Rivers fed by melting snow and glaciers supply water to over one-sixth of the world’s population–well over a billion people. But these sources of water are quickly disappearing: the Himalayan glaciers that feed rivers in India, China, and other Asian countries could be gone in 25 years (after this story appeared in print this claim was retracted by scientists: see correction). Such effects of climate change no longer surprise scientists. But the speed at which they’re happening does. “The earth appears to be changing faster than the climate models predicted,” says ­Daniel Schrag, a professor of earth and planetary sciences at Harvard University, who advises President Obama on climate issues.

Atmospheric levels of carbon dioxide have already climbed to 385 parts per million, well over the 350 parts per million that many scientists say is the upper limit for a relatively stable climate. And despite government-led efforts to limit carbon emissions in many countries, annual emissions from fossil-fuel combustion are going up, not down: over the last two decades, they have increased 41 percent. In the last 10 years, the concentration of carbon dioxide in the atmosphere has increased by nearly two parts per million every year. At this rate, they’ll be twice preindustrial levels by the end of the century. Meanwhile, researchers are growing convinced that the climate might be more sensitive to greenhouse gases at this level than once thought. “The likelihood that we’re going to avoid serious damage seems quite low,” says Schrag. “The best we’re going to do is probably not going to be good enough.”

This shocking realization has caused many influential scientists, including Obama advisors like Schrag, to fundamentally change their thinking about how to respond to climate change. They have begun calling for the government to start funding research into geoengineering–large-scale schemes for rapidly cooling the earth.

Strategies for geoengineering vary widely, from launching trillions of sun shields into space to triggering vast algae blooms in oceans. The one that has gained the most attention in recent years involves injecting millions of tons of sulfur dioxide high into the atmosphere to form microscopic particles that would shade the planet. Many geoengineering proposals date back decades, but until just a few years ago, most climate scientists considered them something between high-tech hubris and science fiction. Indeed, the subject was “forbidden territory,” says Ronald Prinn, a professor of atmospheric sciences at MIT. Not only is it unclear how such engineering feats would be accomplished and whether they would, in fact, moderate the climate, but most scientists worry that they could have disastrous unintended consequences. What’s more, relying on geoengineering to cool the earth, rather than cutting greenhouse-gas emissions, would commit future generations to maintaining these schemes indefinitely. For these reasons, mere discussion of geoengineering was considered a dangerous distraction for policy makersconsidering how to deal with global warming. Prinn says that until a few years ago, he thought its advocates were “off the deep end.”

It’s not just a fringe idea anymore. The United Kingdom’s Royal Society issued a report on geoengineering in September that outlined the research and policy challenges ahead. The National Academies in the United States are working on a similar study. And John Holdren, the director of the White House Office of Science and Technology Policy, broached the idea soon after he was appointed. “Climate change is happening faster than anyone previously predicted,” he said during one talk. “If we get sufficiently desperate, we may try to engage in geoengineering to try to create cooling effects.” To prepare ourselves, he said, we need to understand the possibilities and the possible side effects. Even the U.S. Congress has now taken an interest, holding its first hearings on geoengineering in November.

Geoengineering might be “a terrible idea,” but it might be better than doing nothing, says Schrag. Unlike many past advocates, he doesn’t think it’s an alternative to reducing greenhouse-gas emissions. “It’s not a techno-fix. It’s not a Band-Aid. It’s a tourniquet,” he says. “There are potential side effects, yes. But it may be better than the alternative, which is bleeding to death.”

The idea of geoengineering has a long history. In the 1830s, James Espy, the first federally funded meteorologist in the United States, wanted to burn large swaths of Appalachian forest every Sunday afternoon, supposing that heat from the fires would induce regular rainstorms. More than a century later, meteorologists and physicists in the United States and the Soviet Union separately considered a range of schemes for changing the climate, often with the goal of warming up northern latitudes to extend growing seasons and clear shipping lanes through the Arctic.

In 1974 a Soviet scientist, Mikhail Budyko, first suggested what is today probably the leading plan for cooling down the earth: injecting gases into the upper reaches of the atmosphere, where

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.

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.

