Did lead cause global cooling?
By Jeff Hecht Previous generations unwittingly found a way to cool the Earth, but it’s an approach we won’t want to reprise. Research suggests that particles of lead from gasoline exhaust may have offset warming in the 20th century. It’s well known that particles in the atmosphere such as mineral dust, pollen, heavy metals and even bacteria can act as seeds for the nucleation of ice crystals. These crystals form clouds that can affect the Earth’s energy balance by reflecting the sun’s rays back into space, for example. Dan Cziczo and colleagues of the Pacific Northwest National Laboratory in Richland, Washington, created artificial clouds in the laboratory to explore the ice nucleation efficiency of various particles. Over a third of the ice nuclei generated contained lead, suggesting it is a highly-efficient nucleator. They found similar proportions of lead in atmospheric mineral dust samples collected in Switzerland. Cziczo argues that lead “supercharges” ice-nucleating dust particles in the atmosphere. According to his calculations, global infrared emission would be 0.8 watts per square metre higher if all atmospheric ice crystals contained lead compared with none. Before leaded fuel was phased out from road vehicles last century, the atmosphere contained substantially more leaded particulates than today, says Cziczo. This may have helped offset greenhouse warming from about 1940 to 1980, when global temperatures rose little before rising steeply. Journal reference: Nature Geoscience (DOI:10.1038/NGEO499) (in press) More on these topics: