The Sahara Desert is a vast expanse of sand and barren rocky areas that occupies much of northern Africa. It is renowned for its incredibly harsh environment, which includes extreme temperatures, near-constant sunshine and a lack of water.
The Sahara Desert is perhaps less known for being a huge generator of atmospheric dust, which is transported in enormous quantities from the Sahara and deposited up to a few thousand miles away, carried by large winds that blow over an incredibly long distance.
How much Saharan dust is produced and transported around the world is currently threatened, due to predicted impacts from climate change.
What is Saharan dust?
Saharan dust is a mixture of sand and dust particles that originate from the Sahara Desert in North Africa and are transported by winds to other regions, including the Caribbean, South America, and the southern United States.
These dust storms can travel thousands of miles across the ocean.
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Saharan Dust on the Move
During the summer months, plumes of Saharan dust are blown from the African continent, often occurring a few days apart from each other.
The dust consists of fine dust particles, found in hollows or on desert flats that, on rare occasions, hold water which then dries out, leaving behind silt and other fine sediments. Dust particles are rarely derived from sand dunes, as these are too heavy and require very strong winds to loft them and keep them in the air for an extended period of time.
After being picked up by the wind, much of the dust travels over an incredibly long distance – the small size of the particles means that they are rather light, and can therefore remain suspended above the Earth’s surface for hundreds, if not thousands, of miles.
What is the Saharan Air Layer
This layer of wind borne dust is know as the Saharan Air Layer. This seasonal mass of extremely dry, dusty air forms above the Sahara Desert
Strong wind currents also keep the dust particles suspended over such distances, with a great example being the north-easterly trade winds, which blow from the Sahara toward the Americas.
The dust particles are usually transported at a height of between one and three miles above the surface of the globe, meaning that they can freely travel for a long distance without settling, and without any interference from many natural landscape features. Only large-scale mountain ranges can disrupt the airflow at such a height.
Saharan Dust Plumes
Saharan dust plumes are often sent westward across the Atlantic Ocean, regularly reaching (and often settling) over the Americas, although some of the dust travels even further afield.
Although a significant portion of the particles settle in the tropical Atlantic to the east of Central America, the prevailing north-easterly trade winds typically blow the dust toward the northern half of South America, Central America and the Caribbean.
A large percentage of the dust settles over these areas, and when it does, it is rather noticeable, with large-scale reductions in visibility occurring temporarily. For example, the leading edge of a dust plume blew over Puerto Rico in June 2020, after being carried for 5,000 miles. Some of the plume settled on the island, but the rest of it was blown toward North America.
Dust plumes do not always blow toward the west – some plumes carry the particles northward, toward Europe. In February 2021, two large plumes carried dust across much of Europe, coating the Alps in murky beige-colored skies, and even reaching as far north as Scandinavia.
La Calima
Around the Canary Islands and some parts of Spain, a meteorological event known as “La Calima” happens at specific times of the year. La Calima is defined by the presence of a warm and dusty air mass that originates from the Sahara Desert in North Africa.
During a La Calima event, fine dust particles and sand are carried by strong winds from the Sahara Desert and transported across the Atlantic Ocean to the affected regions. As a result, the atmosphere becomes laden with suspended dust particles, reducing visibility and causing hazy and dusty conditions.
The term “La Calima” is of Spanish origin and is often used in the Canary Islands, where this phenomenon is quite common. It can have various effects on the environment and human health. The increased levels of airborne dust can lead to respiratory issues, particularly for individuals with pre-existing respiratory conditions. It can also impact air quality and reduce the clarity of the sky, affecting visibility.
Does Saharan dust reach the United States?
Saharan dust can indeed reach the United States, although its impact is typically more pronounced in the southeastern parts of the country, such as Florida and the Gulf Coast states. The satellite image below shows Saharan dust drifting towards the Caribbean and the southeastern United States on June 27, 2018.
Aerosol Atmospheric Rivers
Dust plumes that travel far distances can often be carried by atmospheric rivers. Research recently published in the journal Atmospheric Research found that 78% of atmospheric rivers over northwestern Africa resulted in extreme dust transport events in Europe.
These dust-laden storms have been given the name “aerosol atmospheric rivers”. (Related: Haboobs in the United States)
Most recently on March 15, 2022, Storm Celia resulted in a plume of dust from North Africa that traveled to Europe, blanketing the region with sand.
