A giant mushroom cloud of steam and ash explodes out of the Mount Pinatubo volcano in the Philippines in 1991.
A giant mushroom cloud of steam and ash explodes out of the Mount Pinatubo volcano in the Philippines in 1991. - 
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KAI RYSSDAL: We've been doing some what-iffing around the issue of climate change this week. What if we can't get greenhouse gases under control? Plan B, the series we're wrapping up today, has explored some of the ways we're adapting, just in case our best ideas to stop global warming aren't good enough. Some of those ideas sound pretty far out, like generating a giant cloud out in space to block the rays of the sun, or launching what amounts to a really big umbrella.

Nate DiMeo reports the proposals are almost too risky to try, and too risky not to.

NATE DIMEO: Ken Caldeira wasn't having any of it. A few years ago the Stanford climate scientist was at a big conference, and he sat and listened while prominent colleagues said we could stop global warming by blocking the sun with a gigantic space shade.

KEN CALDEIRA: These ideas basically seemed kind of crazy to me at the time, and we were saying, well this will never work.

So he set out to prove that it wouldn't, but then something went wrong.

CALDEIRA: Each time we tried to show why this was such a bad idea, the results of our studies indicated that it would basically work.

Not perfectly, but the assumption held up. You could cool down the Earth by blocking some sunlight. Since the 60s, people way out on the fringes had been proposing ways to do it. They called it "geoengineering." Most of the ideas sound like they come straight out of science fiction, things like reflecting the sun with millions of mirrored satellites, but there's one idea that's beginning to gain traction among respected scientists, and it comes straight from Mother Earth.

Sixteen years ago Mt. Pinatubo erupted in the Phillipines, killing hundreds and blanketing much of the surrounding area in ash and debris, but some of those particles never touched ground. Instead, they swirled in the upper atmosphere for more than two years. Those particles diffused heat from the sun, and the Earth cooled down. Now, with projections about climate change worsening, people are starting to ask very seriously if a simulated volcanic eruption should be "Plan B" for reversing global warming.

SCOTT BARRETT: The economics of geoengineering, when I began to look into them, were simply astonishing.

Scott Barrett spent years trying to figure out ways to get the world to work together to reduce emissions and stop climate change, but the Johns Hopkins economist came to a depressing conclusion, human society, with its 200-odd sovereign states, its imperfect capitalism and its convoluted trade regimes, wasn't going to get its act together to stop climate change. Among the many, many stumbling blocks -- the potential price tag for all the steps you would need to take to reduce global emissions runs in the tens of trillions of dollars. But a simulated volcano?

BARRETT: Costs range, I've seen from any where from $1 billion a year to up to maybe $50 billion a year. Now that's a lot of money, but even $50 billion a year, that's only a month or so in Iraq, so it's actually doable, even for a single country.

But it could be dangerously doable.

CALDEIRA: Throwing particles into the stratosphere is one of the few things a politician could do quickly.

Ken Caldeira says we should think of a simulated volcano as an emergency response. You know, in case of global warming, break glass, but he says he's still got some serious reservations.

CALDEIRA: While our models suggest that this could help, it also has the potential to really screw things up profoundly.

RAYMOND PIERREHUMBERT: It's more like trying to fit a new, untested kind of engine on a 747 while it's completely full, and you're flying across the Atlantic, and you have to get it right the first time.

Raymond Pierrehumbert studies planetary climates at the University of Chicago. Bottom line, bad things happen when you mess with the Earth's atmosphere. For one thing, the benefits of a cooled down world wouldn't be evenly distributed.

PIERREHUMBERT: So, if there's a conflict between people in Texas who would like to geoengineer their climate so people aren't dying in the summer, and people in India who would like to hold on to their monsoon rains, who gets to decide or adjudicate that?"

BARRETT: Geoengineering is almost too easy to do.

Economist Scott Barrett says there's real danger in giving countries, and even corporations the ability to put their fingers on the global thermostat, but for all the potential problems with geoengineering, Barrett says we do need to explore the possibilities. He's calling for an international agreement on experimentation. A few months ago Ken Caldeira wrote an Op-Ed in the New York Times. He called for 1 percent of climate change research funding to go to geoengineering, but he says he has to walk a fine line.

CALDEIRA: I do feel somewhat conflicted about it, in that I know it will be used by some people as a reason to not reduce emissions.

But he says we can't keep our heads in the sand and pretend geoengineering doesn't exist. We already tried that with climate change.

I'm Nate DiMeo for Marketplace

KAI RYSSDAL: Plan B was a co-production with the Center for Investigative Reporting.

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