planes or releasing it from a big chimney, would have an almost immediate impact on temperature. And it would cost a thousand times less than even the most optimistic scenarios for cutting emissions. A small group of scientists began looking into how this kind of geo-engineering could be done most efficiently and with the fewest side effects.

Over the past two decades geo-engineering began to include other ways of fixing climate, including new spins on the Pinatubo effect. Using sulfur dioxide or other materials, they aim to reflect sunlight back into outer space. One would boost a series of mirrors into orbit, shading Earth from sunlight, but at a cost that would likely bankrupt the planet. In the 1990s, the controversial inventor of the hydrogen bomb, Edward Teller, proposed floating reflective particles of metal in the atmosphere, adding a Dr. Strangelove air to the geo-engineering field.

The other, more publicly acceptable form of geo-engineering would focus on removing carbon from the atmosphere and storing it underground. Known as carbon capture and storage (CCS), this idea is behind today's experimental clean-power plants, which are attracting lots of research and funding. But clean-coal plants will only reduce future emissions, which does not address the root of the problem. Among all the uncertainties that still surround climate change, one thing has become clear: the scary durability of carbon, which will hang in the air for a thousand years, continuing to warm the planet no matter how drastically future emissions are cut. So there is a growing urgency behind the geo-engineer's dream: to change the climate by artificial means, either sucking the existing carbon out of the air or cooling the air with solar reflectors.

Geo-engineering labored on the lunatic fringe of climate policy until recently. Experts shunned its ideas as mad science, and for fear that it would undermine the campaign to cut carbon emissions. People are not going to make hard sacrifices to combat global warming if they get the impression that a quick engineering trick can erase the threat. Besides, the very idea of engineering climate change spooks people. If science can't reliably predict the weather, how can it reliably engineer the global climate? The 20th century saw many less-ambitious efforts to reshape the earth—by diverting rivers, for example—end in disaster.

Now, however, many scientists are starting to take geo-engineering seriously, if only out of desperation. As more and more climate specialists come to believe that even current levels of carbon pollution are warming the globe more rapidly than previously thought, the case for developing an emergency earth-rescue plan is getting difficult to resist. Nobel laureates Paul Crutzen and Thomas Schelling have endorsed the need for a climate-engineering plan. The British Royal Society has begun to study the options. The U.S. National Academies of Science just convened a conference in which climate engineering was discussed, and it's scheduled a meeting in June to hammer out details. "It still hasn't shown up on politicians' agendas," says Princeton climate scientist Michael Oppenheimer, "but scientists are now talking about it as an option."

Oppenheimer is working on a team that will produce a National Academies of Science report recommending a geo-engineering option in climate policy. Oppenheimer is on the cautious side, supporting theoretical research only, with no field experiments that might have a lasting impact on the environment. Other scientists, however, are beginning to press the issue. "A lot of people don't like the idea of solving our problems with technology," says Harvard climatologist Dan Schrag. "If you're sitting in an ivory tower, it's easy to say we have to all change our behavior. But what if we don't? Do you really want to kiss the earth goodbye?"

The most eloquent argument for geo-engineering as a Plan B is the failure of Plan A—emissions cuts. The Kyoto agreement calls for a 5.2 percent reduction of emissions below 1990s levels by 2012. Of the 40 countries that signed the agreement in 2001, 21 have seen carbon emissions increase since then. That includes Japan, which hosted the talks. Although Britain, Germany and France have managed to make reductions, none is currently on track to meet its Kyoto target. And Kyoto didn't include China or the United States, the world's No. 1 and 2 carbon emitters.

The campaign to expand and tighten Kyoto now looks likely to fall short, too. In December, negotiators will gather in Copenhagen and attempt to extend the Kyoto agreement, which expires in 2012. They also hope to impose more demanding cuts in emissions, and to bring China and the U.S. onboard. The hope is that Copenhagen will cap the amount of carbon in the atmosphere at a level low enough to prevent temperatures from rising more than 2 degrees Celsius. If temperatures go higher, many climate scientists worry that the effects will exact an unacceptable toll on the environment, exacerbating drought and flooding, endangering coastal areas from sea-level rise, hurting agriculture and leading to loss of biodiversity. No one is certain how sharp the cuts in greenhouse-gas emissions need to be, but the best guess is a reduction of 80 percent below current levels by 2050. That's a radical goal for Europe, Japan and the United States. For China and India, which are trying to grow their way out of poverty, it's almost unthinkable.

The glacial pace of global politics may be no match for the pace of climate change. Just because the earth's climate system is a big, slow-moving machine with lots of inertia, it doesn't mean a catastrophe would also be slow in coming. One of the increasingly frightening scenarios is a snowball effect, in which, for example, carbon begins to seep out of melting permafrost, or warming oceans raise atmospheric humidity, amplifying the greenhouse effect of carbon. Perhaps most important is the growing consensus that the gas we've already emitted will go on warming the earth for centuries. The landmark 2007 report from the U.N.'s Intergovernmental Panel on Climate Change estimated that by the end of the century, temperatures could rise by between 2 and 5 degrees Celsius. An increasing number of scientists are coming to believe in the worst-case scenario. A 5-degree spike by 2100— a brief instant in geologic time—would almost certainly prove to be a disaster for civilization. That's why many scientists have begun to urge serious consideration to geo-engineering schemes that only a few years ago seemed absurd and dangerous.

