Figure 3 The amount of sulfur introduced into the atmosphere by human activities is compared with emissions from natural sources. The burning of fossil fuels and metal smelting, the major contributors, have increased substantially in the past century. Airborne sulfur rapidly combines with oxygen to form sulfur dioxide which, as with volcanic emissions (Fig. 2), forms sulfuric acid droplets that cool the surface temperature, compensating some of the long-term warming that is expected from the concurrent accumulation of greenhouse gases. A teragram (10¹² grams) is roughly a million tons. Adapted from R. Charlson et al., Science, vol 255, pp 423-430, 1992.