CAMBRIDGE, Mass. -- Some geoengineering schemes to fight climate change would probably succeed in cooling the planet, scientists said here Friday -- but whether we should ever deploy them is still an open question.
Researchers who gathered at the Massachusetts Institute of Technology outlined a stark list of potential side effects of different climate engineering approaches, including further depleting the ozone layer, inducing drought and turning the blue sky white.
At the same time, many experts said geoengineering could be a planetary "Plan B," an option to exercise if cutting greenhouse gas emissions can't stave off dangerous climate change.
"Even if we cut emissions, we have a lot of carbon dioxide already in the air," said David Keith of the University of Calgary. "We don't know exactly how bad the climate response will be, and we have to think clearly about how we manage the risk posed by CO2 already in the air."
An ongoing MIT research project into the risks posed by different levels of greenhouse gas emissions suggests that even steep cuts won't guarantee the world will stay under the 2-degree-Celsius climate guardrail espoused by many political leaders.
Stabilizing the level of greenhouse gases in the atmosphere at the equivalent of 550 parts per million of CO2 -- a goal that's "not easy," according to Ron Prinn, director of MIT's Center for Global Change Science -- would give the world just a 25 percent chance of limiting temperature rise to 2 degrees between 1990 and 2090.
"Even with a very tough and expensive target, we are still at risk," Prinn said. "Hence, I think it's legitimate to begin thinking about geoengineering as something that should be on the table."
While many proposed geoengineering approaches evoke low-budget science fiction -- using a giant garden hose to spew sulfur particles into the sky, launching an array of mirrors into space or planting acres upon acres of carbon dioxide-munching plastic trees -- the idea of using technology to artificially cool the planet is gaining ground.
The American Meteorological Society and the U.K. Royal Society have issued cautious calls for more research, though warning that geoengineering approaches shouldn't supplant efforts to cut greenhouse gas emissions. The National Academy of Sciences is investigating geoengineering as part of its wide-ranging "America's Climate Choices" study. And the House Science and Technology Committee is holding the first-ever congressional hearing on geoengineering later this week, in the midst of Congress' attempts to fashion a new U.S. climate policy.
Yet stumbling blocks abound, beginning with the fact that there's no standard definition of what constitutes "geoengineering." A case can be made, some attendees at the MIT meeting said, that humans have already geoengineered the climate by burning fossil fuels.
But most of the discussion in Cambridge centered on two main approaches to cool the planet: those that try to increase the amount of sunlight reflected into space, and those that seek to suck carbon dioxide out of the atmosphere.
The first category includes proposals to shoot sulfate particles into the upper atmosphere, creating a cooling haze that mimics the effects of a volcanic eruption, and a similar plan to use thousands of special ships to spew sea salt into the sky, encouraging the formation of clouds. Scientists believe many approaches in this category could cool the Earth rapidly -- but they might produce unacceptable side effects.
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Proponents of sulfate geoengineering, one of the most discussed geoengineering plans, say Mother Nature has already provided their "proof of concept."
They point to the 1991 eruption of Mount Pinatubo in the Philippines. That blast sent 10 million tons of sulfur into the stratosphere and cooled the Earth's surface by 0.9 degrees Fahrenheit for about a year. Research suggests a smaller amount of sulfate -- between 0.5 million to 5 million tons -- would be enough to counteract the warming brought about if the current amount of CO2 in the atmosphere doubled.
But experts say the Mount Pinatubo example also hints at reasons to be cautious. Studies suggest the 1991 eruption depleted the Earth's ozone layer and temporarily altered Asia's monsoon, inducing drought.
And while deflecting sunlight would cool the planet, it wouldn't remove CO2 from the atmosphere, so ocean acidification would continue.
"The risks are so high with some of these reflection options," said Tim Lenton, a professor of Earth system science at the University of East Anglia. "We need to do more research, but we need to reserve them for use in case of emergency."
There are also engineering challenges to overcome. Because sulfate doesn't linger in the atmosphere, temperatures would shoot up quickly if injections ended before the amount of carbon dioxide in the atmosphere stabilized at a safe level -- meaning that the world would likely have to commit to continue geoengineering for generations.
Using existing military planes and tankers to deliver those injections would cost "several billion" dollars per year, while other options -- like artillery or balloons -- would be even more expensive, according to a study published last week in Geophysical Research Letters.
Joyce Penner, a University of Michigan scientist who studies aerosols, said there's also a concern that sulfate particles of the type likely to be used in geoengineering schemes tend to clump together into larger particles that can actually warm the atmosphere rather than cool it.
Yet Penner also said she's not ready to rule out geoengineering. Particle pollution from factories and vehicle engines is already cooling the atmosphere, but as countries work to improve their air quality, that cooling effect will lessen.
"We should expect that protection to go away," she said. "That's one reason to consider geoengineering."
Climate engineering options that seek to pull carbon dioxide from the air seem less risky, experts said Friday, but also less effective. They include fertilizing the oceans with iron to spur the growth of algae that consume CO2, enriching soil with charcoal, or planting forests of artificial carbon-eating trees.
Overall, though, scientists at the MIT meeting said current understanding of geoengineering's peril and promise is rudimentary.
'Precious little' science has been done
"The thing that's always frustrated me," said Philip Boyd, a professor of ocean biochemistry at the University of Otago in New Zealand, is that geoengineering "has great press coverage. It has that science fiction component that makes good copy. But there's been precious little or no science done."
David Keith, the University of Calgary scientist, agreed. "The actual number of real, serious science done on this topic is pitifully small," he said.
Scientists' research wish list includes exploring the environmental, economic and social effects of different geoengineering schemes, as well as the engineering needed to make them happen and their effectiveness if deployed.
Answers to some questions depend on improving "major uncertainties" in climate models, said David Battisti, a professor of atmospheric sciences at the University of Washington. They don't do well at simulating the behavior of clouds, sea ice and oceans, areas that must be improved to give useful predictions of climate engineering's effects.
"In 20 to 30 years, models will get there," Battisti said. "Yet at the rate CO2's increasing, we'll need geoengineering in 20 to 30 years. ... If we're serious about this -- and I do think we need to be serious about this -- we need a large international development effort."
There are also questions how geoengineering would play out politically, since many climate fixes have side effects that are unevenly distributed or hard to control.
Fertilizing one ocean basin with iron, for example, would likely spur algal growth that would use up nutrients destined for other ocean basins, said Boyd, the University of Otago professor.
"Suddenly a fishery starts to fall over there," he said. "Does that happen because of an experiment down here?"
Boyd said he's about to publish a study that predicts many geoengineering proposals would increase the potential for conflict, in part because documenting their effectiveness and assigning blame if things go wrong would be difficult tasks.
He and others also noted that some climate engineering options, like delivering sulfate particles to the stratosphere, appear cheap enough that a large corporation or an individual country could deploy them without any international input.
"The fact that it's cheap automatically means the policy challenge is control," said Keith, the University of Calgary professor. "The challenge is to control early actors."
But in the end, if more conventional efforts to blunt climate change don't succeed, whether to proceed with geoengineering may become an easier question to answer.
"The most dangerous approach," said Keith, "is to assume geoengineering will work if we need it to -- without doing the research to prove it."
Correction: The two organizations calling for more research into geoengineering but stipulating that it should not take the place of emissions-cutting efforts are the American Meteorological Society and the U.K. Royal Society; a previous version of the story incorrectly stated that the former of the two organizations was the American Geophysical Union.