Sahara Dust

Dr Anjani Ganase, marine scientist, discusses the effects of Sahara Dust. Whether it is beneficial or harmful, the research is still all up in the air!

Hazy skies, irritated eyes and dust everywhere is a sign that Sahara Dust is here. These days, with the advent of satellite technology capable of tracking the Saharan air layer across the Atlantic Basin, meteorologists can warn those that may be particularly sensitive to the dust and suffering from respiratory diseases. The Sahara Dust actually originates from multiple sources in the Sahel region in Africa that fringes the Sahara desert. The weather patterns in this area determine the transport of hundreds of million tonnes of dust a year across the Atlantic Ocean to the Americas. Periods of drought in the Sahel savannahs are typically associated with higher transportation of Saharan dust across the Atlantic Ocean. These dust-laden trade winds tend to be seasonal and are known as the Harmattan in West Africa. Interestingly, greater plumes of Sahara Dust associated with hot and dry atmospheric conditions prevent the formation of clouds and the intensification hurricanes off the African coast bound for the Caribbean (Bietler 2007). The composition of the Sahara Dust is essentially quartz, clay, minerals such as iron oxide plus nutrients like phosphorus and iron. However, in addition to these, the dust is known to carry bacteria and fungi.

June 2014, winds pushed a river of dust from western Africa across the Atlantic Ocean. Image of the Day for July 5, 2014 Instrument: Suomi NPP — VIIRS NASA images by Norman Kuring, NASA’s Ocean Color web. (West is the top of image)

Before satellite technology, quantities of Sahara Dust have been collected and recorded in Barbados since the 1960s and in Miami since the 1970s. Periods of higher concentrations of dust are typically related to drier seasons and extended periods of droughts in the Sahel. The 1970s and 1980s, when drought was more severe, corresponded with greater concentrations as recorded in Barbados; while low concentrations are found in the 1990s and 2000s. 

So what does the future look like with regard to the intensities of Sahara Dust in relation to climate change? The easy answer is that scientists are uncertain. Recently developed climate models have shown that climate change may result in the weakening of the tropical wind circulations and therefore in less intense plumes of Sahara Dust across the Atlantic Basin. Yet, physical evidence from Barbados shows consistently higher concentrations of Sahara Dust in recent years, even though conditions in the Sahel region are getting “greener” (Prospero and Mayol-Bracero 2013). Scientists speculate that this may have less to do with the climate, and more to do with human activities; specifically intensified land clearing which may be responsible for the changes in the patterns of Sahara Dust being transported. 

The transport of the Sahara Dust to the Caribbean and the South America has been responsible for many ecological phenomena. The nutrients and minerals carried by the Sahara Dust enrich many ecosystems including the Amazon, as well as oceanic ecosystems. In the ocean, the growth of phytoplankton that forms the base of the ocean food web is typically limited by the absence of iron, as so the inclusion of this from Sahara Dust results in a spike in ocean productivity. In addition to this, as much of the algae that blooms settle on the seafloor when it dies, the Sahara Dust indirectly increases carbon sequestration in the ocean. 

Sahara Dust plume reaching the Amazon Basin. NASA's Goddard Space Flight Center

Unfortunately, Sahara Dust has also been linked to ecological disturbances on Caribbean coral reefs. Sequential outbreaks of disease on Caribbean reefs – the first being the white band disease that wiped out branching corals through out the Caribbean in the 1970s, the die off of sea urchins regionally in 1980s, and the wide spread of our sea fans in the early 1990s have been shown to correspond with the high influx of Sahara Dust. It may explain the rapid region wide impact of these diseases. However, the dust is not the only driving factor of these outbreaks, sea surface temperatures also corresponded with the disease outbreaks. Therefore it is likely that the combination of warmer waters, higher nutrients and even the inclusion of introduced pathogens brought on the winds may have resulted in these widespread infections across the Caribbean (Hunter and Cervonne 2017).

Specifically, the infection of sea fans on Caribbean coral reefs was caused by a soil fungal pathogen (Aspergillus species) that quickly resulted in region-wide infection. Although some scientists relate this infection to pollution from land use, the seemingly ubiquitous infection, even at remote coral reefs away from land pollution supports the idea of the rapid spread of the fungus through the transport of the Saharan Dust. There is some dispute subject to experimental research as to whether the specific pathogen is found in the Saharan region. More research needs to be done. 

The research on Sahara Dust and its impacts on different marine and terrestrial ecosystems is still in its infancy. However, as satellite technology continues to improve and we get better views of planet earth, we quickly learn how connected were we are in this global ecosystem.

REFERENCES:

Bietler, J. (2007) Saharan dust versus Atlantic hurricanes 

https://cdn.earthdata.nasa.gov/conduit/upload/1181/NASA_SOP_2007_Saharan_dust_versus_Atlantic_hurricanes.pdf

Dunion JP, Velden CS. The impact of the Saharan air layer on Atlantic tropical cyclone activity. Bulletin of the American Meteorological Society. 2004 Mar;85(3):353-66.

Evan AT, Flamant C, Gaetani M, Guichard F. The past, present and future of African dust. Nature. 2016 Mar; 531 (7595):493.

Hunter H, Cervone G. Analysing the influence of African dust storms on the prevalence of coral disease in the Caribbean Sea using remote sensing and association rule data mining. International journal of remote sensing. 2017 Mar 19;38 (6):1494-521.

Lovett R. African dust keeps Amazon blooming. Nature, doi. 2010;10.

Prospero JM, Mayol-Bracero OL. Understanding the transport and impact of African dust on the Caribbean Basin. Bulletin of the American Meteorological Society. 2013 Sep; 94(9):1329-37.