FireA new study from Stanford University explores the link between biomass burning and climate change. The findings reveal greater impacts on atmospheric temperatures and human health than previously thought.

Biomass burning refers to both natural wildfires and the purposeful burning and clearing of forest areas for agricultural cultivation. It has long been understood that these fires emit carbon into the atmosphere, though their effect on human health has not been so easily quantified. That was one of the goals of Stanford Civil and Environmental Engineering Professor Mark Z. Jacobson, whose paper on the subject was recently published in the Journal of Geophysical Research: Atmospheres.

“We calculate that 5 to 10 percent of worldwide air pollution mortalities are due to biomass burning,” says Jacobson. “That means that it causes the premature deaths of about 250,000 people each year.”

Most carbon emissions are anthropogenic, or man-made, says Jacobson. Much of this comes in the form of carbon dioxide. But there are other types, which are referred to by color. The burning of forests releases soot particles, known as black carbon, and other types of associated substances, known as brown carbon. There are also gray and white types of carbon created by ash.

Coal-fired power plants, automobiles, concrete factories and cattle farms all emit carbon. Combined with burning biomass, total anthropogenic carbon emissions now equal 39 billion tons per year. Approximately 18 percent of that (8.5 billion tons) comes from burning forests.

As Jacobson explains, black and brown carbon generated by biomass burning actually create more global warming per unit weight than any other carbon sources. They do this in two distinct ways. First, atmospheric carbon seeps into the water droplets that form clouds and accelerate the absorption of solar energy. The carbon particles that float between droplets also absorb scattered sunlight. Together, these particles reduce the humidity in clouds, causing greater cloud dissipation and increasing the amount of sunlight on land and water.

The second method of warming occurs when carbon lands on snow. “Ice and snow are white, and reflect sunlight very effectively,” says Jacobson. “But because carbon is dark it absorbs sunlight, causing snow and ice to melt at accelerated rates. That exposes dark soil and dark seas. And again, because those surfaces are dark, they absorb even more thermal energy from the sunlight, establishing an ongoing amplification process.”

Gray and white carbon have the opposite effect and actually reflect sunlight, but Jacobson accounts for that in his final calculations. Over the next 20 years, the effect of carbon emissions will be a 2 degree Celsius rise in temperature. The cooling effect of white and gray ash will reduce this by a little over 1 degree celsius, resulting in a net warming gain of 0.9 degrees Celsius. According to Jacobson, biomass burning will account for 0.4 degrees of that increase.

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