Why South African wildfires can be really cool
Smoke in clouds counteracts the greenhouse effect, new research reveals
It might sound counter-intuitive but South African wildfires cool the climate.
That’s the contention of atmospheric physicists who say smoke drifting west over the Atlantic Ocean makes low-level clouds more reflective so that less solar radiation reaches Earth.
This leads to a net cooling effect, which is contrary to the previous understanding and has implications for global climate models, the scientists write in Proceedings of the National Academy of Sciences.
Until now a layer of pollution between clouds and the sun was thought to diminish the cooling effect of clouds by absorbing light that would otherwise be reflected. But the new research, using lasers aboard the International Space Station, found smoke and cloud layers are much close to each other than previously observed.“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,” said one of the researchers, Xiaohong Liu of the University of Wyoming.
One of his colleagues, Zhibo Zhang from the University of Maryland, said tiny smoke particles called aerosols affected how clouds were “seeded” because they acted as nuclei around which moisture condensed.
“With more seeds many small cloud droplets replace fewer large droplets, which then collectively reflect more light and increase the cooling effect,” said Zhang.
The scientists found that in smoky conditions there were almost twice as many “seeds” per cubic centimetre, but Zhang said this was not an argument in favour of fires.
“Aerosols are a very local phenomenon, and they are also shortlived,” he said. “The lifetime of carbon dioxide and other greenhouse gases is hundreds of years.”Liu said computer models suggested that carbon dioxide from human activities since the Industrial Revolution was responsible for a greenhouse effect of 1.66 watts per square metre of the Earth’s surface.
Smoke from fires in South Africa, however, produced a cooling effect of seven watts per square metre over the southeast Atlantic during the annual fire season between July and October. “Our group is the first to quantify this brightening effect,” said Liu.
A light detection and ranging (LiDAR) system aboard the ISS used laser pulses to calculate the gap between the aerosol layer and stratocumulus cloud decks about 1km above the Atlantic – the largest semi-permanent gathering of clouds in the world.
The launch of a new Nasa satellite in 2020 will help the scientists as they develop their work because it will be able to detect polarised light, which LiDAR cannot do, and enable the creation of three-dimensional models of the interactions between aerosols and clouds.
In the meantime the scientists plan to test existing climate models against their new finding. “Now that we know there are two competing mechanisms, and the seeding effect is winning, we can see whether climate models consider these processes properly when they predict the weather and climate in this area,” said Zhang.
The team’s goal is to refine global climate models by improving the scientific understanding of clouds. Researcher Zhifeng Yang is analysing data from a satellite that stays put in the sky, rather than orbiting Earth, to get a more accurate understanding of how cloud cover changes in daily cycles.