Scientists already know how to cool the planet quickly. The secret is geoengineering: specifically, using very fine particles in the stratosphere to reflect sunlight (see "The Geoengineering Gambit"). The direct cost of shading the planet this way could be less than a few hundredths the cost of reducing carbon dioxide emissions. If reflecting sunlight is fast and cheap, why struggle with all the problems of collective action to achieve emission reductions? Why not wait until we have a climate problem and then simply fix it?

Over the past half-century, people have "fixed" a number of other problems with environmental implications. We have reversed rivers in Russia, inadvertently destroying the Aral Sea in the process; we have built roads and encouraged farming in tropical areas, inadvertently depleting the soil and destroying millions of acres of rain forest. If, with typical shortsightedness and hubris, we count on geoengineering to save the planet, can we be sure that the outcome will be what we intend?

Despite the mistakes of the past, the answer is not to treat geoengineering like chemical and biological weapons research, surrounding it with a global taboo. If a country experiencing a prolonged drought, for example, seeks to engineer the planet's climate unilaterally, we will need to be familiar with the potential consequences in order to muster informed counterarguments. And if our more extreme climate-change predictions become reality and a sudden climate emergency puts billions of people at risk, the world should not find itself collectively embarking on a crash program of geoengineering in ignorance.

We need to know much more about geoengineering. Until recently, most scientists and research managers have been reluctant to do research in this area, for fear that knowing how to engineer the climate would encourage people to do it. But today, the risks of avoiding research outweigh the risks of pursuing it.

We need to take two steps now. First, we should establish a loosely coördinated international program aimed at researching how to shade the planet, how much it would cost, and what the intended and unintended effects would be. This research should also address what the rising atmospheric concentration of carbon dioxide means for terrestrial and oceanic ecosystems, since reflecting sunlight will do nothing to stop it. About one third of emitted carbon dioxide is absorbed by the oceans, which become more acidic as a result; they are already 30 percent more acidic today than they were in preindustrial times. If current emissions continue, most coral reefs could be gone by the end of the century, along with all the ecosystems they support.

Second, we need to get the foreign-­policy community working on a collective approach to regulating geoengineering. My colleagues and I have started that process with two international workshops involving climate scientists and foreign-policy experts. Further informal discourse will lay the groundwork for a formal framework.

M. Granger Morgan is head of the Department of Engineering and Public Policy at Carnegie Mellon University.