Despite significant pyrotechnics and air travel disruption last year, the Icelandic volcano Eyjafjallajokull simply didn't put that many aerosols into the stratosphere. In contrast, the eruption of Mount Pinatubo in 1991, put 10 cubic kilometers of ash, gas and other materials into the sky, and cooled the planet for a year. Now, research suggests that for the past decade, such stratospheric aerosols—injected into the atmosphere by either recent volcanic eruptions or human activities such as coal burning—are slowing down global warming. "Aerosols acted to keep warming from being as big as it would have been," says atmospheric scientist John Daniel...