New Scientist: Previous generations unwittingly found a way to cool the Earth, but it’s an approach we won’t want to reprise. Research suggests that particles of lead from gasoline exhaust may have offset warming in the 20th century. 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 …