Can we engineer a cooler earth?

Launch myriad mirrors into space to deflect a fraction of sunlight from reaching Earth. Seed the stratosphere with sulfur or other particles to cut some of the sun’s rays. Bioengineer trees to soak up huge amounts of carbon dioxide from the air. Scatter unmanned self-powered ships to roam the world’s oceans funneling sea spray high in the sky to help form protective clouds.

Thinkers have posed a number of creative ideas on how to protect the planet from global warming. But they’ve been dismissed by most environmentalists and many in the scientific community as science-fiction whimsy, at best. At worst, critics say, these schemes might have unexpected and potentially disastrous consequences or distract from the effort to cut greenhouse-gas emissions.

But today, attitudes show signs of shifting as meaningful efforts by governments to cut emissions have proved elusive. More and more scientists and environmentalists, despite their continuing reservations, are seeing “geoengineering” projects as a necessary backup plan. In June, the top scientific academies in 13 countries, including the United States, joined in a call for more aggressive action against global warming, including serious consideration of geoengineering.

At the same time, the Group of Eight leading economic powers meeting in Japan failed to set any near-term goals to reduce emissions. The group’s soft, conditional goals for 2050 will be too little, too late, many environmentalists say.

“The reality is that de-carbonization is not happening fast enough,” says Jamais Cascio, an environmentalist and futurist in northern California.

The need for geoengineering is “almost certain,” he says.

The attitude among tech-friendly environmentalists, sometimes called “Bright Greens,” has been shifting in favor of geoengineering, Mr. Cascio says. “This is by no means anyone’s first choice, but it is better than the alternative,” he says, which is unmitigated warming of the planet.

“I think that you’ll see quite a few relatively desperate nation-states willing to try something like [geoengineering] simply to avoid global disaster,” Cascio says. Since such efforts are very likely, in his view, the role of environmentalists will be to “make sure we do it in the way that is most responsible,” he says.

Opponents remain unpersuaded and point to a litany of potential problems with geoengineering schemes. Chief among them is that efforts to engineer humanity’s way out of the climate challenge are likely to distract from the hard work of mitigation: cutting greenhouse-gas emissions.

“To me, that [argument] doesn’t make sense,” says Samuel Thernstrom, a resident fellow studying public policy and geoengineering at the American Enterprise Institute (AEI) in Washington. No political leaders have said they would drop emission cuts in favor of geo­engineering, nor do opinion polls indicate the public supports that idea, he says. In fact, Mr. Thernstrom argues, geo­­engineering is more likely to have the opposite effect. If a US president says we’ve got to start thinking about blocking the sun to cool the earth, “People are going to start taking mitigation [emission cuts] really seriously,” he says.

Geoengineering faces legal hurdles. Would nations or private enterprises undertake the projects? Would an international agreement need to be reached? Might countries work at cross purposes?

“What if India wanted it a couple of degrees colder, and Russia didn’t mind it a couple of degrees warmer?” asks Alan Robock, an environmental sciences professor at Rutgers University in New Jersey. Last spring, Dr. Robock published a paper entitled “20 reasons why geoengineering may be a bad idea.”

Such projects could also have military applications and as such could violate an existing global treaty that bans altering the climate for hostile purposes, he says. If the effects are salutary in one part of the world, but harmful in another, who decides what will be done? Any scheme also could bring with it unintended consequences and hard-to-quantify costs. Seeding the atmosphere with sulfur particles, for example, is likely to turn the sky whiter. “How do you quantify no more blue skies” as a cost, Robock asks. (One compensation: The number of fiery red and yellow sunsets would increase.)

A recent study using computer models showed that putting sun-deflecting mirrors in space would cool the Earth, but wouldn’t return it to the way it was before human-generated global warming began.

“Some places get warmer, some places cool down ... some places get wetter, some places get drier,” says lead author Dan Lunt, a climate modeler at Britain’s University of Bristol. He calls the new climate that would emerge “Sunshade World,” an Earth in which CO2 levels remain high but temperatures are moderated. The closest equivalent to that condition last occurred during the Cambrian period about 500 million years ago, the paper says.

The most talked about proposal would send sulfur or other fine particles high into earth’s atmosphere using airplanes, balloons, or perhaps even artillery shells to block out a tiny fraction of the sunlight. “The aerosol idea frightens people a lot,” Thernstrom says. Sulfur is a pollutant, and studies show it would slightly increase acid rain over the poles. The polar ozone holes would close more slowly, with some ill effects. “But compared to the effects of uncontrolled warming, that’s not that big a concern,” he says.

Blocking sunlight, adds futurist Cas­cio, “is at best a delay of the worst ­temperature-related consequences of global warming in order to give us more time for de-carbonization.”

Any long-term ap­­proach to solving global warming, Thern­strom says, almost certainly will have three aspects: emissions reductions, geoengineering, and steps to adapt to an altered climate. “The question is, ‘What is the ratio among those three pieces?’ ”

Schemes to slightly dim sunlight also wouldn’t solve the problem of ocean acidification, caused by airborne CO2 entering seawater. More-acidic oceans would harm coral reefs and upset ocean ecology, with possible far-reaching effects. Ocean acidification is “at least as big” a problem as that of CO2 in the air, Cascio says.

Despite the new buzz around geoengineering, including a recent seminar at AEI, some opponents are adamant. Ray­­mond Pierrehumbert, a professor of geophysics at the University of Chicago, has proposed a 10-year moratorium on research into geoengineering, to ensure humanity isn’t tempted to try this option.

But a new consensus seems to be forming around the idea of stepping up research, even as differences remain over when, if ever, to deploy such schemes. Ro­­bock, who maintains strong reservations, also favors research. “We have to know if it’s reasonable or not, how long it might work, what the problems would be, how much it might cost,” he says.

The US government now spends between $2 billion and $3 billion on global warming research, and will probably spend more under the next president. If just $100 million of that over five years were spent on geoengineering research, “We would learn an awful lot,” Thernstrom says.

“The potential payoff is very large. If mitigation doesn’t work, and we have every reason to believe it’s not likely to [work] in the short term,... you kind of want to have a Plan B.”