Northwestern Africa has always been a prodigious source of airborne dust. But dust supplies really took off when commercial agriculture came to the region in the 19th century, a new analysis suggests.
Climate exerts a strong influence on African dust emissions: When precipitation is lower than normal, atmospheric concentrations of dust increase. But new data gleaned from sediments deposited on the North Atlantic seafloor suggest that since the mid-1800s, human activity — especially agriculture along the southern fringe of the Sahara, a region known as the Sahel — has played a big role too, says Stefan Mulitza, a paleoclimatologist at the University of Bremen in Germany. He and his colleagues report their findings in the July 8 Nature.
To reach their conclusion, the researchers analyzed two sediment cores taken from the seafloor about 30 kilometers from the mouth of the Senegal River. That material contains both fine-grained, iron-rich sediment carried there by the river and silicon-rich dust blown to the site by prevailing winds, says Mulitza. One core, which measures more than 5 meters long, covers the last 3,200 years, the team’s radiocarbon dating suggests. The other core, only 43 centimeters long, provided a high-resolution look at sediment that accumulated in recent years.
The researchers found that for most of the last 3,200 years, changes in precipitation were strongly correlated with dust emissions. But since the early 1800s — a time that coincided with the arrival of commercial agriculture in the western Sahel — dust emissions have increased substantially, says Mulitza. Early that century, Portuguese settlers in Africa began farming maize, which was soon replaced by millet and sorghum. But dust emissions really skyrocketed when farmers began growing groundnuts such as peanuts during the region’s “cash crop revolution,” which dates to the 1840s.
The uppermost layers of seafloor sediment also chronicle an increase in windblown dust reaching the site after 1968, the onset year of an extended drought in the Sahel, says Mulitza. This trend is also reflected in direct measurements of African dust by instruments on the Caribbean island of Barbados, a data check that helps validate findings from before the instrumental record.
“This is potentially a very important study,” says Natalie Mahowald, an atmospheric scientist at Cornell University. The new findings back up her team’s recent models, which show a doubling of dust emissions from northern Africa during the 20th century. Those analyses suggest that increased dust flux has significantly affected global climate, she notes. Determining the modern-day balance of dust emission sources could help scientists better understand past climate as well.
It’s possible that some of the fine-grained material in the cores, which Mulitza and his colleagues categorized as river-carried sediment, actually is lightweight dust blown to the site from more distant parts of northern Africa, she notes. Nevertheless, she adds, the team’s data make up the first long-term, high-resolution record of dust flux from that region in recent centuries.
“The data make indeed a strong case for the role of humans in dust production in the Sahel,” says Ina Tegen, an atmospheric scientist at the Leibniz Institute for Tropospheric Research in Leipzig, Germany.