Ken Caldeira devised simulations to discredit geoengineering. But the surprising results converted him. *

Photo: Todd Hido * It was one of the largest public demonstrations in US history. On June 12, 1982, an estimated 750,000 protesters thronged Central Park in New York City, chanting "No nukes!" and bearing signs reading "Reagan is a bomb — both should be banned" and "Arms are for embracing." Some demonstrators called for unilateral US disarmament, others for renewing arms control talks with the Soviet Union. It was a diverse coalition that had been pulled together by Ken Caldeira, a 25-year-old activist and computer geek. Back then he was paying the rent doing software consulting on Wall Street, but his passion for the environment would eventually lead him to become one of the nation's leading experts on global warming.

Around the same time, at the Lawrence Livermore National Laboratory near San Francisco, Lowell Wood — then 41 and a protégé of the brilliant and controversial hydrogen bomb inventor Edward Teller — was leading a secretive team of young geniuses called the O Group. They weren't merely working with the nukes that Caldeira and his fellow peaceniks reviled; they were dreaming up new and expanded uses for them. One plan called for channeling the energy of a hydrogen bomb into laser blasts that could theoretically destroy enemy ballistic missiles from outer space. It sounded crazy, but Wood and Teller's ideas inspired President Reagan's famous March 23, 1983, "Star Wars" speech introducing the Strategic Defense Initiative, the bane of arms-control advocates everywhere.

What's surprising, then, is that today, 25 years later, Caldeira, the left-wing environmentalist, calls Wood, the Cold Warrior and Star Wars proselytizer, "one of my best friends." Recently, they have collaborated on strategies for a process known as geoengineering: the large-scale, deliberate modification of the planet to counteract the consequences of ever-increasing concentrations of atmospheric greenhouse gas. The global climate crisis has made for strange bedfellows, and Caldeira's passage from devout environmentalist to would-be geoengineer has led him into a partnership that his younger self would have scorned.

Geoengineering schemes sound like they're pulled straight from pulp sci-fi novels: Fertilize the oceans with iron in order to sequester carbon dioxide; launch fleets of ships to whip up sea spray and enhance the solar reflectivity of marine stratocumulus clouds; use trillions of tiny spacecraft to form a sunshade a million miles from Earth in perfect solar orbit. They all may seem impractical, but among a small but growing set of climate scientists, one idea that Wood and Teller started pushing in the late 1990s (before Teller's death in 2003) is gaining acceptance: Inject sulfur dioxide into the stratosphere to reflect a portion of the sun's rays back into space, thus cooling the planet.

Ken Caldeira is a self-described blabbermouth with disheveled curly hair and a habit of modestly turning his voice up at the end of sentences so even his most definitive utterances sound like questions. On a recent morning in his Carnegie Institution office on the Stanford campus, he's dressed casually in a sweater and jeans, eagerly examining results from a new climate model run that was designed to explore possible consequences of geoengineering. As Caldeira tinkers with the software, he jokes that his talent with computers basically ensured that he would spend more time in an office than out leading research expeditions. A career in science, he says, "was all a ploy for me to get into the rain forests. But it never really worked."

Caldeira's transit from antinuke activist to climate scientist grew out of his fondness for ecotourism. During the '80s, he joined occasional research expeditions into the Mexican rain forest; he pitched in by writing software to help researchers map out species distributions. Caldeira loved the trips, but he soon realized that to get himself beneath the canopy more regularly, he'd need a PhD. So he took night classes at New York University under Martin Hoffert, a physicist best known today for his skepticism about how quickly cleaner energy sources can actually replace dirty fossil fuels.

In 1990, a year short of his doctorate, Caldeira went to Leningrad to study with Russian climate scientist Mikhail Budyko, one of the first champions of geoengineering. In the 1970s, Budyko had suggested an early version of the basic Teller-Wood idea — decrease the global temperature by shooting sunlight-scattering particles into the stratosphere. While he was in Leningrad, Caldeira's philosophical outlook still predisposed him to distrust such interventions, but years later that would change.

