Black Carbon (BC) has recently emerged as a major contributor to global climate change, possibly second only to CO₂ as the main driver of change. BC particles strongly absorb sunlight and give soot its black color. BC is produced both naturally and by human activities as a result of the incomplete combustion of fossil fuels, biofuels, and biomass. Primary sources include emissions from diesel engines, cook stoves, wood burning and forest fires. Reducing CO₂ emissions is essential to avert the worst impacts of future climate change, but CO₂ has such a long atmospheric lifetime that it will take several decades for CO₂ concentrations to begin to stabilize after emissions reductions begin. In contrast, BC remains in the atmosphere for only a few weeks, so cutting its emissions would immediately reduce the rate of warming, particularly in the rapidly changing Arctic. Moreover, reduced exposure to BC provides public health co-benefits, especially in developing countries. Technologies that can reduce global BC emissions are available today.