If global warming really takes off, some researchers think we should consider pumping sulfate aerosols high up in the atmosphere to shade the earth and cool it back down. The approach, one of a group of strategies known as geoengineering, sounds mad. But it has the attraction of being cheap. According to one estimate, it would cost less than a billion dollars a year, pocket change in a world economy of $70 trillion (see “A Cheap and Easy Plan to Stop Global Warming” and “Climate Change: The Moral Choices”).

No one really knows, however, what putting large amounts of sulfate up in the stratosphere will do, especially in the long term. We have a general idea of the short term reaction, because we’d basically be doing what big volcanic eruptions naturally do. But the sulfur from those eruptions falls out of the sky after a couple of years. Geoengineering would have to go on for hundreds or thousands of years while we wait for carbon dioxide levels to come back down.

One outstanding question is how much geoengineering would impact rainfall. According to some models, the news is good. Global warming would increase rainfall, on average. Geoengineering would reduce it, and things would even out—it would just bring precipitation levels back to normal.

But a study in the Journal of Geophysical Research suggests that geoengineering could significantly reduce precipitation, possibly enough to have an impact on crops and drinking water.

Here’s one reason why. Higher carbon dioxide levels cause plants to partially close pores in their leaves, which decreases the amount of water vapor they release. Without geoengineering, high levels of carbon dioxide would also make the earth warmer and increase evaporation, which would offset the decrease in water vapor from plants. But shading the earth slows down evaporation, so there’s a net decrease in rainfall.

The study is far from the last word on geoengineering. For one thing, it looked at extreme levels of CO2 that could still be avoided. At lower levels the reduction in rainfall is smaller and might not even be noticed. There are also many other factors other than average rainfall that matter, including effects of sulfate aerosols on the ozone layer. But given the study’s findings suggest that at the very least we need to learn more about about the potential side effects of geoengineering.