Global warming inadvertently curbed in past by lead pollution, scientists find

Science Daily: Lead pollution in the air stimulates the formation of ice particles in clouds. A team of scientists from the USA, Germany and Switzerland has found that particles containing lead are excellent seeds for the formation of ice crystals in clouds. This not only has a bearing on the formation of rain and other forms of precipitation but may also have an influence on the global climate. This is because the heat given off from the earth's surface is more efficiently radiated into space by ice clouds (cirrus) with lead-containing particles than has been hitherto realized.

In comparison to clouds with a low lead content, clouds with a high lead content thus actually help cool the earth. Over the last twenty years, there has been a continuing decrease in the rate of anthropogenic lead emissions. This may mean that the greenhouse effect is now even more pronounced because lead-containing clouds once previously helped limit it.

At the Sphinx Observatory, a Swiss research station on the Jungfraujoch at an altitude of 3,580 meters, scientists from various institutions, including the Universities of Frankfurt and Mainz, and the Max Planck Institute for Chemistry in Mainz, investigated the chemical composition of clouds in the winters of 2006 and 2007. "What mainly interested us was the question of how ice particles form. Water particles in the atmosphere do not simply freeze at zero degrees.

On the contrary, at temperatures as low as minus 37 degrees they still need an ice nucleus, for example an aerosol particle, before ice formation is triggered," explains Professor Joachim Curtius of the Institute for Atmosphere and Environment (IAU) at the Goethe University Frankfurt. The same principle is also employed for snow guns, and in this case proteins derived from Pseudomonas bacteria are sometimes used as the ice-forming nuclei – a controversial application….

Crepuscular rays through a cloud, shot by Saperaud (I think), Wikimedia Commons, under the terms of the GNU Free Documentation License, Version 1.2