||Miércoles 1 de Octubre de 2008
|Geoengineering: How to cool Earth-- at a price
Por Robert Kunzig
When David W. Keith, a physicist and energy expert at the University of Calgary in Alberta, gives lectures these days on geoengineering, he likes to point out how old the idea is. People have been talking about deliberately altering climate to counter global warming, he says, for as long as they have been worrying about global warming itself. As early as 1965, when Al Gore was a freshman in college, a panel of distinguished environmental scientists warned President Lyndon B. Johnson that carbon dioxide (CO2) emissions from fossil fuels might cause “marked changes in climate” that “could be deleterious.” Yet the scientists did not so much as mention the possibility of reducing emissions. Instead they considered one idea: “spreading very small reflective particles” over about five million square miles of ocean, so as to bounce about 1 percent more sunlight back to space—“a wacky geoengineering solution,” Keith says, “that doesn’t even work.”
In the decades since, geoengineering ideas never died, but they did get pushed to the fringe—they were widely perceived by scientists and environmentalists alike as silly and even immoral attempts to avoid addressing the root of the problem of global warming. Three recent developments have brought them back into the mainstream.
First, despite years of talk and international treaties, CO2 emissions are rising faster than the worst-case scenario envisioned as recently as 2007 by the Intergovernmental Panel on Climate Change. “The trend is upward and toward an ever increasing reliance on coal,” says Ken Caldeira, a climate modeler at the Carnegie Institution for Science in Stanford, Calif.
Second, ice is melting faster than ever at the poles, suggesting that climate might be closer to the brink—or to a tipping point, in the current vernacular—than anyone had thought.
And third, Paul J. Crutzen wrote an essay. The 2006 paper in the journal Climatic Change by the eminent Dutch atmospheric chemist, in which with heavy heart he, too, urged serious consideration of geoengineering, “let the cat out of the bag,” Keith says. Crutzen had won the Nobel Prize in Chemistry for his work on the destruction of atmospheric ozone in 1995; if he was taking geoengineering seriously, it seemed, everyone needed to.
By November 2007 Keith and Harvard University geophysicist Daniel P. Schrag had no trouble convincing top climate scientists to join zealous geoengineers at a workshop in Cambridge, Mass. At the end, all agreed that more research was necessary—some because geoengineering truly excites them, some because they consider it the lesser of two evils, and some because they hope to drive a stake through its heart. But still there was a consensus: geoengineering is back.
Geoengineering schemes fall into two categories, corresponding to the two knobs you might imagine twiddling to adjust the earth’s temperature. One knob controls how much sunlight—or solar energy, to be more precise—reaches the planet’s surface; the other controls how much heat escapes back into space, which depends on how much CO2 is in the atmosphere. Schemes for removing CO2 from the atmosphere, say,by fertilizing the oceans with iron, would strike closer to the root of the problem. But they would inevitably take decades to have much of an effect. In contrast, a sunshade could, in principle, stop global warming immediately—albeit only for as long as it was maintained. Sunshade ideas thus address what some scientists see as the extreme urgency of the climate problem. “If the Greenland ice sheet started to collapse tomorrow, and you’re president of the United States, what do you do?” Schrag asks. “You don’t have a choice.”
So far, however, relatively little research has been done on any of the approaches or on their potentially substantial and unpredictable side effects. “There’s a lot more talk than work,” Caldeira says. “Most of the research has been at the hobby level.” Some ideas do not merit much more than that—scattering reflective particles over a large part of the ocean, for instance, would inevitably pollute it, and the particles would probably wash up on beaches fairly quickly. But others are harder to dismiss.
Dismissing the basic rationale behind geoengineering is harder still. Few investigators today suggest that blocking the sun is a substitute for stopping the rise of atmospheric CO2 or that geoengineering can fix the CO2 problem by itself. They argue instead that it might give us time for the revolution needed to convert the world to carbon-neutral energy sources. “The reason I think geoengineering should be considered,” says Tom M. L. Wigley of the National Center for Atmospheric Research (NCAR), “is I don’t think we are going to save the planet with the emissions-reductions approaches that are on the table. No one is taking the magnitude of the technological challenge seriously.”
