Dwindling desert dust spells danger for the Amazon
Modelling finds a worrying link between climate change, the Sahara and South America’s rainforest. Michael Lucy reports.
Global warming may cut the amount of dust blown into the atmosphere from the Sahara Desert by up to 100 million tonnes a year, starving the Amazon rainforest of nutrients and turning up the heat in the north Atlantic.
Rising temperatures will mean less wind and hence less dust, according to a paper posted on the arXiv preprint server by NASA earth scientist Tianle Yuan and colleagues. Previous attempts to predict future dust levels have had limited success, but the new paper argues they have ignored the key factor: the temperature difference between the north and south Atlantic.
Airborne dust is a surprisingly large player in the planet’s climate system: it both absorbs and scatters radiation from the sun, serves to seed clouds by providing nuclei for water droplets to grow around, and – when it finally falls back to the surface – can provide key minerals such as iron and phosphorus for plants and marine life. The amount of dust in the atmosphere in turn depends on climatic conditions such as temperature, rainfall, and wind speeds.
The vast Sahara Desert of north Africa and the semi-arid Sahel region on its southern border are major sources of atmospheric dust. In an average year they contribute more than 180 million tonnes. Most of the dust is carried west over the Atlantic by the trade winds, and much of it falls into the sea, where it provides vital iron for phytoplankton to grow.
Around 27 million tonnes blows all the way to the Amazon basin in South America, delivering phosphorus that contributes to the riotous lushness of the rainforest.
As the world warms, climate scientists expect that the northern part of the Atlantic ocean will get hotter faster than the southern part. Southern hemisphere winds will rush northward to balance out the temperature contrast, meaning that the area where air circulation from the two hemispheres meets – a blustery band of latitude known as the intertropical convergence zone, or ITCZ – will drift northward.
This all adds up to weaker winds over the Sahara, resulting in less dust in the air. Depending on future carbon dioxide emissions, Yuan’s team calculate that the amount of dust could drop by as much as 60% by the end of the twenty-first century.
The authors checked their model against historical records and palaeoclimatic evidence, and found that temperature differences between the north and south Atlantic are correlated with levels of African dust over the last 17,000 years. They also found a decreasing trend in the amount of dust since 1980.
The decline in dust levels may become a self-reinforcing cycle. Another effect of the airborne dust is to provide shade that cools the north Atlantic. As the dust dwindles, clearer air will mean warmer seas – which in turn means less dust.