Inadvertent climate modification due to anthropogenic lead

Aerosol particles can interact with water vapour in the atmosphere, facilitating the condensation of water and the formation of clouds. At temperatures below 273 K, a fraction of atmospheric particles act as sites for ice-crystal formation. Atmospheric ice crystals—which are incorporated into clouds that cover more than a third of the globe1 —are thought to initiate most of the terrestrial precipitation2. Before the switch to unleaded fuel last century, the atmosphere contained substantial quantities of particulate lead; whether this influenced ice-crystal formation is not clear. Here, we combine field observations of ice-crystal residues with laboratory measurements of artificial clouds, to show that anthropogenic lead-containing particles are among the most efficient ice-forming substances commonly found in the atmosphere3. Using a global climate model, we estimate tha