In 1993, the former peace activist accepted a research post at that haven of bombmakers, the Lawrence Livermore National Laboratory. Caldeira was running high-powered computer models to study the climate, but when he was offered a security clearance that would have allowed fuller access to the lab's resources, he tried to stay true to his old principles by declining. As a result, he didn't see much of Wood. "There's an outer fence at Livermore, and then there's an inner fence," Caldeira says. "Lowell worked inside the fence."

The two got to know each other in 1998, just after the adoption of the Kyoto Protocol, at a conference in Aspen, Colorado. The topic was strategies for stabilizing the climate system, and Wood had been invited to speak about his work with Teller on geoengineering.

Roll over the numbers to learn more.### Cooling the Globe

Many climate specialists see so-called geoengineering techniques as a way to bring down global temperatures if other attempts to combat global warming fail. One approach is to inject sulfur dioxide into the stratosphere, where it reacts to form particles that block sunlight.

Roll over the numbers in the infographic to learn more.Infographic: Ryan VulkUnlike many political conservatives in the late '90s, Wood and Teller took climate change seriously. But they doubted people would ever give up enough of their costly energy-consumption habits to prevent climate-associated risks (a cynical point of view that seems to have been borne out a decade later). Wood and Teller were just as dismissive of global greenhouse gas treaties like Kyoto as they had been of arms-control agreements during the '80s. They thought the only solution lay with technology: direct, aggressive intervention, either in the upper atmosphere or low Earth orbit, essentially to turn down the volume knob on solar radiation.

In Aspen, Wood didn't pull any punches. "He was being outrageous," Caldeira remembers with amusement. "He said something like, 'If we want to solve the global warming problem, we could just have a nuclear war.'" Even the title of his presentation was provocative: "Geoengineering and Nuclear Fission as Responses to Global Warming." By spending only about $1 billion per year, Wood argued, we could put enough particles in the stratosphere to reduce sunlight by about 1 percent — and that's all we'd need to start reversing global warming.

Wood must have known his talk was going to be controversial. Geoengineering had a long history but a checkered reputation. In a 1965 environmental report that discussed climate change (yes, back then), President Johnson's Science Advisory Committee offered a single solution to the problem, and it was a fix that we would now call geoengineering: Spread reflective particles over the oceans.

But as the environmental movement gained momentum, it became decidedly uncool to propose high tech interventions. Geoengineering also suffered by association with several undistinguished military ventures into weather modification. In 1971, news broke that the Pentagon had attempted to alter the weather in Vietnam for military purposes by seeding clouds with silver iodide crystals. The episode prompted an international outcry and, later, a United Nations convention against weather interference for hostile purposes.

Wood's talk didn't generate much enthusiasm among the scientists in Aspen that day. "I thought it would never work," Caldeira says. In fact, he teamed up with another Livermore colleague, Bala Govindasamy, to prove Wood wrong by conducting a thorough climate simulation, the first of its kind. "The intent was to put an end to all the geoengineering talk," Caldeira says. Because changes in the intensity of sunlight affect the climate quite differently than do changes in greenhouse gas concentrations — sunlight varies by season and by the diurnal cycle and falls most strongly over the tropics and equator, whereas carbon dioxide traps heat both day and night across the globe — he had a hunch that the two might not really offset one another very well, especially on a regional or seasonal basis.

When the results came back, Caldeira had disproved his own hypothesis. Geoengineering really did seem to operate more or less like a planetwide thermostat. As the resulting paper noted, Wood's proposal might have any number of adverse environmental and ecological consequences, but it also might offer real benefits. "Melting of Greenland and Antarctic ice caps and the consequent sea level rise," Caldeira and Govindasamy wrote in the spring 2000 issue of Geophysical Research Letters, "is less likely to occur in a geoengineered world."

