Enlarge Image Seeding the skies. Geoengineering approaches include spreading aerosols in the stratosphere, where they could block a small fraction of the sun's rays. Credit: Wikimedia Commons / Blink SAN DIEGO—The prospect of cooling the planet manually—geoengineering—was the subject of several presentations here today, including two presentations that involved new findings on the controversial idea. The scene was the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW). It turns out that there’s a lot scientists don’t know about various cooling methods, but that’s changing steadily. And researchers are learning lessons that suggest that some...