"Under colonial rule West African agriculture transformed into the export-oriented cultivation of cash crops," said Stefan Mulitza of the University of Bremen, Germany. "The steepest increase in dust deposition parallels the advent of widespread groundnut crops in Senegal, Nigeria and Gambia in the 19th century."

Mulitza and colleagues from the University of Bremen, the Royal Netherlands Institute for Sea Research, and the Alfred Wegener Institute for Polar and Marine Research, Germany, believe that commercial cultivation led agricultural lands to expand into forests and woodlands, exposing more soil to wind erosion.

The researchers used marine sediment cores taken 30 km off the coast of Mauritania to analyse levels of dust production for the past 3200 years. The cores contained sediment washed into the sea by the Senegal River and also dust from the western Sahara blown into the sea. The two types of deposit were distinguishable because the particles transported by the river contained high levels of aluminium and iron, and they were generally smaller than 10 microns in diameter. The dust from the Sahel and western Sahara, in contrast, is relatively rich in silicon and up to 200 microns in size.

Comparison with oxygen isotope records of precipitation in West Africa from Lake Bosumtwi in Ghana indicate that, prior to the 19th century, dust production increased as conditions were drier. From 1200 BC to 200 AD, the bulk of the core was made up of river deposits. Dust deposition then gradually increased until 900 AD, when it stabilized, before increasing again from the 14th century. Finally, the rate of increase rose steeply after the early 19th century. The region experienced drying trends from 100–900 AD and from 1400–1700 AD.

"Less rain, more dust: this obvious relation can be observed at least until the 18th century," said Mulitza. Since then rainfall and dust deposition seem to have been, at least partly, decoupled. "For the most recent interval of our core we observe a rise of dust deposition along with increased rainfall on land."

After about 1700 AD, precipitation in the region returned to intermediate levels. The droughts in the 1970s and 1980s were devastating and they contributed to increased atmospheric-dust concentrations but they were not as large as the multi-century-long droughts of the past, say the researchers.

Direct measurements of airborne African dust concentrations only became available in the mid-1960s – from a station in Barbados. Nowadays scientists can also make use of the satellite data that came onstream in the late 1970s.

"This change in African dust generation happened in the 19th century and was therefore not visible in the sparse instrumental data of atmospheric-dust concentration that only go back to the 1960s," said Mulitza.

According to Mulitza, it's plausible that the increased human-induced dust production also contributed to locally drier conditions in the Sahel. "More dust in the atmosphere leads to less solar radiation and a cooler surface," he said. "This might cause a substantial reduction of rainfall during the monsoon period." The Sahel has been experiencing desertification for the last four centuries.

"In coming years it will be very important to further understand the role of African dust in the climate system and to incorporate these processes into climate models," said Mulitza.

The researchers reported their work in Nature.