A new research has suggested that particles of lead from gasoline exhaust may have offset warming in the 20th century, causing global cooling.
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.
According to a report in New Scientist, 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 meter 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," said Cziczo.
This may have helped offset greenhouse warming from about 1940 to 1980, when global temperatures rose little before rising steeply.