Wildfires and agricultural burning play a much bigger role in climate change and human health than previously thought, new calculations suggest.
Biomass burning—whether accidental wildfires or deliberate burning of forests to create agricultural lands—has long been known to affect both climate change and public health.
“We calculate that 5 to 10 percent of worldwide air pollution mortalities are due to biomass burning,” says civil and environmental engineering professor Mark Z. Jacobson of Stanford University. “That means that it causes the premature deaths of about 250,000 people each year.”
Exposure to biomass burning particles is strongly associated with cardiovascular disease, respiratory illness, lung cancer, asthma and low birth weights.
The research, published in the Journal of Geophysical Research: Atmospheres, is based on a three-dimensional computer model simulation of the impacts of biomass burning.
Carbon, of course, is associated with global warming. Most carbon emissions linked to human activity are in the form of carbon dioxide gas, CO2. But other forms of carbon include methane and the particles generated by fires—tiny bits of soot, called black carbon, and motes of associated substances, known as brown carbon.
Jacobson explained that total anthropogenic, or human-created, carbon dioxide emissions, excluding biomass burning, now stand at more than 39 billion tons annually. That incorporates everything associated with non-biomass-burning human activity, from coal-fired power plants to automobile emissions, from concrete factories to cattle feedlots.
Jacobson, the director of Stanford’s Atmosphere/Energy Program, says that almost 8.5 billion tons of atmospheric carbon dioxide—or about 18 percent of all anthropogenic carbon dioxide emissions—come from biomass burning.
But Jacobson’s research also demonstrates that it isn’t just the CO2 from biomass burning that’s a problem. Black carbon and brown carbon maximize the thermal impacts of such fires. They essentially allow biomass burning to cause much more global warming per unit weight than other human-associated carbon sources.
Sun And Clouds
Black and brown carbon particles increase atmospheric warming in three ways. First, they enter the minuscule water droplets that form clouds. At night, that’s not an issue. But during the day, sunlight scatters around within clouds, bathing them in luminescence.
When sunlight penetrates a water droplet containing black or brown carbon particles, Jacobson says, the carbon absorbs the light energy, creating heat and accelerating evaporation of the droplet. Carbon particles floating around in the spaces between the droplets also absorb scattered sunlight, converting it to heat.
“Heating the cloud reduces the relative humidity in the cloud,” Jacobson says.
This causes the cloud to dissipate. And because clouds reflect sunlight, cloud dissipation causes more sunlight to transfer to the ground and seas, ultimately resulting in warmer ground and air temperatures.
Ice And Snow
Finally, carbon particles released from burning biomass settle on snow and ice, contributing to further warming.
“Ice and snow are white, and reflect sunlight very effectively,” Jacobson says. “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.”
Jacobson notes that some carbon particles—specifically white and gray carbon, the variants associated with some types of ash—can exert a cooling effect because they reflect sunlight. That must be weighed against the warming qualities of the black and brown carbon particles and CO2 emissions generated by biomass combustion to derive a net effect.
Jacobson says the sum of warming due to all anthropogenic greenhouse gases—CO2, methane, nitrous oxide, chlorofluorocarbons, and some others—plus the warming due to black and brown carbon will yield a planetary warming effect of 2 degrees Celsius over the 20-year period simulated by the computer. But light-colored particles—white and gray particles primarily—reflect sunlight and enhance cloudiness, causing more light to reflect.
“The cooling effect of these light-colored particles amounts to slightly more than 1 degree Celsius,” says Jacobson, “so you end up with a total net warming gain of 0.9 degree Celsius or so. Of that net gain, we’ve calculated that biomass burning accounts for about 0.4 degree Celsius.”
Significant Heat Effect
Jacobson’s model also tracks the impact of the direct heat produced by combusting biomass.
“The direct heat generated by burning biomass is significant, and contributes to cloud evaporation by decreasing relative humidity,” Jacobson points out. “We’ve determined that 7 percent of the total net warming caused by biomass burning—that is, 7 percent of the 0.4 degree Celsius net warming gain—can be attributed to the direct heat caused by the fires.”
Biomass burning also includes the combustion of agricultural and lumber waste for energy production. Such power generation often is promoted as a “sustainable” alternative to burning fossil fuels. And that’s partly true as far as it goes. It is sustainable, in the sense that the fuel can be grown, processed, and converted to energy on a cyclic basis. But the thermal and pollution effects of its combustion–in any form–can’t be discounted.
“The bottom line is that biomass burning is neither clean nor climate-neutral,” Jacobson says. “If you’re serious about addressing global warming, you have to deal with biomass burning as well.”