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Geoengineering Steps Toward Reality

Once viewed skeptically, plans for intentional intervention in earth’s climate gain traction

by Cheryl Hogue
November 23, 2009 | A version of this story appeared in Volume 87, Issue 47

Credit: LLBL/C&EN
Geoengineering options include technologies to reflect sunlight into space and to remove carbon dioxide from the air.
Credit: LLBL/C&EN
Geoengineering options include technologies to reflect sunlight into space and to remove carbon dioxide from the air.

Launching hundreds of billions of 2-foot-wide reflectors into Earth’s orbit. Outfitting an armada of oceangoing ships to gin up clouds. Blasting tons of sulfur into the stratosphere with artillery.

These and other geoengineering schemes that might weaken the effects of human-induced global warming may seem like science fiction. In the past, they’ve been dismissed as far too risky or imprudent for mainstream scientists or policymakers to consider seriously.

Today, however, geoengineering proposals are starting to get lots of attention. Geoengineering increasingly seems to offer viable, science-backed options for averting the worst of predicted global warming. It doesn’t offer a silver bullet solution, however. The technologies involve trade-offs that are likely to generate powerful opponents, and their deployment may be influenced by international treaties over outer space and Earth’s atmosphere and oceans.

Perhaps most responsible for bringing the idea of geoengineering into the public consciousness is the just-released best-selling book “SuperFreakonomics.” It offers quirky slants on issues from drunken driving (arguing that it’s better for the boozed up to get behind the wheel than walk short distances) to prostitution (not as profitable as in decades past because of the increase in casual sex). Written by economist Steven D. Levitt and journalist Stephen J. Dubner, the book argues that geoengineering is an easier, less-expensive way to deal with climate change than transforming the world’s economy to low-carbon energy.

And now, scientists and policymakers alike are taking a hard, calculated look at intervening in vast natural systems that are essential to life. They point out, however, that for the past 150 or so years, societies have been involved in an unintentional geoengineering experiment by raising the level of atmospheric carbon dioxide.

Most scientists who are proponents of geoengineering to cool the planet are prefacing any discussion with a powerful caveat: These techniques hold potential only as stopgap measures. This places their discussions in a decidedly different camp from the authors of “SuperFreakonomics.”

The scientists’ argument is that technologies might be deployed temporarily to help cool Earth. Geoengineering, they emphasize, cannot replace emission cuts, which are an essential part of any plan to address climate change.

Backing this stance is a group no less scientifically august than the U.K. Royal Society. The organization issued a report in September declaring that geoengineering techniques may be the only strategy left in the playbook if political talks on reducing global greenhouse gas emissions stall. Those political talks have not yet halted, but their progress is slow (C&EN, Nov. 9, page 37).

Policy discussions over what to do about climate change have evolved over the years, according to Ronald G. Prinn, director of Massachusetts Institute of Technology’s Center for Global Change Science. Initially, talks focused solely on mitigation—reducing greenhouse gas emissions. Then, deliberations began to include adaptation—figuring out how to live with effects of global warming, such as rising sea levels, that are likely to happen with just the concentrations of greenhouse gases now in the atmosphere.

A decade ago, proposals on geoengineering to stave off climate change were regarded as “a little bit off the deep end,” Prinn said at a recent symposium at MIT on geoengineering. But they are gaining more currency as the concentration of greenhouse gases continues to rise and predictions about the likely effects of climate change have gotten graver.

“We can and must stop emissions,” warned David Keith, a professor of chemical and petroleum engineering and of economics at the University of Calgary. But Keith also argued that geoengineering is necessary to dampen the effects of anthropogenic climate change.

Geoengineering is needed, Keith told the MIT meeting, because the atmosphere already contains enough long-lived greenhouse gases to cause severe changes in climate for centuries to come.

Predictions of climate change include decreased precipitation in already-dry parts of the world, including the U.S. southwest. Averting droughts in these regions is likely to become a major impetus for policymakers to pursue geoengineering projects, said Thomas R. Karl, director of the National Climatic Data Center at the National Oceanic & Atmospheric Administration.

Also, as the effects of climate change continue to unfold, policymakers may come under increasing pressure to take action to temper them, Karl told those at the MIT gathering.

For instance, rising sea levels or high temperatures that hamper the growth of food crops may make geoengineering attractive to governments and some businesses as well, said David S. Battisti, a professor of atmospheric sciences at the University of Washington, in Seattle. “Someone’s going to make a lot of money off this,” he said at the symposium.

Not all ideas for geoengineering are likely to be effective, and some would be less expensive to install than others, said Timothy Lenton, a professor of earth systems science at the University of East Anglia, in England. He has taken a first stab at analyzing the practicality and cost of proposed geoengineering projects.

Geoengineering proposals fall into two main categories, Lenton said at the symposium. One is reducing the amount of solar radiation that enters the atmosphere and gets trapped by greenhouse gases. The other comprises efforts to strip CO2 out of the air.

Ideas for reflecting sunlight back into space include launching mirrors with an area of about 0.3 m2 into space around Earth, injecting aerosols into the stratosphere, and promoting the formation of clouds.

