Every cloud has a silver lining, or should that be a lead lining?

We could have inadvertently stalled the greenhouse effect by historically including lead in petrol, a European and US team of scientists have concluded.

Writing in the current edition of Nature Geosciences, Ulrike Lohmann and her colleagues show that lead is one of the most potent water droplet-forming agents in the atmosphere.

Clouds form when rising warm wet air expands sufficiently to cool, allowing water molecules to coalesce.  At very low temperatures they can do this spontaneously but usually this occurs at much higher temperatures because the water molecules begin to cluster around particles of dust, pollen and even dandruff and bacteria.  These particles are collectively known as nucleation sites, and lead is the king.

The team made the discovery by sampling clouds collected in Switzerland and also by recreating clouds in the laboratory using ambient air samples.  When they fed their cloud samples into an analyser they found that the majority of the water droplets were associated with lead particles.  Presumably the chemistry of lead makes it an ideal rain-maker, but the implication of the research is that by choosing to lubricate the valves of engines by adding lead to petrol we may have made a significant different to global weather and global warming.

The lead particles, say the scientists, could have triggered increased cloud formation by boosting the effectiveness of nucleating particles in the air.  And because clouds reflect heat back out into space, this could have kept the Earth artifically cool despite rising CO₂ levels. The effect, say the researchers, could have been sufficient to send back out to space nearly 1W per square metre that would otherwise have hit the ground and warmed the Earth.  But now that we've eschewed lead in favour of a healthier atmosphere we may be about to reap the costs of the CO₂ seeds we've sown in the past...