Was global cooling caused by leaded fuel?

According to a news item in New Scientist (Volume 202 Issue 2707, page 14, April 2009), previous generations unwittingly found a way to cool the Earth, but it’s a polluting approach we are unlikely to want to repeat.

Pure water vapour freezes around dust, pollen and even bacteria at higher temperatures than it can as ice alone. This “nucleation” effect allows warmer clouds to form, causing them to radiate more of Earth’s heat into space than cold clouds formed in cleaner air.

Dan Cziczo of the Pacific Northwest National laboratory in Richland, Washington, and colleagues, created artificial clouds from aerosol particles collected in Switzerland. They found that while only 8 per cent of these particles contained lead, it was present in 44 per cent of the particles that seeded ice.

Cziczo suggests that lead “supercharges” ice- nucleating dust particles. The team calculates that the Earth’s global infrared emission would be 0.8 watts per square metre higher if all atmospheric ice crystals contained lead than if none did (Nature Geoscience, DOl: 1O.1038/ngeo499). By comparison, human-made CO2 has reduced infrared radiation by around 1.5 watts per square metre since pre-industrial times.

Before leaded fuel was phased out from road vehicles, the atmosphere contained many more leaded particles than today. This may have helped offset greenhouse warming between 1940 and 1980, says Cziczo.

Another example of how complex the equations for climate change are, and how many unknowns are involved in the mechanisms.