How Much Saharan Dust is Transported?
Around 180 million tons of Saharan dust are transported across the Atlantic each year. Just one strong wind event running across part of the Sahara can loft a few million tons of dust into the air, which can make up a large portion of a single plume.
Large quantities of dust are deposited in areas far away from their origin. A study by the American Geophysical Union (Yu et al., 2015) found that – based on an eight-year average – 102, 20 and 28 tons of dust were deposited into the tropical Atlantic Ocean, Caribbean Sea and Amazon Rainforest respectively. This demonstrates the sheer amount of dust that is transported away from the Sahara, with large amounts being deposited in each region.
How Climate Change Will Affect Saharan Dust Plumes?
Scientific studies strongly suggest that climate change will restrict the rate and extent of Saharan dust plumes.
A 2021 study by the National Aeronautic and Space Administration (NASA) used a combination of computer modelling and satellite data to predict the extent of future dust plumes. They concluded that, over the next century, Saharan dust plumes will reduce to a 20,000-year low.
This is concerning as dust particles are rather high in nutrients; they contain large amounts of iron and phosphorus. A portion of these nutrients are deposited onto the Amazon Rainforest, which keeps the soil rich in natural fertilizer and helps its plants and trees to thrive. As the Amazon is one of the Earth’s largest carbon sinks, and sequesters vast amounts of carbon each year, a reduction in vegetation is quite alarming in terms of mitigating climate change.
Furthermore, if global temperatures increase, the sea surface temperature of the Atlantic increases with it, which is predicted to slow the easterly winds, meaning that less dust will be able to reach the Amazon. This therefore sets up a feedback mechanism, where less dust reaching the Amazon results in higher atmospheric carbon dioxide concentrations, and the cycle continues even further.
How Northern African Rainfall Patterns Can Affect Saharan Dust Plumes
A change in northern African rainfall patterns could also limit the amount of Saharan dust plumes. Weakened easterly winds, located over the Sahel region and the southern edge of the Sahara, mean that rainfall bands located to the north of the Equator will be able to extend further north into the Sahara, compared with the rainfall patterns of today.
This will have the effect of dampening the ground, thereby reducing the rate at which dust dries, and limiting how much dust can be lofted into the air. This is another factor that prevents large dust quantities from reaching the Amazon Rainforest.
In addition, the light-brown color of atmospheric dust means that, during plumes, sunlight is reflected from the Earth’s surface. On the contrary, the dark blue color of the ocean absorbs heat from the Sun – therefore, a reduction in plumes means that more heat is absorbed, thus increasing the temperature of both the sea surface and the atmosphere.
References
Chakraborty, S., Guan, B., Waliser, D. E., da Silva, A. M., Uluatam, S., & Hess, P. (2021). Extending the Atmospheric River Concept to Aerosols: Climate and Air Quality Impacts. Geophysical Research Letters, 48(9), e2020GL091827. https://doi.org/10.1029/2020GL091827
Francis, D., Fonseca, R., Nelli, N., Bozkurt, D., Picard, G., & Guan, B. (2022). Atmospheric rivers drive exceptional Saharan dust transport towards Europe. Atmospheric Research, 266, 105959. https://doi.org/10.1016/j.atmosres.2021.105959
Streiff, L. (2021, April 19). NASA study predicts less saharan dust in future winds. NASA. https://www.nasa.gov/feature/esnt/2021/nasa-study-predicts-less-saharan-dust-in-future-winds
Voiland, A. (2016, February 25). Saharan dust sweeps over the Iberian Peninsula. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/87566/saharan-dust-sweeps-over-the-iberian-peninsula
Voiland, A. (2018, June 30). Here comes the saharan dust. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/92358/here-comes-the-saharan-dust
Yu, H., Chin, M., Yuan, T., Bian, H., Remer, L. A., Prospero, J. M., … & Zhao, C. (2015, December). Saharan Dust Fertilizing Atlantic Ocean and Amazon Rainforest via Long-range Transport and Deposition: A Perspective from Multiyear Satellite Measurements. In AGU Fall Meeting Abstracts (Vol. 2015, pp. EP42A-05). https://ui.adsabs.harvard.edu/abs/2015AGUFMEP42A..05Y/abstract
This article was originally written on September 1, 2021 by Julian Marks and contains updates by Caitlin Dempsey.