Even in a best-case scenario, where side effects

And if the geoengineering had to stop–say, for environmental or economic reasons–the higher levels of greenhouse gases would cause an abrupt warm-up. “Even if the geoengineering worked perfectly,” says Raymond Pierrehumbert, a professor of geophysical sciences at the University of Chicago, “you’re still in the situation where the whole planet is just one global war or depression away from being hit with maybe a hundred years’ worth of global warming in under a decade, which is certainly catastrophic. Geoengineering, if it were carried out, would put the earth in an extremely precarious state.”

Figuring out the consequences of various geoengineering plans and developing strategies to make them safer and more effective will take years, or even decades, of research. “For every dollar we spend figuring out how to actually do geoengineering,” says Schrag, “we need to be spending 10 dollars learning what the impacts will be.”

To begin with, scientists aren’t even sure that sulfates delivered over the course of decades, rather than in one short volcanic blast, will work to cool the planet down. One key question is how microscopic particles interact in the stratosphere. It’s possible that sulfate particles added repeatedly to the same area over time would clump together. If that happened, the particles could start to interact with longer-wave radiation than just the wavelengths of electromagnetic energy in visible light. This would trap some of the heat that naturally escapes into space, causing a net heating effect rather than a cooling effect. Or the larger particles could fall out of the sky before they had a chance to deflect the sun’s heat. To study such phenomena, David Keith, the director of the Energy and Environmental Systems Group at the University of Calgary, envisions experiments in which a plane would spray a gas at low vapor pressure over an area of 100 square kilometers. The gas would condense into particles in the stratosphere, and the plane would fly back through the particle cloud totake measurements. Systematically altering the size of the particles, the quantity of particles in a given area, the timing of their release, and other variables could reveal key details about their microscale interactions.

Yet even if the behavior of sulfate particles can be understood and managed, it’s far from clear how injecting them into the stratosphere would affect vast, complex climate systems. So far, most models have been crude; only recently, for example, did they start taking into account the movement of ice and ocean currents. Sulfates would cool the planet during the day, but they’d make no difference when the sun isn’t shining. As a result, nights would probably be warmer relative to days, but scientists have done little to model this effect and study how it could affect ecosystems. “Similarly, you could affect the seasons,” Schrag says: the sulfates would lower temperatures less during the winter (when there’s less daylight) and more during the summer. And scientists have done little to understand how stratospheric circulation patterns would change with the addition of sulfates, or precisely how any of these things could affect where and when we might experience droughts, floods, and other disasters.

If scientists could learn more about the effects of sulfates in the stratosphere, it could raise the intriguing possibility of “smart” geoengineering, Schrag says. Volcanic eruptions are crude tools, releasing a lot of sulfur in the course of a few days,

“So far we’re thinking about a very simplistic thing,” Schrag says. “We’re talking about injecting stuff in the stratosphere in a uniform way.” The effects that have been predicted so far, however, aren’t evenly distributed. Changes in evaporation, for example, could be devastating if they caused droughts on land, but if less rain falls over the ocean, it’s not such a big deal. By taking advantage of stratospheric circulation patterns and seasonal variations in weather, it might be possible to limit the most damaging consequences. “You can pulse injections,” he says. “You could build smart systems that might cancel out some of those negative effects.”

Rather than intentionally polluting the stratosphere, a different and potentially less risky approach to geoengineering is to pull carbon dioxide out of the air. But the necessary technology would be challenging to develop and put in place on large scale.

In his 10th-floor lab in the Manhattan neighborhood of Morningside Heights, Klaus Lackner, a professor of geophysics in the Department of Earth and Environmental Engineering at Columbia University, is experimenting with a material that chemically binds to carbon dioxide in the air and then, when doused in water, releases the gas in a concentrated form that can easily be captured. The work is at an early stage. Lackner’s carbon-capture devices look like misshapen test-tube brushes; they have to be hand dipped in water, and it’s hard to quickly seal them into the improvised chamber used to measure the carbon dioxide they release. But he envisions automated systems–millions of them, each the size of a small cabin–scattered over the countryside near geologic reservoirs that could store the gases they capture. A system based on this material, he calculates, could remove carbon dioxide from the air a thousand times as fast as trees do now. Others at Columbia are working on ways to exploit the fact that peridotite rock reacts with carbon dioxide to form magnesium carbonate and other minerals, removing the greenhouse gas from the atmosphere. The researchers hope to speed up these natural reactions.

