On the Contrary, South African Wildfires Have a Climate Cooling Effect
While there’s growing interest in investigating the fingerprints of climate change on extreme weather events, it’s often challenging for scientists to parse out its influence versus other natural and human factors—and, as experts have warned time and again, no single weather event can be attributed solely to the effects of climate change. This may be particularly true for wildfires, which are heavily influenced by human land-use and management practices in addition to the weather.
However, scientists at the University of Wyoming have now discovered how biomass smoke originating from South Africa that drifts over the southeast Atlantic Ocean significantly enhances the brightness of low-level clouds there- creating a reflective process that actually helps cool the Earth and counteract the greenhouse effect.
“If you change the particles, you are changing the composition of the cloud,” says Xiaohong Liu, a UW professor in the Department of Atmospheric Science and the Wyoming Excellence Chair in Climate Science. “For our study, we found the smoke comes down and can mix within the clouds. The changed clouds are more reflective of sunlight. Brighter clouds counteract the greenhouse effect. It creates cooling.”
The cloud gathering
plays a significant role in moderating global climate temperature, by reflecting solar radiation back out into space. Its ability to do this is in direct proportion to its whiteness, expressed as a measurement known as albedo. Previous studies have shown that aerosols can disturb radiation in the top atmosphere by scattering and absorbing solar radiation as well as altering the cloud properties by changing the lower tropospheric stability.This new study does not dispute that phenomenon. However, more dominantly, the new study found that smoke and cloud layers are closer to each other than previously thought. This makes the clouds more reflective of light and, thus, accelerates the clouds’ cooling effect. This is due to the tiny aerosol particles from the smoke that serves as the nuclei for the formation of cloud droplets.
This cooling effect is larger per square meter than the warming created by greenhouse gases, the researchers said. According to Liu, carbon dioxide emitted by human activities since the Industrial Revolution creates a greenhouse warming effect of 1.66 watts per square meter that is uniform across the globe.
However, the cooling effect of the smoke, in the southeast Atlantic at least, is much larger than this, coming to 7 watts per square meter during the annual fire season—which runs from July through October. Some of these fires are started intentionally to clear land, although many are wildfires.
They release huge quantities of smoke, containing aerosols, which travels westward over the southeast Atlantic, interacting with clouds located approximately 0.6 miles (1 km) above sea level. As the aerosols mix into the clouds, the amount of water that condenses increases, boosting their brightness. This results in substantial cooling of the Earth in this region, according to the study.
“Marine stratocumulus clouds cover nearly one-quarter of the ocean surface and thus play an extremely important role in determining the global radiative balance,” the researchers wrote in their study published in the Proceedings of the National Academy of Sciences on March 5. “Our results highlight the importance of realistically representing the interactions of stratocumulus with biomass burning aerosols in global climate models in this region.”
