Developing the technology to suck planet-warming carbon dioxide back out of the atmosphere is an expensive but promising approach that may be necessary to help prevent the worst effects of climate change, according to the first of two reports released this morning by the research arm of the National Academy of Sciences.

But according to the second report, proposals to cool the planet on the cheap by reflecting sunlight are so risky that even serious study of them should be undertaken only in preparation for an emergency.

Together the two reports from the National Research Council (NRC) offer the most comprehensive U.S. examination yet of "geoengineering"—the intentional intervening in the climate system in an attempt to forestall some of the impact of global warming.

"The world is in a very tough situation, and there's no magic bullet here, unfortunately," said Paul Falkowski, a biochemistry professor at Rutgers University, who worked on the reports.

An NRC committee of experts from across disciplines was asked by several U.S. government science and intelligence agencies to evaluate geoengineering proposals. The ideas range from anodyne (planting trees to capture CO₂) to potentially alarming (injecting sulfate particles or other aerosols into the atmosphere to reflect sunlight and cool the planet).

Committee members were blunt in their first recommendation: The world should focus first and foremost on curbing fossil fuel emissions rather than on any kind of geoengineering.

"I think it's going to be easier and cheaper to avoid making a mess than it will be to make a mess and then try to clean it up later," said committee member Ken Caldeira, a climate scientist at Stanford University's Carnegie Institution for Science. "If we end up having to build a fix that's on the scale of our energy system, why not just retool our energy system?"

Six years after a report from the Royal Society in the United Kingdom reached many of the same conclusions, the American scientists decided to issue two reports—to distinguish as forcefully as they could between two very different approaches that for years have been lumped together under the heading "geoengineering."

The first, CO₂ removal, the committee characterized as worthy and "almost inevitable." The second, using aerosols or other means to reflect solar radiation, would be "irrational and irresponsible" if done as anything but a last-ditch effort to prevent a global famine or other emergency.

"We were clearly trying to send a message that we didn't want to paint CO₂ removal with the same geoengineering label," said committee member Steve Fetter, associate provost at the University of Maryland's School of Public Policy.

View Images A reforestation project in Peru illustrates a more benign approach to geoengineering: removing CO₂ from the atmosphere by, for example, planting trees. Photograph by Gabby Salazar, National Geographic Creative

Removing CO₂

Even the Intergovernmental Panel on Climate Change (IPCC) has suggested in its most recent report that CO₂ removal may become an essential tool. With atmospheric CO₂ at 400 parts per million and rising, the world seems likely to overshoot the target of 450 parts per million needed to keep the global average temperature from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit)—a level many scientists consider a danger threshold.

IPCC scenarios that avoid dangerous climate change typically assume that we'll reduce CO₂ emissions to zero and develop a way of reducing atmospheric CO₂ by the second half of this century.

There are various ideas about how to do that. The simplest is planting forests, which store CO₂ in wood and soil as they grow. Another is burning wood or other plant matter for energy, then capturing and burying the CO₂ before it exits the smokestack. A third idea is to develop chemical scrubbers, like the ones used to purify air on submarines, that remove CO₂ directly from the air.

"I personally think it will be necessary," Fetter said, meaning CO₂ removal in general. "CO₂ concentrations already are too high and increasing, and it's hard to see a realistic scenario in which we can limit and stabilize greenhouse gases at a level that doesn't pose a threat without employing some form of CO₂ removal."

The problem, he said, is that "all the things we can do that are cheap, like planting trees, are limited in their capacity."

CO₂ removal is also not a quick fix. The volume required to make a difference would be enormous—humans now emit more than 36 billion metric tons of CO₂ a year—and the benefits would take a long time to appear. At the moment, the NRC report concluded, no one approach to CO₂ removal can be relied upon to make a huge dent without enormous front-end costs.

"You really need to spread your bets over a variety of techniques," said committee member Scott Doney, a marine geochemist at Woods Hole Oceanographic Institution.

Another reason that CO₂ removal would almost certainly become necessary is that some parts of the energy system may use fossil fuels for a long time to come.

"It's really hard, for example, to make carbon-net-zero airplanes," said committee member Granger Morgan, a professor of engineering and public policy at Carnegie Mellon University. "But if you can scrub CO₂ out of the atmosphere for a reasonable price, that might be a strategy."

Blocking the Sun

The second NRC report released today details the far more fraught idea of increasing the planet's albedo—its reflectivity—so that more sunlight gets bounced back into space.

An example of this approach would be to use high-flying planes to inject sulfate particles into the stratosphere—essentially mimicking the effects of volcanoes such as Mount Pinatubo, whose massive eruption in 1991 cooled the planet by about one degree Fahrenheit.

Though that type of geoengineering would be far cheaper than CO₂ removal, the NRC report said, it would not address the underlying problem: the accumulation of CO₂ in the atmosphere. Nor would it stop the oceans from acidifying as they absorb CO₂.

What's more, any scheme to increase Earth's reflectivity would pose enormous unknown ecological and political risks, the report said. If it were done as an alternative to reducing CO₂ emissions, it would have to be done forever, since catastrophic global warming might ensue if it were halted.

"It's not ready for prime time," said Doney. "The committee strongly recommends not moving forward at this time."

Still, the committee gingerly recommended taking steps toward doing careful research on the topic, calling for a global discussion on setting research parameters.

In an emergency, such as a massive global famine, some way of cooling the planet quickly might be needed to provide a temporary reprieve. Rogue states might also decide to try doing that on their own, which would require that mainstream scientists understand the potential consequences well enough to recommend a response.

"I'm terrified of the idea," said committee member and climate scientist Ray Pierrehumbert of the University of Chicago. "But even if we all think it's a really, really bad idea, there are still good reasons to want to know more."

"Despair Is Not an Option"

Despite the complexity of the climate problem, several committee members said they remained optimistic.

Falkowski pointed out that technology can, and often has, changed overnight. The time between the drilling of the first oil well and an America with cars and airplanes was only about 60 years, he said, which suggests we're capable of remaking our energy system again in the next 60 years.

"The way I look at it is, Despair is not an option," Pierrehumbert said. "It's going to be really, really hard to avoid 2 degrees of warming. Barring some technological miracle, we'll probably blow right past it. But the really, really bad things start to happen between 2 and 4 degrees, and we still have a pretty good window of avoiding 4 degrees.

"By doing our very, very best," Pierrehumbert said, "and if we do manage to get CO₂ removal going, we might then be able to bring it back down under 2 degrees in a century or so."