Climate-cooling aerosols reach the upper atmosphere as a result of human activity, such as coal burning, as well as from volcanic eruptions that eject large quantities of sulphur dioxide. Discerning the source of these particles can be difficult, but a recent break in volcanic activity makes clear just how rapidly atmospheric concentrations of anthropogenic aerosols are increasing.
A team led by David Hofmann of the US National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado, tracked upper-atmosphere aerosol levels since 1991 — when the last major volcanic eruption took place — using lidar measurements made at the Mauna Loa Observatory in Hawaii and at the Boulder research station. They found strong seasonal trends in aerosol concentrations, with a wintertime peak, which they attribute to alternating wind regimes in the tropics. Overlaying this seasonal cycle was a four- to seven-per-cent increase in aerosols between 2000 and 2009.
The most likely source of the rise is a steep increase in coal burning, mainly in China, where sulphur emissions have climbed 5.2 per cent per year since 2002. As China's coal use is expected to grow 3.2 per cent per year in the next two decades, atmospheric aerosol levels are unlikely to decline any time soon.