In the study, Shindell and colleagues added chemical interactions between aerosols and greenhouse gases such as methane and carbon monoxide to a century-long model of climate change. They wanted to see the effects on each gas's "Global Warming Potential," or individual contribution to global warming. Methane played a bigger role than expected, suggesting that climate treaties such as the 1997 Kyoto Protocol need to consider it more carefully, the study says. - Meryn Stol