A new study indicates soot, known as black carbon, plays a far greater role in global warming than previously believed and is second only to CO2 in the amount of heat it traps in the atmosphere. Reducing some forms of soot emissions — such as from diesel fuel and coal burning — could prove effective in slowing down the planet’s warming. It rises from the chimneys of mansions and from simple hut stoves. It rises from forest fires and the tail pipes of diesel-fueled trucks rolling down the highway, and from brick kilns and ocean liners and gas flares. Every day, from every occupied continent, a curtain of soot rises into the sky.