“When people say we shouldn’t even explore this issue, it scares me,’’ Hunt said. He pointed out that carbon emissions are heavy, and finding a place to deposit them will not be easy. “Roughly speaking, the CO 2 we generate weighs three or four times as much as the fuel it comes from.” That means that a short round-trip journey—say, eight hundred miles—by car, using two tanks of gas, produces three hundred kilograms of CO 2 . “This is ten heavy suitcases from one short trip,’’ Hunt said. “And you have to store it where it can’t evaporate.

“So I have three questions, Where are you going to put it? Who are you going to ask to dispose of this for you? And how much are you reasonably willing to pay them to do it?” he continued. “There is nobody on this planet who can answer any of those questions. There is no established place or technique, and nobody has any idea what it would cost. And we need the answers now.”

Hunt stood up, walked slowly to the window, and gazed at the manicured Trinity College green. “I know this is all unpleasant,’’ he said. “Nobody wants it, but nobody wants to put high doses of poisonous chemicals into their body, either. That is what chemotherapy is, though, and for people suffering from cancer those poisons are often their only hope. Every day, tens of thousands of people take them willingly—because they are very sick or dying. This is how I prefer to look at the possibility of engineering the climate. It isn’t a cure for anything. But it could very well turn out to be the least bad option we are going to have.’’

The notion of modifying the weather dates back at least to the eighteen-thirties, when the American meteorologist James Pollard Espy became known as the Storm King, for his (prescient but widely ridiculed) proposals to stimulate rain by selectively burning forests. More recently, the U.S. government project Stormfury attempted for decades to lessen the force of hurricanes by seeding them with silver iodide. And in 2008 Chinese soldiers fired more than a thousand rockets filled with chemicals at clouds over Beijing to prevent them from raining on the Olympics. The relationship between carbon emissions and the earth’s temperature has been clear for more than a century: in 1908, the Swedish scientist Svante Arrhenius suggested that burning fossil fuels might help prevent the coming ice age. In 1965, President Lyndon Johnson received a report from his Science Advisory Committee, titled “Restoring the Quality of Our Environment,” that noted for the first time the potential need to balance increased greenhouse-gas emissions by “raising the albedo, or the reflectivity, of the earth.” The report suggested that such a change could be achieved by spreading small reflective particles over large parts of the ocean.

While such tactics could clearly fail, perhaps the greater concern is what might happen if they succeeded in ways nobody had envisioned. Injecting sulfur dioxide, or particles that perform a similar function, would rapidly lower the temperature of the earth, at relatively little expense—most estimates put the cost at less than ten billion dollars a year. But it would do nothing to halt ocean acidification, which threatens to destroy coral reefs and wipe out an enormous number of aquatic species. The risks of reducing the amount of sunlight that reaches the atmosphere on that scale would be as obvious—and immediate—as the benefits. If such a program were suddenly to fall apart, the earth would be subjected to extremely rapid warming, with nothing to stop it. And while such an effort would cool the globe, it might do so in ways that disrupt the behavior of the Asian and African monsoons, which provide the water that billions of people need to drink and to grow their food.

“Geoengineering” actually refers to two distinct ideas about how to cool the planet. The first, solar-radiation management, focusses on reducing the impact of the sun. Whether by seeding clouds, spreading giant mirrors in the desert, or injecting sulfates into the stratosphere, most such plans seek to replicate the effects of eruptions like Mt. Pinatubo’s. The other approach is less risky, and involves removing carbon directly from the atmosphere and burying it in vast ocean storage beds or deep inside the earth. But without a significant technological advance such projects will be expensive and may take many years to have any significant effect.

There are dozens of versions of each scheme, and they range from plausible to absurd. There have been proposals to send mirrors, sunshades, and parasols into space. Recently, the scientific entrepreneur Nathan Myhrvold, whose company Intellectual Ventures has invested in several geoengineering ideas, said that we could cool the earth by stirring the seas. He has proposed deploying a million plastic tubes, each about a hundred metres long, to roil the water, which would help it trap more CO 2 . “The ocean is this giant heat sink,’’ he told me. “But it is very cold. The bottom is nearly freezing. If you just stirred the ocean more, you could absorb the excess CO 2 and keep the planet cold.” (This is not as crazy as it sounds. In the center of the ocean, wind-driven currents bring fresh water to the surface, so stirring the ocean could transform it into a well-organized storage depot. The new water would absorb more carbon while the old water carried the carbon it has already captured into the deep.)

The Harvard physicist Russell Seitz wants to create what amounts to a giant oceanic bubble bath: bubbles trap air, which brightens them enough to reflect sunlight away from the surface of the earth. Another tactic would require maintaining a fine spray of seawater—the world’s biggest fountain—which would mix with salt to help clouds block sunlight.

