Increase in African dust flux at the onset of commercial agriculture in the Sahel region

Abstract

The Sahara Desert is the largest source of mineral dust in the world. Emissions of African dust increased sharply in the early 1970s (ref. 2), a change that has been attributed mainly to drought in the Sahara/Sahel region caused by changes in the global distribution of sea surface temperature. The human contribution to land degradation and dust mobilization in this region remains poorly understood, owing to the paucity of data that would allow the identification of long-term trends in desertification. Direct measurements of airborne African dust concentrations only became available in the mid-1960s from a station on Barbados and subsequently from satellite imagery since the late 1970s: they do not cover the onset of commercial agriculture in the Sahel region approximately 170 years ago. Here we construct a 3,200-year record of dust deposition off northwest Africa by investigating the chemistry and grain-size distribution of terrigenous sediments deposited at a marine site located directly under the West African dust plume. With the help of our dust record and a proxy record for West African precipitation we find that, on the century scale, dust deposition is related to precipitation in tropical West Africa until the seventeenth century. At the beginning of the nineteenth century, a sharp increase in dust deposition parallels the advent of commercial agriculture in the Sahel region. Our findings suggest that human-induced dust emissions from the Sahel region have contributed to the atmospheric dust load for about 200 years.

West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad

“…Deflatable lacustrine sediments in the higherelevation parts of the Bodélé Basin may have been periodically exposed, either between the Early and mid-Holocene highstands or during more severe later drying, allowing mineral dust production. Certainly, records from Nigerian lakes (52) and a West African marine core (53) indicate an increase in dust flux after ∼2 ka, which is consistent with our reconstruction of a drying Bodélé Basin after 2.4 ± 0.1 ka. It is also possible that higher-elevation parts of the Bodélé Basin contained diatomite, which has now been completely deflated.…”

“…Therefore, the present-day mode and scale of dust production from the Bodélé Basin could not have occurred between ∼15 and 1 ka. Many studies exclude biogenic contributions from their dust flux calculations (52,53), thereby removing the main diatomaceous component of present-day Bodélé dust. However, both studies cited above (52,53) still record an increase in dust flux over the last 2 millennia, which is consistent with the release of mineral (not diatomaceous) dust from the Bodélé Basin in response to drying after 2.4 ± 0.1 ka.…”

“…Many studies exclude biogenic contributions from their dust flux calculations (52,53), thereby removing the main diatomaceous component of present-day Bodélé dust. However, both studies cited above (52,53) still record an increase in dust flux over the last 2 millennia, which is consistent with the release of mineral (not diatomaceous) dust from the Bodélé Basin in response to drying after 2.4 ± 0.1 ka. Nonetheless, the existence of a small standing-water body in the lowest-elevation parts of the Bodélé Basin would have limited the emission of present-day quantities of diatomaceous dust until after 1 ka.…”

Where She Blows! A Ten Year Dust Climatology of Western New South Wales Australia

“…Analysis of a 3200 year marine core off West Africa at the century scale shows a marked increase in dust activity at the beginning of the nineteenth century. This increase was linked to the advent of commercial agricultural activity in the Sahel region [16]. While these very long term studies provide clear evidence of variable dust emissions, and are even able to link increased dust with periods of land use change, the temporal scale is too coarse to inform about land management practices and the spatial scale is limited.…”

Dust may cool polar regions

“…An important question for future climate predictions is the applicability of this study to current conditions. While dust emissions have increased in some regions over the last centuries 6 and may further increase due to a drying of the subtropics and land-use change, the midlatitude hydrological cycle is projected to intensify, which may offset increased emissions through enhanced wash-out. In addition, reduced sea-ice cover over the Arctic implies a different surface reflectivity than during the LGM, which will alter the resulting dust radiative forcing.…”