A fifty-year famine provoked by drought threatens millions in the sub-Saharan Sahel region of Africa with crop failure, cattle loss, starvation and death. As policy makers work to provide relief for the drought-ravaged nations, scientists in Europe are investigating what caused a seemingly-minor ‘dry spell’ to snowball into a drought so severe that it now threatens political stability in sub-Saharan Africa. Airborne dust, created by commercial farming practices introduced by Portuguese settlers between the 18th and the 19th centuries, may be the culprit.
New research shows that dust emission from the West African Sahel has increased almost exponentially since the early part of the 18th century. Many factors influence the rate at which dust is released from the Sahel. Long periods of drought dry the soil, leading to increased dust emission. Some agricultural practices, like extensive cash crop farming, loosen the topsoil, releasing even more dust. Dust from North Africa is carried by trade winds across the Atlantic Ocean in seven days, mainly during the summer months. While in transit, the larger and denser dust particles settle out near the African coast. Lighter particles are lofted into higher levels of the atmosphere and travel longer distances – as far as the Caribbean and South America. In fact, windblown African dust is a major contributor to the iron rich soils of the Amazon.
Figure 1. A massive cloud of dust billows off the coast of West Africa, as seen by satellite. (Photo courtesy of NASA)
Scientists can measure the amount of dust emitted from North Africa and deposited in the ocean. Mud samples collected from the bottom of the Atlantic Ocean bordering the Western coast of Africa have a distinctive composition that can be used to track variations in dust emission over centuries. Scientists can use this data to infer changes in rainfall, wind patterns and human activities, all of which contribute to dust emission from the Sahel region. Current studies show that, for the past three thousand years, dust emission from West Africa was regulated by frequently occurring droughts in the region. High dust emission is characteristic of extensive droughts, whereas low dust emission is a feature of wetter periods.
Sahel region droughts spanning several decades are the consequence of warming ocean temperatures in the tropics and atmospheric circulation patterns over the North Atlantic. However, scientists have used coastal mud samples to measure the increase in North African dust emission over the last 150 years, and it is far greater than can be explained by changes in the amount of rainfall. Researchers believe this recent increase in dust emission may reflect the introduction of aggressive commercial farming practices to the Sahel around the mid 19th century. These farming practices release huge quantities of dust into the atmosphere – an amount that was grossly underestimated until recently. African dust is carried by wind as far as Western Europe and the United States, where it also impacts air quality. These findings may confirm what scientists have long suspected: that dust emitted from human activities like farming can increase the severity and duration of naturally occurring droughts.
How can the increase in dust emission amplify droughts in the Sahel region? During transit, dust acts as an additional layer in the atmosphere. The dust layer causes absorption of incoming solar radiation. This absorption causes localized heating in the atmosphere, but leaves the ocean surface cooler, since the sun’s rays are partially blocked by the dust layer. The heating of the atmosphere caused by the dust particles impedes atmospheric mixing, which is necessary for rainfall. Additionally, dust in the atmosphere changes cloud properties, further preventing them from raining out. Less rainfall causes drier conditions in the already arid sub-Saharan Africa. The resultant decades-long dry spells have had catastrophic effects for the expanding agriculture-based economy of the region.
These recent studies have major implications for commercial farming practices, especially in dry regions. For North Africa, scientists suggest that replacing cash crop production with local produce that are more suited to the climate of the region may help reduce dust emissions from the Sahel region. The history of the United States shows that we are not immune to these effects, either. In the southwestern United States, malpractices in farming techniques in the late 1930’s led to the dust bowl and contributed to the biggest economic downturn in American history. Today, the southwest is going through another drought period. These new findings show that dust emissions must be taken into serious consideration when planning commercial agriculture in drought-prone regions around the world.
Atreyee Bhattacharya is a PhD student at Harvard University.
References:
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