The best solution, nearly all scientists agree, would be the simplest: stop burning fossil fuels, which would reduce the amount of carbon we dump into the atmosphere. That fact has been emphasized in virtually every study that addresses the potential effect of climate change on the earth—and there have been many—but none have had a discernible impact on human behavior or government policy. Some climate scientists believe we can accommodate an atmosphere with concentrations of carbon dioxide that are twice the levels of the preindustrial era—about five hundred and fifty parts per million. Others have long claimed that global warming would become dangerous when atmospheric concentrations of carbon rose above three hundred and fifty parts per million. We passed that number years ago. After a decline in 2009, which coincided with the harsh global recession, carbon emissions soared by six per cent in 2010—the largest increase ever recorded. On average, in the past decade, fossil-fuel emissions grew at about three times the rate of growth in the nineteen-nineties.

Although the I.P.C.C., along with scores of other scientific bodies, has declared that the warming of the earth is unequivocal, few countries have demonstrated the political will required to act—perhaps least of all the United States, which consumes more energy than any nation other than China, and, last year, more than it ever had before. The Obama Administration has failed to pass any meaningful climate legislation. Mitt Romney, the presumptive Republican nominee, has yet to settle on a clear position. Last year, he said he believed the world was getting warmer—and humans were a cause. By October, he had retreated. “My view is that we don’t know what is causing climate change on this planet,” he said, adding that spending huge sums to try to reduce CO 2 emissions “is not the right course for us.” China, which became the world’s largest emitter of greenhouse gases several years ago, constructs a new coal-burning power plant nearly every week. With each passing year, goals become exponentially harder to reach, and global reductions along the lines suggested by the I.P.C.C. seem more like a “pious wish,” to use the words of the Dutch chemist Paul Crutzen, who in 1995 received a Nobel Prize for his work on ozone depletion.

“Most nations now recognize the need to shift to a low-carbon economy, and nothing should divert us from the main priority of reducing global greenhouse gas emissions,’’ Lord Rees of Ludlow wrote in his 2009 forward to a highly influential report on geoengineering released by the Royal Society, Britain’s national academy of sciences. “But if such reductions achieve too little, too late, there will surely be pressure to consider a ‘plan B’—to seek ways to counteract climatic effects of green-house gas emissions.’’

While that pressure is building rapidly, some climate activists oppose even holding discussions about a possible Plan B, arguing, as the Norfolk protesters did in September, that it would be perceived as indirect permission to abandon serious efforts to cut emissions. Many people see geoengineering as a false solution to an existential crisis—akin to encouraging a heart-attack patient to avoid exercise and continue to gobble fatty food while simply doubling his dose of Lipitor. “The scientist’s focus on tinkering with our entire planetary system is not a dynamic new technological and scientific frontier, but an expression of political despair,” Doug Parr, the chief scientist at Greenpeace UK, has written.

During the 1974 Mideast oil crisis, the American engineer Hewitt Crane, then working at S.R.I. International, realized that standard measurements for sources of energy—barrels of oil, tons of coal, gallons of gas, British thermal units—were nearly impossible to compare. At a time when these commodities were being rationed, Crane wondered how people could conserve resources if they couldn’t even measure them. The world was burning through twenty-three thousand gallons of oil every second. It was an astonishing figure, but one that Crane had trouble placing into any useful context.

Crane devised a new measure of energy consumption: a three-dimensional unit he called a cubic mile of oil. One cubic mile of oil would fill a pool that was a mile long, a mile wide, and a mile deep. Today, it takes three cubic miles’ worth of fossil fuels to power the world for a year. That’s a trillion gallons of gas. To replace just one of those cubic miles with a source of energy that will not add carbon dioxide to the atmosphere—nuclear power, for instance—would require the construction of a new atomic plant every week for fifty years; to switch to wind power would mean erecting thousands of windmills each month. It is hard to conceive of a way to replace that much energy with less dramatic alternatives. It is also impossible to talk seriously about climate change without talking about economic development. Climate experts have argued that we ought to stop emitting greenhouse gases within fifty years, but by then the demand for energy could easily be three times what it is today: nine cubic miles of oil.

The planet is getting richer as well as more crowded, and the pressure to produce more energy will become acute long before the end of the century. Predilections of the rich world—constant travel, industrial activity, increasing reliance on meat for protein—require enormous physical resources. Yet many people still hope to solve the problem of climate change just by eliminating greenhouse-gas emissions. “When people talk about bringing emissions to zero, they are talking about something that will never happen,’’ Ken Caldeira told me. “Because that would require a complete alteration in the way humans are built.”

Caldeira began researching geoengineering almost by accident. For much of his career, he has focussed on the implications of ocean acidification. During the nineteen-nineties, he spent a year in the Soviet Union, at the Leningrad lab of Mikhail Budyko, who is considered the founder of physical climatology. It was Budyko, in the nineteen-sixties, who first suggested cooling the earth by putting sulfur particles in the sky.

“In the nineteen-nineties, when I was working at Livermore, we had a meeting in Aspen to discuss the scale of the energy-system transformation needed in order to address the climate problem,’’ Caldeira said. “Among the people who attended was Lowell Wood, a protégé of Edward Teller. Wood is a brilliant but sometimes erratic man . . . lots of ideas, some better than others.” At Aspen, Wood delivered a talk on geoengineering. In the presentation, he explained, as he has many times since, that shielding the earth properly could deflect one or two per cent of the sunlight that reaches the atmosphere. That, he said, would be all it would take to counter the worst effects of warming.