Lenton’s analysis calculated that the mirror plan is impractical. Having a significant effect on solar radiation would require thousands of launches per year carrying payloads with hundreds of thousand of reflectors, he found.

Injecting aerosols into the stratosphere to reflect sunlight has the backing of Nobel Prize-winning chemist Paul J. Crutzen (C&EN, Aug. 7, 2006, page 19). The idea seems to be feasible and affordable, Lenton found. It would require the addition of perhaps a few million tons of aerosol-forming sulfur or sulfur-containing compounds to the upper atmosphere each year. The material could be shot into the upper atmosphere by surface-based cannons or released by high-flying planes.

Another strategy to reduce the amount of sunlight striking the lower atmosphere is to increase the mass of clouds that would reflect light into space. This involves seeding clouds with material, such as sea salt, around which water droplets can condense. Implementing the cloud-boosting strategy in oceans could be a feasible plan for cooling Earth, Lenton found. Increasing cloud cover, he said, is more effective than other reflective strategies, such as painting roofs white or putting reflectors in deserts.

Any scheme to reflect more light into space, however, has downsides, Lenton said. A decrease in sunlight at Earth’s surface would have a significant climate effect of its own by weakening the water cycle. Computer models predict that reflection-based geoengineering would promote drought in areas with monsoon climates, including India and Western Africa. And these projects would do nothing about the increasing acidity of oceans, a key result of climate change. Ocean waters absorb increasing amounts of CO2 from the air as atmospheric concentrations of the gas rise.

All reflection-based methods share another key side effect, Lenton said. Deploying any of them would commit future generations to maintaining the projects until atmospheric concentrations of greenhouse gases fall—because stopping them suddenly will trigger rapid global warming.

Other geoengineering methods remove CO2 from the atmosphere. They include planting more trees or fertilizing the oceans to promote algal growth, which would turn CO2 into biomass via photosynthesis, Lenton said. Proposals for “synthetic trees,” which would use physical or chemical processes to capture the gas for storage, would be more expensive to implement than growing algae or real trees, his analysis found.

To keep the CO2 sequestered, plants cultivated specifically to remove CO2 must be harvested and converted to biochar through pyrolysis—heating in the absence of oxygen so that carbon is not oxidized to CO or CO2. Biochar is not easily broken down by biotic or physical processes, Lenton said, and it can be used as a soil amendment or otherwise stored.

But growing plants solely for carbon capture is limited by the amount of suitable land that is not needed for crops or other purposes, such as human habitation or national parks, Lenton pointed out.

Ocean fertilization is intended to produce massive blooms of algae that eventually die and sink to the bottom of the ocean, locking away carbon for eons. However, this technique could cause major disruptions in marine ecosystems, Lenton noted. Plus, its effects on atmospheric CO2 levels are hard to verify.

A major uncertainty, Lenton said, is whether the dead algae would descend to a depth low enough to yield the desired effect. If algae carcasses don’t get below 500 meters in depth, they will liberate CO2 into the surface water when they decompose. The gas can get released again into the atmosphere as the oceans and atmosphere equilibrate their CO2 levels, he explained.

Thus, the effectiveness of all forms of CO2 removal will drop off over time. This is because the ocean will release the gas as atmospheric levels go down, Lenton pointed out.

Other factors complicate the picture for geoengineering. Geoengineering likely will have significant unintended consequences. For instance, injecting reflective aerosols into the stratosphere would essentially end ground-based astronomy, said James R. Fleming, a professor of science, technology, and society at Colby College, in Waterville, Maine.

A further complication is determining who would be allowed to deploy geoengineering projects and who would govern them. A looming challenge will be to constrain “rogue actors,” Keith said. Injecting aerosols into the stratosphere is likely to be so relatively cheap, he explained, that the private sector could carry out these projects if it wanted.

Finally, any geoengineering intervention will have to contend with a slew of existing international treaties that appear to have bearing over geoengineering projects, said Catherine Redgwell, a professor of international law at University College London.

One is the 1967 Outer Space Treaty, which covers exploration and use of outer space, she said at the MIT meeting. Another is the Environmental Modification Convention. That 1977 pact prohibits the hostile use of techniques that modify the dynamics, composition, or structure of Earth, including its atmosphere, or of outer space.

International agreements on environmental protection could also have sway over some geoengineering projects, according to Redgwell. They include the Montreal protocol, a 1987 accord controlling chemicals that deplete the stratospheric ozone layer, she said, pointing out that sulfur-based aerosols help destroy ozone. Another is a regional treaty among European nations, Canada, the U.S., and former Soviet bloc countries governing transboundary air pollution.

Projects to stimulate algal photosynthesis by fertilizing the oceans are covered under the Law of the Sea treaty and the 1972 London Convention, a global accord controlling dumping in the oceans. And parties to the 1992 United Nations pact to protect the diversity of life on Earth have formally asked governments to hold off on any large-scale ocean fertilization attempts until there is “an adequate scientific basis” for doing so.

So as geoengineering moves into the scientific community’s mainstream thinking, it may also wind up in the bailiwick of many existing international bureaucracies. But with the public’s increasing exposure to the idea and with governments unable or unwilling to ratchet down the flow of greenhouse gases into the atmosphere, geoengineering could draw more backers.


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