Particles in the Stratosphere
The geoengineering scheme Crutzen and Wigley both defend is the cheapest and most certain to work; it was proposed as long ago as 1974 by the late Russian physicist Mikhail I. Budyko, then at the Main Geophysical Observatory in Leningrad. The idea is to inject several million tons a year of sulfur dioxide (SO2) into the stratosphere. There it would react with oxygen, water and other molecules to form minute sulfate droplets made up of water, sulfuric acid (H2SO4) and whatever dust, salt or other particles onto which the acid and water condense. Clouds of sulfate droplets would scatter sunlight, making sunsets redder, the sky paler and the earth’s surface, on average, cooler—everyone agrees on all that. In 1991 the volcanic eruption of Mount Pinatubo in the Philippines put 20 million tons of SO2 into the stratosphere, and it had all those effects: it cooled the earth by nearly one degree Fahrenheit for about a year. “So we basically know it works,” Caldeira says. In fact, Caldeira started modeling the idea nearly a decade before Crutzen wrote about it.
By the time Crutzen picked up the thread, the world was readier for geoengineering; it had gotten a degree warmer since Budyko’s paper, and a lot of ice had melted. In the 1990s Edward Teller and his colleagues at Lawrence Livermore National Laboratory had suggested that metallic particles might stay aloft longer and reflect more sunlight, but Crutzen stuck with the more well established idea of injecting SO2. It enabled him to frame his proposal in an appealing way.
By burning fossil fuels, he pointed out, people are already putting 55 million tons of SO2 into the lower atmosphere every year (along with eight billion tons of CO2). According to the World Health Organization, the resulting concentration of SO2 kills 500,000 people a year. It also cools the planet, however—although no one knows by exactly how much—and so as governments enforce antipollution laws, such as the U.S. Clean Air Act, they are making global warming worse. Wouldn’t it make more sense, Crutzen suggested, to loft some of that SO2 into the stratosphere? Up there it would shade us from the sun without killing us.
Budyko’s original idea had been to send planes into the stratosphere burning high-sulfur fuel; Crutzen proposed delivering the SO2 with balloons. Estimates vary of just how much SO2 would be needed to counteract, say, a doubling of CO2 over preindustrial levels. Wigley put the number (generally expressed as the weight of the sulfur alone) at five million tons a year; Crutzen and Philip J. Rasch of NCAR have calculated that 1.5 million tons would do the job—provided the particles were smaller, on average, than the typical volcanic ones, which are less than 0.2 micron across.
All those estimates are small compared with the amount of SO2 we have already put in the lower atmosphere—and by the scale of the CO2 problem, they are tiny. The annual amount of SO2 needed, Caldeira remarks, is roughly what you could push through a fire hose. Crutzen estimated that his scheme would cost between $25 billion and $50 billion a year, which amounts to between $25 and $50 for each citizen of the developed countries. That is less than the average American spends on lottery tickets, and the return would be far more certain: a cooler planet—at least on a globally averaged basis.
All Climate Change Is Local
Yet the regional temperature pattern is what matters most. On that score, according to climate modeler David S. Battisti of the University of Washington, sun-blocking SO2 and heat-trapping CO2 are not well matched. CO2 warms the planet day and night, summer and winter. As ice melts on sea and land, replacing a white and cold surface with a dark and warmer one, the CO2 warming is amplified near the poles. In contrast, a stratospheric sulfate sunshade would block the sun only when and where the sun was shining; it would have no direct effect at all during polar winter. One would thus expect it to cool the tropics more than the poles—just the opposite of what is needed to restore climate to its preindustrial state.
Surprisingly, the few model simulations done so far suggest the effects of a sulfate sunshade are not that simple. “What we found is that it actually did a pretty good job” of reversing the warming trend in global climate, Caldeira says. By cooling the poles enough during the summer to maintain sea ice, the sunshade triggers the same powerful feedback that amplifies CO2 warming, but in reverse.
