At a recent meeting in Japan of the U.N. Convention on Biological Diversity, diplomats tried to set some rules for future geoengineers. They issued what some are calling a moratorium on all geoengineering activities until the science is clear and there are global regulations in place.

If you want to see what geoengineering might look like, go back to 1991, to the eruption of Mt. Pinatubo, in the Philippines.

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The volcano spewed almost 20 million tons of sulfur dioxide into the stratosphere. Those particles can reflect sunlight back into space, and for a while, that's exactly what happened. Temperatures around the world dropped by an average of half a degree.

It turns out you don't need a volcano to get the same effect. Scientists could use airplanes to inject sulfur dioxide directly into the stratosphere and bring down global temperatures. What's more, says David Keith who directs the University of Calgary's Energy and Environmental Systems Group, it would be pretty easy to do.

"It takes so little material to alter the whole planet's climate," he says. "The costs of doing it are just absurdly cheap."

The Mt. Pinatubo effect, as it's called, is just one way scientists believe they might use technology to counteract climate change, or at least its effects. Another way would be growing algae in the ocean to suck up carbon dioxide.

But in order to have a real impact on climate change, the project would have to take place on a massive scale. And that could have all sorts of unintended consequences. For example, rainfall patterns could change. That could mean drought and starvation for thousands of people in East Africa, according to Diana Bronson of the ETC Group, which has been skeptical of geoengineering.

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