The atmospheric concentration of carbon dioxide increased steadily between 1998 and 2008, but Earth’s temperature declined. Why? Chair of Geography and Environment Robert Kaufmann, along with three colleagues from other institutions, recently published a paper in Proceedings of the National Academy of Sciences that answers this question.
Contrary to claims by climate skeptics, the reasons are consistent with the theory of climate change, and involve both natural and human causes. Human activities both warm and cool the planet. People add carbon dioxide to the atmosphere, which warms the planet. But opposing this effect, burning coal emits aerosols, which cool the planet by reflecting light back to space.
During the early 2000s, there was a huge increase in sulfur emissions. China doubled coal consumption in only four years. Because of this large increase, the cooling effect of sulfur emissions largely canceled the warming effect of increased concentrations of carbon dioxide. During this period, natural forces cooled the planet by lowering the solar energy input (due to the sunspot cycle) and by changing weather patterns, from El Nino to La Nina. This explanation may portend a period of rapid warming as the Chinese install scrubbers that remove aerosols from coal-fired boilers and solar insulation increases due to the increasing phase of the sun-spot cycle.