But the sulfate sunshade could have serious drawbacks on other grounds. SO2, like CO2, would not just affect the planet’s temperature; it would change winds and precipitation as well, in ways that are not yet foreseeable. As less sunlight reached the earth’s surface, there would be less evaporation, particularly in the tropics, which would probably make rain and freshwater scarcer than they are today. The eruption of Mount Pinatubo seems to have done just that: according to an analysis by Kevin E. Trenberth and Aiguo Dai, both at NCAR, the amount of precipitation on land and the volume of river runoff dropped dramatically in the year after the eruption. At the same time, less evaporation should lead to moister soils. And Caldeira’s modeling suggests that adding SO2 to the atmosphere along with the CO2 leads to smaller changes in precipitation than adding the CO2 alone—in short, that geoengineering would still be an improvement over business as usual.
Whether or not there is less of it, the rain is likely to become more acidic if we put millions of tons of sulfuric acid into the stratosphere. Globally, the acid increase will probably be small—because we are already putting so much SO2 into the lower atmosphere—but as Alan Robock of Rutgers University has pointed out in the Bulletin of the Atomic Scientists, some acid rain might fall in pristine areas that have been spared so far.
Return of the Ozone Hole?
A more serious worry is stratospheric ozone. Chlorine atoms that reach the upper atmosphere, the legacy of the chlorofluorocarbons long used as coolants and spray propellants, dig a hole in the Antarctic ozone layer every spring and let ultraviolet (UV) sunlight flood in. The chemical reactions that destroy ozone, however, take place only below a certain temperature threshold and only on the surfaces of stratospheric particles—including tiny droplets of sulfuric acid. As chlorofluorocarbons are phased out under the 1987 Montreal Protocol, the ozone hole is getting both smaller and shallower. But if more sulfuric acid is pumped into the stratosphere, it could act as a catalyst that could delay ozone recovery.
Sure enough, the Pinatubo “experiment” caused some ozone loss but not very much. According to Simone Tilmes of NCAR, however, the small size of the effect is misleading, because the winters following the eruption happened to be mild. In a colder winter, Tilmes says, the ozone destruction at the poles would have been far more severe. Even worse for the ozone, the greenhouse gases that cause global warming actually tend to cool the stratosphere by trapping heat closer to the surface.
By Tilmes’s calculations, if we were to start injecting SO2 into the stratosphere in the next few years, the recovery of the Antarctic ozone hole would be delayed by between 30 and 70 years. In cold years an ozone hole would appear in the high northern latitudes as well, bathing cities there in cancer-causing UV radiation. As Rasch points out, however, Tilmes’s results may represent a “worst-case scenario”; she combined the amount of SO2 needed to counteract a doubling in CO2 decades from now with the amount of chlorine that is in the stratosphere today—even though chlorine is steadily decreasing.
The effect of SO2 on ozone thus remains uncertain, like just about every aspect of sulfate geoengineering. We could start doing it next year, but aside from cooling the planet globally we would have no real idea what we were doing—much as we did not know what we were doing to the ozone layer when the world began using chlorofluorocarbons in refrigerators and underarm deodorant. Crutzen acknowledged this iron law of unintended consequences in his essay, writing: “The chances of unexpected climate effects should not be underrated, as clearly shown by the sudden and unpredicted development of the Antarctic ozone hole.”
Sea Mist in the Troposphere
In the lower atmosphere, SO2 does not just scatter sunlight and cause respiratory disease: it creates clouds where there were none, and it brightens existing ones, the so-called aerosol indirect effect. Climate scientists think this effect already cools the planet at least as much as direct scattering by aerosol particles. Ship tracks—linear clouds of engine exhaust—illustrate the phenomenon vividly: they persist for days and extend for hundreds of miles as the ship steams along. Satellite photographs record the sunlight they reflect back to space.
John Latham’s idea for cooling the planet is essentially to whiten existing marine clouds by lacing them with lots of ship tracks—but made in a cleaner way. Latham, a retired English cloud physicist, thinks spraying microscopic drops of seawater into the sky from a fleet of unmanned sailing vessels could do the trick.
The basic mechanism of the aerosol indirect effect is simple enough. The amount of sunlight reflected by a cloud depends on the surface area of the water drops that make up the cloud. “If instead of having a few big drops, you have a lot of little drops, then for the same amount of water [condensing from the vapor phase into droplets], there’s more surface area,” Latham explains. In principle, adding particles to the atmosphere makes for more but smaller drops, hence whiter and more reflective clouds.