The paper helped propel Wood's idea into the scientific mainstream and turned Caldeira and Wood into unlikely new allies. Since then, increasingly bad climate news has only heightened interest in the proposal. The summer of 2007 saw a new low in the extent of Arctic sea ice, while data from the Greenland ice sheet is similarly ominous; if it were to melt entirely, sea levels would rise 20 feet, submerging coastal cities like New York and Shanghai. Meanwhile, a new climate-modeling study by Caldeira and another colleague found that in order to stabilize the climate now, we'd have to go to zero emissions almost immediately — something nobody thinks is even remotely possible. All of which is forcing scientists to consider geoengineering more seriously as a way to fight climate change.

There's no better evidence for the growing acceptance of geoengineering than a 2006 essay in the journal Climatic Change by the atmospheric chemist Paul Crutzen, who shares a 1995 Nobel Prize for work on ozone formation and decomposition. The continuing failure of governments to move on global warming, Crutzen says, makes open discussion of geoengineering essential. "The very best would be if emissions of the greenhouse gases could be reduced so much that the stratospheric sulfur release experiment would not take place," he wrote. "Currently, this looks like a pious wish."

The stratospheric sulfate experiment has already had its proof of concept — courtesy of planet Earth. On June 15, 1991, Mount Pinatubo, which for months had been rumbling, belching, and terrorizing the main Philippine island of Luzon, finally blew its top in an explosion so powerful that it carried 500 feet of the mountain's peak along with it. It was the second-largest volcanic eruption of the 20th century, 10 times the size of the Mount Saint Helens explosion in 1980 and the first of its scale to occur with modern scientific technologies in place — especially satellites — to measure the global environmental and climatic effect.

Pinatubo's eruption didn't just unleash huge mud slides and lava flows; it also fired an ash stream 22 miles into the air, injecting 20 million tons of sulfur dioxide into the stratosphere. Over the following months, a massive haze gradually dispersed across the globe. Meanwhile, the sulfur dioxide component underwent chemical reactions to form a particulate known as sulfate aerosol (in essence, droplets of water and sulfuric acid), which absorbs sunlight and reflects some of it back into space.

The climatic effect of this volcanic eruption was rapid, dramatic, and planetary in scale. In a year, the global average temperature declined by half a degree Celsius, and researchers observed less summer melt atop the Greenland ice sheet.

Of course, that got scientists thinking. Not only could we mimic volcanoes by seeding the stratosphere with extra sulfur, but if we were really clever, we could design particles to do an even better job at scattering sunlight. University of Calgary climate scientist and geoengineering expert David Keith has suggested that we might ultimately find a particle that can be placed still higher up in the atmosphere, in the region called the mesosphere, above the ozone layer, where it would cause fewer problems.

The evidence from Pinatubo showed that such an intervention will definitely cool the planet. Furthermore, it would work quickly and wouldn't alter the atmosphere permanently: Depending upon the starting elevation, stratospheric sulfate aerosol will stay in the atmosphere for only a year or two. (Though this could also be seen as a drawback: If you cool the planet artificially by injecting sulfur and then stop suddenly, things warm back up more quickly than before.)

The next question, of course, is how to get the particles up there. Various proposals have suggested using artillery, balloons, suspended hoses, military jets, or even converted 747s. Then there is the question of where to deposit the sulfur. There are different elevations to consider, as well as planetary location. A number of scientists, most recently Wood and Caldeira in a yet-unpublished paper, propose dispensing the gas over the Arctic — after all, that's where global warming is felt most powerfully and where cooler temperatures would help restore sea ice and stabilize Greenland.

Al Gore isn't buying it, and neither are mainstream green organizations. "People have this fantasy right now that maybe this will save us," says Alan Robock, a Rutgers University climate scientist and geoengineering critic, "but it looks like it's going to cause more problems than it will solve." Indeed, ever since that first 2000 study, nearly all geoengineering simulations that Caldeira has run point to unwanted consequences.

Caldeira's response is that it's hard to see how those consequences would be anywhere near as nasty as simply letting global warming go unchecked. But the more geoengineering becomes a matter of public debate and concern, the more the downsides of a remade world come under scrutiny. First, there's the fear that injecting sulfate into the stratosphere could destroy much-needed ozone, which also declined markedly after Pinatubo.

