Engineering a cooler Earth

Researchers brainstorm radical ways to counter climate change

None of the scientists in the room so much as blinked when David Keith suggested saving the world with spy planes spraying sulfuric acid.

ENGINEERING A COOLER EARTH Giant air-capture machines, like artificial trees, could cleanse the atmosphere of excess carbon dioxide. Michael Morgenstern
COOL BUT DIFFERENT
FIVE WAYS TO SAVE THE PLANET | The United Kingdom’s Royal Society has rated geoengineering techniques according to cost, effectiveness (cooling power) and timeliness (considering both how quickly a technology could be deployed and how fast it would cause cooling). T. Dube

Keith, a physicist at the University of Calgary in Canada, was facing an audience not likely to be shocked: nearly 200 other researchers, some of whom had their own radical ideas for fighting global warming. His concept was to spray a mist of sulfuric acid high in the stratosphere to form particles called sulfate aerosols, which would act like a sprinkling of tiny sunshades for the overheating Earth.

Keith’s idea may sound outrageous, but it is just one of many proposals for bumping the global thermostat down a couple of degrees by tinkering directly with the planet’s heating and cooling systems. Plans to cool the Earth range from shading it to fertilizing it, from seeding clouds to building massive supersuckers that filter greenhouse gases from the air. The schemes are all part of a growing field known as geoengineering: a subject once taboo for all but the scientific fringe, but now beginning to go mainstream.

So far the tinkering happens mainly in computer models, where researchers are trying to figure out geoengineering’s potential side effects. Yet some technologies are in the prototype stage, governments are starting to consider geoengineering seriously and budding geoengineers are working out how to proceed safely, and ethically, with real-world experiments. 

“It truly is asking giant questions which nobody really knows the answers to,” Keith says — “like how we manage the whole Earth.”

In March, Keith and other experts met in a dimly lit chapel-turned-auditorium at the Asilomar resort near Monterey, Calif. In 1975, molecular biologists met at the same resort to write landmark guidelines to regulate DNA experiments. This time around, cloud physicists, legal scholars and government bureaucrats debated the relative merits of brightening clouds versus building artificial trees. In the end, the meeting-goers concluded that geoengineering research should cautiously proceed, in case Earth’s climate proves broken beyond the current means of repair: ratcheting down fossil fuel use.

Researchers have kicked around the idea of large-scale climate manipulation since at least the 1960s, when Soviet scientists suggested damming the Bering Strait as part of a scheme to warm Siberia and free shipping lanes of sea ice. But mainstream scientific attention began only about five years ago.

In a 2006 editorial in Climatic Change, atmospheric chemist Paul Crutzen suggested that sulfate aerosols might be used to intentionally cool the planet. Coming from a scientist who had shared the Nobel Prize for helping explain how man-made chemicals ate away the Earth’s protective ozone layer, the idea gained some traction.

Geoengineers poked their heads from the closet tentatively at first. But soon, ideas multiplied.

Quick and dirty

Keith’s spray planes fall into one of the most controversial categories of geoengineering: solar radiation management, meant to reflect sunlight away from the Earth. As geoengineering schemes go, solar radiation management would be relatively cheap and fast. It would also be effective: Blocking just 2 percent of the sun’s rays, scientists estimate, could cool the planet by about 2 degrees Celsius, roughly balancing the warming effect of doubling carbon dioxide above preindustrial levels.

One obvious way to reflect light would be to hang something shiny between the Earth and the sun. Such proposals feel distinctly like science fiction, ranging from a fine mesh of aluminum threads hung in space to a swarm of reflective disks launched in stacks of a million every minute for 30 years.

A more popular idea among scientists is to pump tiny reflective particles high into the stratosphere, the atmospheric layer between about 10 and 50 kilometers up. To scatter light effectively, the particles need to be solid. But dispersing solid bits without clumping is nearly impossible, so one idea is to spray sulfur gases that turn into solids. Keith’s sprayers would use a mist of liquid H2SO4, or sulfuric acid, which forms small particles of the right size effectively.

Volcanoes, whose emissions have long been known to cause temporary chills, inspired the idea of using sulfate aerosols as climate coolers. The 1991 eruption of Mount Pinatubo in the Philippines released about 20 million metric tons of sulfurous gases and cooled the planet by half a degree for more than a year. Geoengineers wouldn’t need to spray that much, because particles shot directly into the stratosphere would cool more efficiently than volcanoes. To reflect 2 percent of incoming light, geoengineers would need to spray between 1 million and 5 million metric tons of sulfur each year.

A single fire hose to the stratosphere, running constantly, could potentially deliver enough. The cost? An estimated $10 billion per year to cool the planet 2 degrees.

The environmental effects are an even bigger question, scientists say. For one thing, sulfur spraying would lead to whiter daytime skies and bolder, redder sunsets, because of the way the sun’s rays scatter off aerosol particles. 