The irony is that the more respectable geo-engineering option, carbon capture, is also by far the more expensive and less likely to counteract a steep rise in temperatures. It's easy to see why the idea of cleaning the air is far less controversial than the idea of cooling it—because cleaning involves no massive release of new gases or hardware, and thus far less risk that the experiment will run out of control. Columbia University climate scientists Wallace Broecker and Klaus Lackner have argued that the same technology used for capturing the carbon in coal plants could be trained on the atmosphere at large. But the task of vacuuming up this much carbon would be mind-bogglingly huge. Each year roughly 30 billion metric tons of carbon dioxide are released by the world's industries and autos. If converted to liquid form, it would take less than four years to fill an underground space with the volume of Lake Geneva.

And that doesn't take into account the 1.8 percent yearly rise in emissions, or the billions of tons of carbon dioxide that have already accumulated in the atmosphere for the past 100 years (there's no reliable estimate of how much that is). Scientists still think there's enough porous rock deep beneath the earth's surface to accommodate all the liquid carbon dioxide we can pump, but getting it there would take many years and cost billions. Assuming the cost of removing carbon eventually falls to $50 a ton (it now costs $200 per ton), the bill for removing only the current year's emissions would reach $150 billion.

The idea of engineering climate isn't new. In 1965 a report fell on President Lyndon Johnson's desk exploring ways to manipulate climate to compensate for a rise in temperatures. (Oddly enough, the report never mentioned cutting emissions.) The idea made the rounds in scientific circles over the next decades, but by the 1990s it had disappeared from discussion, mainly because policymakers were trying to build a consensus for emissions cuts. "It became so un-PC we couldn't talk about it," David Keith, a physicist at the University of Calgary, told a TED conference in 2007. "It just sank below the surface; we weren't allowed to speak about it."

In 2006 Crutzen, a chemist, broke the ice with a paper in the journal Climate Change. He repeated an idea that originated with Russian physicist Mikhail Budyko, who proposed in 1974 using planes to release sulfur dioxide, or SO2, into the atmosphere, where it would react with water and other molecules to form sulfate particles—the same stuff as volcanic ash. (Crutzen preferred using weather balloons to carry the gas aloft.) Crutzen pointed out that the amount of SO2 you'd need to lower temperatures significantly is surprisingly small—nowhere near the 20 million tons that Pinatubo released, much of which was wasted near the ground where it had no effect on temperatures. By Crutzen's reckoning, it would take about 1.5 million tons to counteract the effects of a doubling of CO2 concentrations from preindustrial levels to 550 parts per million (today's level is 385ppm, but it will certainly rise by the time any geo-engineering scheme goes into effect). Others have put the figure as high as 5 million tons—still nothing that couldn't be handled by a fleet of airplanes for a few billion dollars. The low price is rather astonishing. Britain's authoritative 2006 report on the cost of cutting emissions enough to stabilize temperatures put the price tag at about 1 percent of the world's annual GDP; other estimates run as high as 4 percent. The options for cooling the planet by deflecting solar rays are far cheaper: for one thousandth of 1 percent of GDP, "you could bring on an ice age," says Keith.

Geo-engineering, say critics, would create many nasty side effects. One of the drawbacks to SO2 is that it destroys the ozone layer, exposing people in the Southern Hemisphere to deadly ultraviolet radiation. There's some disagreement over how severe this destruction would be. Mount Pinatubo increased the ozone hole over the South Pole only slightly, but some studies say a large release of SO2 could increase the southern hole and perhaps even cause one to appear in the north as well. One way around the problem is to go slowly—release SO2 a bit at a time, study how the climate responds and try a bit more. If there was big ozone damage, the experiment could be halted and the SO2 would quickly dissipate. This should provide some comfort to people frightened by the idea of engineers permanently tampering with the atmosphere.

Keith is working on designing particles that are more efficient at cooling than sulfates, but without the side effects. Because sulfates tend to settle on the ground after a few months, they'd need to be replenished regularly. Keith's engineered particle would absorb the sun's energy unevenly, causing one side to heat more quickly than the other and to drift upward. Such a particle might be released on the ground and rise up on its own accord. It might be built in such a way that it rises higher than the ozone layer—to the mesosphere, 100 kilometers up—where it would reflect light but leave the ozone intact. For safety reasons, these particles could also have preordained lifetimes. "It's something we're developing," he says. "It may not work. But it's almost certain that if engineers put their minds to this, something could be made to work better than sulfates."

The most devastating side effect could be political. Success in lowering temperatures—or even the knowledge that scientists had the means to do so—might decrease the political will to make costly emissions cuts. Not even the most zealous advocate of geo-engineering argues for using it in lieu of cutting and capturing carbon. Using geo-engineering to keep temperatures artificially low while carbon levels continued to rise would be doubling down the risk of rapid warming should the air-cooling project suddenly stop, or need to be halted, for any reason.

The debate over whether to re-create the Pinatubo effect could eventually turn out to be moot. The technology is potentially so cheap that virtually any nation, or at least a middle-rate power, could undertake a climate-cooling project on its own. And as the grand river-diversion projects of the former Soviet Union and China have shown, certain kinds of regimes are far more confident of their ability to reshape the environment. It's entirely possible that someone is going to use the technology eventually, especially in countries where droughts and other climate-related weather become a political issue. "If one country can do it, another can," says Scott Barrett, an economist at Johns Hopkins University. "Along with climate change, there'll be an increase in political tensions."

The notion of engineering the climate may be frightening, but it may also be unavoidable, which is why policymakers would do well to start thinking about it now. If global warming does accelerate in coming years, any scheme to stop it may start to look safer than the alternative.