When soot embeds itself in snow and ice, it absorbs sunlight, making Earth’s nether regions less brilliantly reflective. Less sunlight gets reflected out into space and more energy stays behind, warming the oceans and land. Then the snow begins to melt, exposing even more carbon and intensifying the effect. That would help account for the greater warming at the poles, where average temperatures have risen 2.5 degrees Celsius in the past 50 years, compared to just over a quarter degree overall. The retreat of the glaciers–usually blamed entirely on greenhouse gases–may, in fact, have been promoted by black carbon.
The soot hypothesis doesn’t mean that global warming will go away as soon as we start cleaning the air. Meteorologists regularly report record-setting average temperatures (2003 ranks as the third warmest year since at least 1861), and greenhouse gases are probably the main cause, they say. What’s more, because soot promotes the melting of snow and ice, it may exacerbate the problem of rising seas.
The good news is that reducing the amount of soot in the air by installing exhaust filters on diesel trucks and buses and better managing forest fires is probably easier than getting Americans to give up SUVs. Fires are the source of about half the world’s soot, Hansen estimates.
Meanwhile, another NASA study published last week in the journal Science found that fires around the world account for much of the variation in the atmospheric concentration of greenhouse gases. Understanding this variation will help climatologists predict future changes in temperatures. Perhaps instead of wondering if the world will end in fire or ice, we should worry about how these elements get along.