Another question, at least at first, was whether spraying sulfur in the stratosphere would, like sulfur pollution from power plants, cause problems with acid rain and air quality. But where in the atmosphere sulfur is released makes a big difference, says physicist Jason Blackstock of the International Institute for Applied Systems Analysis in Laxenburg, Austria. “The sulfate particles that we would put in the stratosphere would be the same as those which cause acid rain now,” he says. “But by putting them higher, they stay up longer because they’re above the clouds, which means they aren’t raining out as acid rain all the time.”

A different problem is that aerosols can damage the stratospheric ozone layer. Chemicals known as chlorofluorocarbons, or CFCs, once common in spray cans and refrigerants, have depleted the ozone layer globally, and in particular over Antarctica. Now that CFCs are being phased out by a global treaty, ozone has been slowly recovering. But spraying enough sulfate aerosols in the stratosphere to push back the effects of global warming by 40 years would delay the recovery of the Antarctic ozone hole by about 30 years, according to a 2009 paper in the Journal of Geophysical Research-Atmospheres.

An even more worrying potential effect is on precipitation, says meteorologist Alan Robock of Rutgers University in New Brunswick, N.J. Injecting sulfur gas could reduce rain delivered during the Asian and African summer monsoons, which 2 billion people rely on for growing food, by as much as 15 percent, Robock and his colleagues reported in the Journal of Geophysical Research in 2008. That would make the new average rainfall “the equivalent of the worst monsoon year now, and weather would produce bad monsoon years with precipitation much lower than that,” he says.

In principle, injecting sulfur in the right way could minimize precipitation changes, says climate modeler Ken Caldeira of the Carnegie Institution for Science in Stanford, Calif. Injecting aerosols uniformly around the globe would result in less disruption of global precipitation patterns than focusing injection at the poles, a strategy some have suggested for saving the ice caps, Caldeira reported in February in San Diego at a meeting of the American Association for the Advancement of Science. In fact, he said, a geoengineered climate would seem more like the one we’re used to than would a climate with doubled CO2 levels. Still, he noted, “stratospheric aerosols are likely to cause some damage in some places.”

Seeing white

Another way to reflect light would be to harness the Earth’s built-in solar reflectors, clouds. And where there are clouds, there could be more clouds — or at least whiter ones, some researchers say.

One leading geoengineering idea is to spray a mist of seawater into clouds over the ocean to make them whiter and brighter. Sea-salt particles in the spray would add more “seeds” to the air on which water vapor could condense, amplifying the natural cloud-forming process.

A fleet of 1,500 remote-controlled spray ships could whiten clouds enough to offset the warming created by doubled CO2, says engineer Stephen Salter of the University of Edinburgh in Scotland. He and John Latham of the National Center for Atmospheric Research in Boulder, Colo., even developed a working prototype spray ship.

One challenge: along with clouds comes rain. So cloud seeding can also affect precipitation patterns, as new computer model studies by Latham and colleagues show. In simulations of cloud seeding across 20 to 70 percent of the ocean’s surface area, less precipitation fell at the equator but more fell over the Amazon region on average over a 20-year period, the team reported last year in Environmental Research Letters. Other work by Caldeira and his colleagues suggests that brightening ocean clouds would generally increase rainfall over the ocean but decrease it over land.

Models by Philip Rasch, a climate scientist at the Pacific Northwest National Laboratory in Richland, Wash., who works with Latham’s team, also show that high levels of cloud seeding could cool the Arctic enough to restore disappearing sea ice. But the team can’t find a way to maintain sea ice, temperature and precipitation at desired levels at the same time. “You could optimize the planet to produce a sea-ice distribution that looks like today’s, but then temperature and precipitation would be off,” Rasch says.

What’s more, reflecting light is no permanent solution. Like a celebrity addicted to painkillers, a planet hooked on sulfur particles or cloud seeding would need to keep its fix coming to stay cool; turn off the juice, and temperatures climb right back up. And any solar management plan does nothing to counteract other ecosystem-wide changes caused by rising CO2 levels, such as the rising acidity of the oceans as they absorb more CO2. So most scientists say these methods would work only as a stopgap to head off the worst effects of warming while greenhouse gas levels are brought down by burning less fossil fuel.

Slow but sure

As an alternative, many researchers champion carbon dioxide removal — taking CO2 from the air using any of a number of materials, such as sodium hydroxide. This solution is essentially permanent, it keeps oceans from acidifying and it has few side effects. But it is expensive and could mean finding storage for billions of tons of carbon-containing stuff each year (SN: 5/10/08, p. 18).

Klaus Lackner of Columbia University and Calgary’s Keith have both designed air-capture systems. Using different industrial processes, each system would absorb and separate out CO2 from the air. That pure stream can be stored in geological formations, as is done in carbon capture and storage systems for power plants being tested in the United States, Canada and elsewhere.

At the Asilomar meeting, Keith announced that he plans to build an air-capture prototype over the next few years. Each unit could lock up 100,000 tons of CO2 per year, at a cost of more than $100 per ton. At that rate, absorbing all the CO2 that the United States emits each year would cost more than $580 billion annually and take 58,000 air-capture units. More realistically, Keith hopes to improve the system’s efficiency and lock up as much carbon as possible while also cutting emissions. Meanwhile, Lackner is developing his capture devices in a 10,000-square-foot Tucson facility through a partnership between Columbia and the company he cofounded, Global Research Technologies. The company estimates that within two years its prototype units will capture a ton a day for less than $100 per ton.