It’s far from clear that these ideas for capturing carbon will be practical. Some may even require so much energy that they create a net increase in carbon dioxide. “But even if it takes us a hundred years to learn how to do it,” Pierrehumbert says, “it’s still useful, because CO₂ naturally takes a thousand years to get out of the atmosphere.”

Several existing geoengineeringschemes, though, could be attempted relatively cheaply and easily. And even if no one knows whether they would be safe or effective, that doesn’t mean they won’t be tried.

David Victor, the director of the Laboratory on International Law and Regulation at the University of California, San Diego, sees two scenarios in which it might happen. First, “the desperate Hail Mary pass”: “A country quite vulnerable to changing climate is desperate to alter outcomes and sees that efforts to cut emissions are not bearing fruit. Crude geoengineering schemes could be very inexpensive, and thus this option might even be available to a Trinidad or Bangladesh–the former rich in gas exports and quite vulnerable, and the latter poor but large enough that it might do something seen as essential for survival.” And second, “the Soviet-style arrogant engineering scenario”: “A country run by engineers and not overly exposed to public opinion or to dissenting voices undertakes geoengineering as a national mission–much like massive building of poorly designed nuclear reactors, river diversion projects, resettlement of populations, and other national missions that are hard to pursue when the public is informed, responsive, and in power.” In either case, a single country acting alone could influence the climate of the entire world.

How would the world react? In extreme cases, Victor says, it could lead to war. Some countries

No current international laws or agreements would clearly prevent a country from unilaterally starting a geoengineering project. And too little is known now for a governing body such as the United Nations to establish sound regulations–regulations that might in any case be ignored by a country set on trying to save itself from a climate disaster. Victor says the best hope is for leading scientists around the world to collaborate on establishing as clearly as possible what dangers could be involved in geoengineering and how, if at all, it might be used. Through open international research, he says, we can “increase the odds–not to 100 percent–that responsible norms would emerge.”

In 2006, Paul Crutzen, the Dutch scientist who won the Nobel Prize in chemistry for his discoveries about the depletion of the stratospheric ozone layer, wrote an essay in the journal Climatic Change in which he declared that efforts to reduce greenhouse-gas emissions “have been grossly unsuccessful.” He called for increased research into the “feasibility and environmental consequences of climate engineering,” even though he acknowledged that injecting sulfates into the stratosphere could damage the ozone layer and cause large, unpredictable side effects. Despite these dangers, he said, climatic engineering could ultimately be “the only option available to rapidly reduce temperature rises.”

At the time, Crutzen’s essay was controversial, and many scientists called it irresponsible. But since then it has served to bring geoengineering into the open, says David Keith, who started studying the subject in 1989. After a scientist of Crutzen’s credentials, who understood the stratosphere as well as anyone, came out in favor of studying sulfate injection as a way to cool the earth, many other scientists were willing to start talking about it.

Among the most recent converts is David Battisti, a professor of atmospheric sciences at the University of Washington. One problem in particular worries him. Studies of heat waves show that crop yields drop off sharply when temperatures rise 3 °C to 4 °C above normal–the temperatures that MIT’s Prinn predicts we might reach even with strict emissions controls. Speaking at ageoengineering symposium at MIT this fall, Battisti said, “By the end of the century, just due to temperature alone, we’re looking at a 30 to 40 percent reduction in [crop] yields, while in the next 50 years demand for food is expected to more than double.”

Battisti is well aware of the uncertainties that surround geoengineering. According to research he’s conducted recently, the first computer models that tried to show how shading the earth would affect climate were off by 2 °C to 3 °C in predictions of regional temperature change and by as much as 40 percent in predictions of regional rainfall. But with a billion people already malnourished, and billions more who could go hungry if global warming disrupts agriculture, Battisti has reluctantly conceded that we may need to consider “a climate-engineering patch.” Better data and better models will help clarify the effects of geoengineering. “Give us 30 or 40 years and we’ll be there,” he said at the MIT symposium. “But in 30 to 40 years, at the level we’re increasing CO₂, we’re going to need this, whether we’re ready or not.”

Kevin Bullis is Technology Review’s Energy Editor.