African fires cool the planet
In a surprise result, smoke from bushfires works to reduce solar radiation reaching the ground. Andrew Masterson reports.
In a counter-intuitive finding, data from the International Space Station (ISS) has revealed that annual massive fires across Africa actually cool, rather than warm, the climate.
This surprising outcome arises from modelling conducted by a team led by atmospheric scientist Xiaohong Liu from the University of Wyoming, in the US, using information gathered by a LiDAR system on board the ISS, and published in the journal Proceedings of the National Academy of Sciences.
The study doesn’t exactly contradict earlier research, which found that the African fires would have a warming effect. Instead, it recalibrates the relative locations of the principle phenomena involved – smoke clouds and water vapour clouds – revealing the existence of a powerful indirect effect that overwhelms the previously calculated outcomes.
Every year, huge fires break out in southern and central Africa. The result is an enormous upwelling of smoke – more specifically, of tiny airborne particles called aerosols – that is then pushed westwards out over the Atlantic Ocean, floating above what constitutes the planet’s largest semi-permanent population of stratocumulus clouds.
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 research has suggested that upwelling smoke from the African fires has a negative effect on the clouds’ albedo, partly because the aerosols decrease the total cloud mass and partly because the smoke, drifting above the clouds, reduces their ability to reflect.
It has also been noted that the fires themselves blacken large areas of land – an effect that decreases planetary albedo, especially in the northern region of the continent, because darker colours absorb rather than reflect sunlight.
Liu and his team now present a significant reappraisal of the relationship between the fire-created aerosol mass and the stratocumulus clouds. Data obtained from the ISS and satellites revealed that the smoke drifts lie only just above the cloud layer, allowing them to interact.
The individual water droplets that make up clouds must each contain a “seed” – a tiny speck of solid matter around which H2O molecules can accrete and adhere. The density of the droplets within a cloud is a critical factor determining its albedo.
The researchers found that because of the close proximity of aerosols created by the fires, the number of water vapour droplets able to form in the Atlantic clouds was significantly increased – bringing a consequent increase in albedo, with more, rather than less, of the sun’s rays bounced away from the ground.
“The seeding effect is winning,” says co-author Zhibo Zhang.
The scientists conclude that albedo effect induced by the increased density of the clouds caused by the particulates launched into the atmosphere by the fires is so great that it outweighs any possible decrease in reflective efficiency caused by haze.
The overall effect of the annual African fires, they write, is one of cooling, not heating.