Myhrvold is not blind to the possibility. He has probably thought about such scenarios in greater scientific detail than any climate doomsayer: a collapse of ma.s.sive ice sheets in Greenland or Antarctica; a release of huge amounts of methane caused by the melting of arctic permafrost; and, as he describes it, "a breakdown of the thermohaline circulation system in the North Atlantic, which would put an end to the Gulf Stream."

So what happens if the doomsayers turn out to be right? What if the earth is becoming dangerously warmer, whether because of our fossil-fuel profligacy or some natural climate cycle? We don't really want to sit back and stew in our own juices, do we?

In 1980, when Myhrvold was a grad student at Princeton, Mount St. Helens erupted back home in Washington State. Even though he was nearly three thousand miles away, Myhrvold saw a thin layer of ash acc.u.mulating on his windowsill. "It's hard not to think about volcanic dust when it's raining down on your dorm room," he says, "although to be honest, my room was messy in many other ways."

Even as a kid, Myhrvold was fascinated by geophysical phenomena-volcanoes, sunspots, and the like-and their history of affecting the climate. The Little Ice Age intrigued him so much that he forced his family to visit the northern tip of Newfoundland, where Leif Eriksson and his Vikings reputedly made camp a thousand years earlier.

The connection between volcanoes and climate is hardly a new idea. Another polymath, Benjamin Franklin, wrote what seems to be the first scientific paper on the topic. In "Meteorological Imaginations and Conjectures," published in 1784, Franklin posited that recent volcanic eruptions in Iceland had caused a particularly harsh winter and a cool summer with "constant fog over all Europe, and [a] great part of North America." In 1815, the gargantuan eruption of Mount Tambora in Indonesia produced "The Year Without a Summer," a worldwide disaster that killed crops, prompted widespread starvation and food riots, and brought snow to New England as late as June.

As Myhrvold puts it: "All really big-a.s.s volcanoes have some climate effects."

Volcanoes erupt all the time, all over the world, but truly "big-a.s.s" ones are rare. If they weren't-well, we probably wouldn't be around to worry about global warming. The anthropologist Stanley Ambrose has argued that a supervolcanic explosion at Lake Toba on Sumatra, roughly seventy thousand years ago, blocked the sun so badly that it triggered an ice age that nearly wiped out h.o.m.o sapiens.

What distinguishes a big-a.s.s volcano isn't just how much stuff it e.j.a.c.u.l.a.t.es, but where the e.j.a.c.u.l.a.t.e goes. The typical volcano sends sulfur dioxide into the troposphere, the atmospheric layer closest to the earth's surface. This is similar to what a coal-burning power plant does with its sulfur emissions. In both cases, the gas stays in the sky only a week or so before falling back to the ground as acid rain, generally within a few hundred miles of its origin.

But a big volcano shoots sulfur dioxide far higher, into the stratosphere. That's the layer that begins at about seven miles above the earth's surface, or six miles at the poles. Above that threshold alt.i.tude, there is a drastic change in a variety of atmospheric phenomena. The sulfur dioxide, rather than quickly returning to the earth's surface, absorbs stratospheric water vapor and forms an aerosol cloud that circulates rapidly, blanketing most of the globe. In the stratosphere, sulfur dioxide can linger for a year or more, and will thereby affect the global climate.

That's what happened in 1991 when Mount Pinatubo erupted in the Philippines. Pinatubo made Mount St. Helens look like a hiccup; it put more sulfur dioxide into the stratosphere than any volcano since Krakatoa, more than a century earlier. In the period between those two eruptions, the state of science had progressed considerably. A worldwide cadre of scientists was on watch at Pinatubo, equipped with modern technology to capture every measurable piece of data. The atmospheric aftereffects of Pinatubo were undeniable: a decrease in ozone, more diffuse sunlight, and, yes, a sustained drop in global temperature.

Nathan Myhrvold was working at Microsoft then, but he still followed the scientific literature on geophysical phenomena. He took note of the Pinatubo climate effects and, one year later, a 900-page report from the National Academy of Sciences called Policy Implications of Greenhouse Warming. It included a chapter on geoengineering, which the NAS defined as "large-scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry."

In other words: if human activity is warming up the planet, could human ingenuity cool it down?

People have been trying to manipulate the weather forever. Just about every religion ever invented has a rain-making prayer. But secularists have stepped it up in recent decades. In the late 1940s, three General Electric scientists in Schenectady, New York, successfully seeded clouds with silver iodide. The trio included a chemist named Bernard Vonnegut; the project's public-relations man was his younger brother Kurt, who went on to become a world-cla.s.s novelist-and in his writing, he used a good bit of the far-out science he picked up in Schenectady.

