Soot May Help Shift Tropics North

Soot may be responsible for the tropics expanding north, according to an analysis involving multiple computer models of the climate. By absorbing sunlight and trapping extra heat in the atmosphere, the tiny, black particles may be helping the poleward march of tropical conditions.

The research will be published in Nature on May 17. (Scientific American is part of Nature Publishing Group.)

The tropics—the belt of land around the equator characterized by abundant rainfall and torrid temperatures —have been expanding for at least 40 years. In fact, the tropics have widened by roughly 0.7 degrees of latitude per decade in recent years, or more than two degrees of latitude since 1979.

The tropics are hot and wet because the sun warms the air more near the equator thanks to a more direct shine. This warmed air rises and then cools, condensing its water vapor, which falls as rain. This zone of heat and wetness has been spreading south thanks to ozone pollution but the cause of its northward spread had been mysterious. But the computer simulations suggest that it is soot (with an assist from the greenhouse gas ozone again) that is helping make the northern subtropics nearly as hot as the tropics themselves.

Only by adding the warming from soot to these computer models did the results begin to mimic what is actually happening, including increased dryness in the subtropics. At present, air raised by the sun’s heat in the tropics often sinks back down as hot dry air at roughly 30 degrees latitude, creating a global belt of deserts. That belt is also beginning to move, along with the jet stream and its attendant storms.

The extra soot and ozone driving this shift in the north is coming from the increase in fossil fuel burning in the northern hemisphere. Eliminating that pollution would not only slow this tropical shift, it would also curb climate change and improve human health—and reducing fossil fuel burning can be accomplished by such simple steps as using cleaner cookstoves or improved internal combustion engines. After all, the soot that doesn’t make it to the atmosphere, often finds a final resting place in human lungs.

Written by David Biello@Scientific American