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.

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.