The 1992 NAS report gave a credibility boost to geoengineering, which until then had largely been seen as the province of crackpots and rogue governments. Still, some of the NAS proposals would have seemed outlandish even in a Vonnegut novel. A "multiple balloon screen," for instance, was meant to deflect sunlight by launching billions of aluminized balloons into the sky. A "s.p.a.ce mirror" scheme called for fifty-five thousand reflective sails to orbit high above the earth.

The NAS report also raised the possibility of intentionally spreading sulfur dioxide in the stratosphere. The idea was attributed to a Belarusian climate scientist named Mikhail Budyko. After Pinatubo, there was no doubt that stratospheric sulfur dioxide cooled the earth. But wouldn't it be nice to not have to rely on volcanoes to do the job?

Unfortunately, the proposals for getting sulfur dioxide into the stratosphere were complex, costly, and impractical. Loading up artillery sh.e.l.ls, for instance, and firing them into the sky. Or launching a fleet of fighter jets with high-sulfur fuel and letting their exhaust paint the stratosphere. "It was more science fiction than science," says Myhrvold. "None of the plans made any economic or practical sense."

The other problem was that many scientists, particularly nature-friendly ones like Ken Caldeira, found the very idea abhorrent. Dump chemicals in the atmosphere to reverse the damage caused by...dumping chemicals in the atmosphere? It was a crazy, hair-of-the-dog scheme that seemed to violate every tenet of environmentalism. Those who saw global warming as a religious issue could hardly imagine a more grievous sacrilege.

But the best reason to reject the idea, Caldeira thought, was that it simply wouldn't work.

That was his conclusion after hearing Lowell Wood give a lecture on stratospheric sulfur dioxide at a 1998 climate conference in Aspen. But being a scientist who prefers data to dogma-even if the environmental dogma in this case lay close to his heart-Caldeira ran a climate model to test Wood's claims. "The intent," he says, "was to put an end to all the geoengineering talk."

He failed. As much as Caldeira disliked the concept, his model backed up Wood's claims that geoengineering could stabilize the climate even in the face of a large spike in atmospheric carbon dioxide, and he wrote a paper saying so. Caldeira, the most reluctant geoengineer imaginable, became a convert-willing, at least, to explore the idea.

Which is how it comes to pa.s.s that, more than ten years later, Caldeira, Wood, and Myhrvold-the onetime peacenik, the onetime weapons architect, and the onetime Viking fanboy-are huddled together in a former Harley-Davidson repair shop showing off their scheme to stop global warming.

It wasn't just the cooling potential of stratospheric sulfur dioxide that surprised Caldeira. It was how little was needed to do the job: about thirty-four gallons per minute, not much more than the amount of water that comes out of a heavy-duty garden hose.

Warming is largely a polar phenomenon, which means that high-lat.i.tude areas are four times more sensitive to climate change than the equator. By IV's estimations, 100,000 tons of sulfur dioxide per year would effectively reverse warming in the high Arctic and reduce it in much of the Northern Hemisphere.

That may sound like a lot but, relatively speaking, it is a smidge. At least 200 million tons of sulfur dioxide already go into the atmosphere each year, roughly 25 percent from volcanoes, 25 percent from human sources like motor vehicles and coal-fired power plants, and the rest from other natural sources like sea spray.

So all that would be needed to produce a globe-changing effect is one-twentieth of 1 percent of current sulfur emissions, simply relocated to a higher point in the sky. How can this be? Myhrvold's answer: "Leverage!"

Leverage is the secret ingredient that distinguishes physics from, say, chemistry. Think back to the Salter Sink, IV's device for preventing hurricanes. Hurricanes are destructive because they gather up the thermal energy in the ocean's surface and convert it into physical force, a primordial act of leverage creation. The Salter Sink ruptures that process by using wave power to continually sink the warm water all through hurricane season.

"A kilogram of sulfur dioxide, emitted by a truck or a bus or a power plant into the troposphere, does much less good for you than in the stratosphere," Myhrvold says. "So you get a huge leverage, and that's a pretty cool thing. That's why Archimedes said, 'If you give me a fulcrum, I can move the world.'"*

So once you eliminate the moralism and the angst, the task of reversing global warming boils down to a straightforward engineering problem: how to get thirty-four gallons per minute of sulfur dioxide into the stratosphere?

The answer: a very long hose.

That's what IV calls this project-a "garden hose to the sky." Or, when they're feeling slightly more technical, a "stratospheric shield for climate stabilization." Considering its scientific forebear and the way it wraps the planet in a protective layer, perhaps it should be called Budyko's Blanket.

For anyone who loves cheap and simple solutions, things don't get much better. Here's how it works. At a base station, sulfur would be burned into sulfur dioxide and then liquefied. "The technology for doing this is well known," says Wood, "because early in the twentieth century, sulfur dioxide was the major refrigerant gas."

