But Jacobson's research also demonstrates that it isn't just the CO2 from biomass burning that's the 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.
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 said, 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 said.
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.
Finally, Jacobson said, 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 said. "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."<
|Contact: Tom Abate|
Stanford School of Engineering