Machines aren’t the only way to absorb CO2. Some researchers are working on fertilizing ocean plankton with iron to stimulate their growth, enhancing natural CO2 uptake. Other scientists are looking into using large pipes to churn nutrient-rich waters to the surface for the same effect.

Several private companies have launched iron fertilization efforts in recent years, but with mixed results. One killed its project when faced with environmental concerns, while another faces continuing scientific questions about how much carbon dioxide is sequestered by plankton sinking into the deep ocean and how much is returned to the atmosphere as plankton decompose.

Moving ahead

Despite such uncertainties, public interest in geoengineering has snowballed in recent months. The U.S. House of Representatives held hearings to learn about geoengineering in November and March, and the Government Accountability Office has launched a major assessment. In September 2009 the Royal Society, the United Kingdom’s leading scientific body, called for £100 million in government funding for geoengineering research over the next 10 years. The society is also leading a study, in cooperation with the Academy of Sciences for the Developing World and the Environmental Defense Fund, that late this year will issue recommendations for regulating geoengineering research.

If funding can be found, several geoengineering technologies could be tested in real-world experiments within a few years. Working with Aurora Flight Sciences in Manassas, Va., Keith’s group has estimated that a civilian version of a U-2 spy plane could be adapted for less than $10 million to spray a ton of sulfur. That amount of sulfur is less than one-millionth the amount proposed for altering climate, so such an experiment would be used only to test the mechanics of spraying and perhaps study atmospheric chemistry.

But spraying even a small amount of sulfur raises major questions, such as what could go wrong and who is responsible if something does. No international authority is in charge of saying whether Keith can do such an experiment. At Asilomar, scientists called for the development of voluntary guidelines because there is no major treaty or law that clearly covers geoengineering.

Even to scientists who take the idea seriously, the prospect of actually fiddling with the planet’s climate on purpose is frightening. “If you’d asked me a decade ago, I would have said that studying these issues is problematic, because the more you know how to do it, the greater the possibility that someone will do it,” says M. Granger Morgan, an applied physicist who heads a research program at Carnegie Mellon University in Pittsburgh that examines solar radiation management. “I don’t want to see the world do this; I want to abate emissions,” he says. “But I think we’re at the point where it would be a mistake not to better understand what might be possible or whether it might work.”

Then again, not fiddling with the climate is just as scary to some. “We know the risks of CO2 are serious too,” Keith said at Asilomar. That makes geoengineering a lot like chemotherapy, in his view.

“Would you prefer to avoid eating carcinogens or have chemotherapy?” he asks. “Everybody would rather avoid the carcinogens, but if you already have cancer you might prefer chemotherapy to dying, even though chemo will leave you sick and make your hair fall out.”

And, of course, chemotherapy doesn’t always work — and sometimes it kills
the patient.


Gauging public understanding

Preliminary results from a survey of 1,001 Americans suggest that few understand the term “geoengineering.” When participants were asked, “Have you heard about geoengineering as a possible response to global warming?”:

74% Said no

25% Said yes but incorrectly thought the term referred to geothermal energy, green building or some other issue

1% Said yes and described geoengineering correctly as a way to artificially cool the planet


Desperate times and desperate measures

As climate researchers have grown increasingly alarmed by vanishing sea ice and faster-than-expected growth in greenhouse gas emissions, geoengineering has become less of a laughing matter in scientific circles. “Emissions reduction alone is not going to cause the Earth to start cooling this century,” says Ken Caldeira of the Carnegie Institution for Science in Stanford, Calif. Even if all CO2 emissions stopped today, he says, temperatures would increase for decades, if not centuries, warming the Earth by at least another half a degree Celsius as oceans slowly release stored heat.

Without a dramatic turnaround in emissions, CO2 levels will probably overshoot targets scientists consider safe, says Steve Schneider, a climate scientist at Stanford University. Today’s atmospheric CO2 level is 380 parts per million, compared with 280 ppm in 1850. Schneider has long advocated a 350 ppm goal — already surpassed — but even 450 ppm, a typical target in international negotiations, is fast approaching (SN: 12/5/09, p. 16). Geoengineering methods might work “as a temporary palliative measure you would use to hold the peak down, while you’re working on carbon removal and tremendously increasing mitigation,” he says.

The idea of using geoengineering to “shave the top” off a temperature peak stirs mixed feelings. “Deep down, I think it’s a bad idea,” says ecologist Rob Jackson of Duke University in Durham, N.C. “But what are we going to do? I can see scenarios now where we don’t have any other choice.”

Scientists don’t agree on whether geoengineering should be seen as part of an ongoing global temperature management plan or only as a possible last-ditch effort. But many do agree that geoengineers need to get cracking now on research and development.

“What happens if in 2040 or 2060 temperature increases are so high that crops are failing throughout tropical regions and billions of people are threatened with famine?” Caldeira says. “We’d better try to understand if there is something we could do, because there’s no other way to realistically stop the Earth from warming during the course of this century.”

From the Nature Index

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