As you say, if people can warm the earth, they may be able to cool it, too (“Every Silver Lining Has a Cloud,” January 31, 2009). Under the banner of geo-engineering, a budding field, proposals for planet-wide projects already abound: dumping iron in the oceans, injecting sulphates into the stratosphere, spraying seawater into the air, constructing giant space-borne parasols to shade the earth from the sun. While you mention all sorts of problems with these last-ditch proposals, such as their effectiveness, side-effects, and costs, you fail to consider one vital factor: the amount of time needed for them to deliver. It has taken people a couple of centuries to warm the earth, and it is likely that it will take them many decades to cool it even under the best of circumstances. The remaining question is how much time we have before it is too late for all remedies, including those of the most intrepid geo-engineering.