The hose, stretching from the base station into the stratosphere, would be about eighteen miles long but extremely light. "The diameter is just a couple inches, not some giant-a.s.s pipe," says Myhrvold. "It's literally a specialized fire hose."

The hose would be suspended from a series of high-strength, helium-filled balloons fastened to the hose at 100-to 300-yard intervals (a "string of pearls," IV calls it), ranging in diameter from 25 feet near the ground to 100 feet near the top.

The liquefied sulfur dioxide would be sent skyward by a series of pumps, affixed to the hose at every 100 yards. These too would be relatively light, about forty-five pounds each-"smaller than the pumps in my swimming pool," Myhrvold says. There are several advantages to using many small pumps rather than one monster pump at the base station: a big ground pump would create more pressure, which, in turn, would require a far heavier hose; even if a few of the small pumps failed, the mission itself wouldn't; and using small, standardized units would keep costs down.

At the end of the hose, a cl.u.s.ter of nozzles would spritz the stratosphere with a fine mist of colorless liquid sulfur dioxide.

Thanks to stratospheric winds that typically reach one hundred miles per hour, the spritz would wrap around the earth in roughly ten days' time. That's how long it would take to create Budyko's Blanket. Because stratospheric air naturally spirals toward the poles, and because the arctic regions are more vulnerable to global warming, it makes sense to spray the sulfur aerosol at high lat.i.tude-with perhaps one hose in the Southern Hemisphere and another in the Northern.

Myhrvold, in his recent travels, happened upon one potentially perfect site. Along with Bill Gates and Warren Buffett, he was taking a whirlwind educational tour of various energy producers-a nuclear plant, a wind farm, and so on. One of their destinations was the Athabasca Oil Sands in northern Alberta, Canada.

Billions of barrels of petroleum can be found there, but it is heavy, mucky crude. Rather than lying in a liquid pool beneath the earth's crust, it is mixed in, like mola.s.ses, with the surface dirt. In Athabasca you don't drill for oil; you mine it, scooping up gigantic shovels of earth and then separating the oil from its waste components.

One of the most plentiful waste components is sulfur, which commands such a low price that oil companies simply stockpile it. "There were big yellow mountains of it, like a hundred meters high by a thousand meters wide!" says Myhrvold. "And they stair-step them, like a Mexican pyramid. So you could put one little pumping facility up there, and with one corner of one of those sulfur mountains, you could solve the whole global warming problem for the Northern Hemisphere."

It is interesting to think what might have happened if Myhrvold was around one hundred years ago, when New York and other cities were choking on horse manure. One wonders if, while everyone else looked at the mountains of dung and saw calamity, he might have seen opportunity.

On balance, Budyko's Blanket is a fiendishly simple plan. Considering the complexity of climate in general and how much we don't know, it probably makes sense to start small. With the fire-hose approach, you could begin with a trickle of sulfur and monitor the results. The amount could be easily dialed up or down-or, if need be, turned off. There is nothing permanent or irreversible about the process.

And it would be startlingly cheap. IV estimates the "Save the Arctic" plan could be set up in just two years at a cost of roughly $20 million, with an annual operating cost of about $10 million. If cooling the poles alone proved insufficient, IV has drawn up a "Save the Planet" version, with five worldwide base stations instead of two, and three hoses at each site. This would put about three to five times the amount of sulfur dioxide into the stratosphere. Even so, that would still represent less than 1 percent of current worldwide sulfur emissions. IV estimates this plan could be up and running in about three years, with a startup cost of $150 million and annual operating costs of $100 million.

So Budyko's Blanket could effectively reverse global warming at a total cost of $250 million. Compared with the $1.2 trillion that Nicholas Stern proposes spending each year to attack the problem, IV's idea is, well, practically free. It would cost $50 million less to stop global warming than what Al Gore's foundation is paying just to increase public awareness about global warming.

And there lies the key to the question we asked at the beginning of this chapter: What do Al Gore and Mount Pinatubo have in common? The answer is that Gore and Pinatubo both suggest a way to cool the planet, albeit with methods whose cost-effectiveness are a universe apart.

This is not to dismiss the potential objections to Budyko's Blanket, which are legion. First of all, would it work?

The scientific evidence says yes. It is basically a controlled mimicry of Mount Pinatubo's eruption, whose cooling effects were exhaustively studied and remain unchallenged.

Perhaps the stoutest scientific argument in favor of the plan came from Paul Crutzen, a Dutch atmospheric scientist whose environmentalist bona fides run even deeper than Caldeira's. Crutzen won a n.o.bel Prize in 1995 for his research on atmospheric ozone depletion. And yet in 2006, he wrote an essay in the journal Climatic Change lamenting the "grossly unsuccessful" efforts to emit fewer greenhouse gases and acknowledging that an injection of sulfur in the stratosphere "is the only option available to rapidly reduce temperature rises and counteract other climatic effects."