Another possible side effect is acid rain. But sulfur dioxide pollution from coal-burning power plants, one of the prime causes of acid rain in the past, never reaches the stratosphere — it remains in the atmosphere's lowest layer, the troposphere, and rains out quickly as a result. The stratospheric sulfate from geoengineering would stay up longer and be more stable, so we would need less of it to begin with, which somewhat weakens the acid rain argument.

But the deepest concern is that at a fundamental level we would be messing with a complex and incompletely understood system, one that — even in the most powerful computer models — can be rendered only partially. There will undoubtedly be unexpected results — the "unknown unknowns," as climate scientists call them. That's why critics like Robock and prominent atmospheric-sciences historian James Rodger Fleming want to avoid "playing God with the elements," to use Fleming's phrase.

Finally, while geoengineering could presumably lower the planet's temperature, it cannot correct the other consequences of mounting carbon dioxide pollution. For instance, as the oceans absorb more CO 2 they acidify, and decreasing ocean pH levels threaten coral reefs and other marine ecosystems. No amount of stratospheric sulfate can reverse that.

Until large-scale experiments are funded, the only way to explore the potential consequences is through computer simulations. By turning down the virtual sun or cranking up the digital carbon, we can create any planetary future we want.

Back at his office at the Carnegie Institution, where he recently received a lifetime research position, I watch Caldeira decipher the results from a new simulation. He designed the study to investigate whether a geoengineered climate in which atmospheric carbon dioxide levels have doubled is significantly worse for plants — in other words, will there be less soil moisture for them? Previous studies suggested that this might be another potentially troubling consequence of geoengineering: If you artificially cool the planet, you also apparently decrease overall precipitation and at a rate that slightly outstrips the corresponding temperature change. So Caldeira wants to know just how bad that would be and, more specifically, whether it would automatically translate into stunted plant growth.

The results finally appear, expressed as images on a map in which the continents are shaded green but trending blue or red in certain places, depending upon soil-moisture changes. Overall, there seems to be little, if any, dramatic consequence. A geoengineered planet, it appears, would have less precipitation but also less evaporation — at least according to this simulation.

Caldeira expected as much. Soil moisture is one of many aspects of a geoengineered world that he has explored since his first study back in 2000, and though there are always some downsides, he continues to be encouraged by the results. Still, he admits that the trajectory of his research leads to a paradox. He doesn't want to deploy any of these solutions immediately — he still views them as last resorts — yet his own ongoing investigations and endorsements could contribute to making them more likely, if not inevitable.

Caldeira may be leery of how such research is altering the debate, but some are already saying, just as Wood and Teller did long ago, that if geoengineering works — why wait? Americans can drive their gas-guzzling SUVs to their hearts' content, and China can build as many coal-powered electricity plants as it wants. The online edition of the conservative magazine National Review recently made the case for geoengineering — in part because it prevents us from having to cut emissions. Similarly, in a recent article in the right-wing journal The National Interest, Wood and a coauthor painted geoengineering as a favorable alternative to the practical difficulties of controlling global carbon gas emissions.

True to his green roots, Caldeira doesn't want people to view geoengineering as a magic bullet to be fired at the expense of cutting emissions. "I'm actually more concerned about the effect of geoengineering on the social and political systems than on the climate system," he says. Caldeira wants geoengineering only if, say, Greenland starts sliding into the ocean.

But at some point, the lure of geoengineering may gradually overwhelm all reservations. Congress is currently considering legislation that would reduce greenhouse gas emissions 56 percent by the year 2050, hardly enough, according to many scientists, to stabilize the climate. And the law regulates only US emissions; getting emerging economies like India and China to cut back presents far greater hurdles.

Meanwhile, Caldeira and Wood keep debating the issue. Despite their differences, they still seem to agree more than they disagree. Sure enough, when our conversation ends, Caldeira checks his phone. Another email from Wood has just come in.

Chris Mooney (moonecc@yahoo.com)* is the author of* Storm World: Hurricanes, Politics, and the Battle Over Global Warming.