Over land these days, the air is loaded with man-made particles, and as a result clouds are thought to be whiter and more reflective than they otherwise would be. But over the ocean the air is filled primarily with natural particles, including seawater droplets blown aloft by foaming waves. By the time the droplets reach 1,000 feet, most of the water has evaporated, leaving particles of salt—but at that altitude water vapor begins to condense again around the particles. The new droplets form low marine stratocumulus clouds, which cover about a quarter of the world’s ocean. Latham’s idea is to brighten such clouds by adding enough airborne salt spray to quadruple the number of water droplets in the clouds.
Stephen Salter, an emeritus engineering professor at the University of Edinburgh, has come up with a scheme that, on paper at least, looks ingenious. “It’s basically a watering can,” Latham says—but the nozzle would be a silicon wafer etched with billions of holes less than a micron across, and it would be mounted on an unmanned, satellite-guided sailing ship. More specifically, the vessel would be a Flettner ship, which has tall, spinning cylinders that resemble smokestacks but act as sails, generating lift because one side is moving with the wind and the other side against it.
In Salter’s concept, turbines spun by water moving past the ship would generate the electricity to keep the cylinders spinning and also to spray seawater out the stacks in 0.8-micron droplets. Salter and Latham estimate that 1,500 ships, each spraying eight gallons a second—and each costing $2 million, for a total of $3 billion—could offset the global warming caused by a doubling of CO2. Half the job could be done, according to modeling results from the Met Office Hadley Center for Climate Prediction and Research in Exeter, England, by deploying ships over just 4 percent of the ocean.
Still, no one has modeled how evenly the cooling would spread around the planet. “You could end up with a polka-dotted world, where there are really cold places and really hot places,” Battisti says. Another concern is drought downwind of the spray vessels; clouds made of many small droplets last longer, which is desirable in a sunshade, but they also produce less rain.
Finally, just how much brighter the new clouds would be is not known. Existing climate models overestimate the effect: according to them, the aerosols in the atmosphere right now should be canceling global warming, which is manifestly not happening. Rasch has thus started modeling Latham’s idea. “This is one of the parts of climate that we understand most poorly,” he says.
Still, as geoengineering schemes go, spraying seawater into the air from wind-powered vessels sounds pretty benign. If anything went wrong, Latham says, you could shut off the spray within days or, at most, a few weeks—whereas sulfuric acid in the stratosphere would stay aloft for years. “It’s definitely worth looking into,” Wigley says. But only a field test could answer some of the questions about the idea—and so far the only support Latham has received has been from the Discovery Channel. In need of good visuals for a documentary series on geoengineering, television producers funded the construction of a small Flettner ship.
A Sunshade in Space
The Discovery Channel has also paid to build something for J. Roger P. Angel: a half-inch-wide disk of silicon nitride ceramic. It is transparent, pierced with many tiny holes, and about a quarter-micron thick—one-fortieth the thickness of Saran Wrap but much stiffer. Angel, director of Steward Observatory Mirror Laboratory at the University of Arizona, is well known as an innovative developer of telescope mirrors and optics, and so his idea for a disk-shaped optical device made out of the same stuff used in high-performance automotive bearings is entirely in character. A couple of years ago his wife asked him if he could do something about climate change. He responded by looking into an old geoengineering proposal that, to put it mildly, lies way outside the box.
That proposal called for placing a sunshade at L1, the inner Lagrangian point, a million miles from the earth in the direction of the sun. (At a Lagrangian point the sun exerts the same pull of gravity as the earth does.) From L1 a sunshade would cast an even shadow over the planet without polluting the atmosphere.
In Angel’s scheme, the space-based sunshade would not be a single spacecraft but trillions of them—each a two-foot-wide disk of silicon nitride equipped with a computer and a navigation system and weighing no more than a gram. (Monarch butterflies, Angel points out, weigh less than a gram, and they navigate thousands of miles to their breeding grounds in Mexico.) These “fliers” would be launched in stacks of a million, one stack every minute or so for 30-odd years, by electromagnetic coil guns that would be more than a mile long and mostly underground. Thus, Angel proposes to keep the weight of the sunshade to less than 20 million tons launched from the earth. Still, for comparison, that is only slightly less than 70,000 times the current mass of the International Space Station—no trivial mass to fling into the heavens.
Highly efficient, ion-propulsion engines would carry each stack from earth orbit to L1, where the fliers would be dealt, like cards flung from a deck, into a cloud 60,000 miles long, pointed at the sun. “Shepherd” satellites patrolling the cloud would set up a local Global Positioning System, and each flier would keep itself from drifting out of the cloud with tiny mirrors acting as solar sails. Solar photons would pass right through the silicon nitride in each flier, but the ones passing through the holes in the flier would come out slightly ahead. The two sets would thus interfere destructively, blocking some of the sunlight that would otherwise reach the earth and scattering around 2 percent of it to either side of the planet.
It is hard to know just how serious Angel is about the idea. “It’s not a quick, cheap fix,” he says cheerfully. In fact, with a price tag loosely estimated at $5 trillion, a space-based sunshade is, according to Wigley, “just completely out of the question.” It would “require such a Herculean effort,” Caldeira says, “that maybe it’s easier to build wind turbines and solar power plants.” Angel himself seems to agree; he spends most of his time these days trying to think of ways to concentrate sunlight and make photovoltaics more efficient. After all, he notes, any sunshade would waste solar power.
Smart vs. Stupid
Geoengineering cannot solve the CO2 problem—in part because the problem is not just one of global warming. If we were to stop global warming with a sunshade, CO2 would continue to seep into the ocean, slowly acidifying it, and in time the ecological consequences would likely be dire. Nevertheless, stopping global warming temporarily might be worthwhile. And sulfate geoengineering, Caldeira says, would be “cheap enough that single actors could do it and bear the cost themselves.” The U.S. could choose to save Greenland’s ice cap (and thus prevent Florida from flooding), China its Himalaya glaciers, Switzerland its ski industry, all without the fuss of negotiating a global climate treaty. Depending on your point of view, that is one of the more appealing or one of the scarier things about geoengineering.
Probably the scariest thing to think about, though, is what would happen if we did not reduce carbon emissions, built a cooling sunshade—and then allowed it to fail. Raymond T. Pierrehumbert, a climate modeler at the University of Chicago, refers to this scenario as “DamoclesWorld.” The thin sulfate thread that holds up the CO2 sword would have to be maintained, year after year, with steady injections of ever increasing amounts of SO2. If it ever snapped, for reasons of war or civil unrest or budget crises, the accumulated CO2 would warm the planet in one fell swoop, creating precisely the emergency the sunshade was intended to prevent, only worse. Caldeira, too, has simulated the collapsing-sunshade scenario, along with H. Damon Matthews of Concordia University in Montreal; they found that the earth might warm at a rate of between four and seven degrees F per decade, 10 times faster than it is warming today. Human history, Pierrehumbert argues, does not inspire confidence that we could forestall such a catastrophe. On a graph he uses to illustrate the result of his own simulations, the point at which geoengineering stops and temperatures soar is labeled “Fall of Rome.”
No one knows today whether geoengineering could ever make sense. Most workers would agree that further research is now inevitable—but their attitudes toward such study vary. To some, such as Wigley, a sunshade could be a rational strategy to buy time for the long labor of converting to a carbon-neutral energy supply. Others fear it would remove the incentive to do that hard work. “It’s extremely unfortunate that this genie has come out of the bottle just at a time when the world seems finally awakening to the seriousness of climate change,” Pierrehumbert told an audience recently at the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. “There is a huge risk that if people begin to see this prematurely as a fallback position, this technology will cut off at the knees actions that are just starting to be taken that make serious reductions in emissions.”
In the end the debate comes down to differing views about human nature—and the power of science to restrain it. “Scientifically it would be utterly stupid just to do geoengineering” without reducing emissions, Wigley says. “If we were to do that, we’d get to the crunch where people realize there aren’t any more fish in the sea. We’re not that stupid. We can be guided by good science.”
Pierrehumbert, like many others, takes a darker view. A bullet point on one of his PowerPoint slides reads simply: “We are quite capable of doing stupid things.”
|| Octubre de 2008. Scientific American Magazine.