This page paraphrases the content of two very good resources on Sahara dust; its composition and relevance to Caribbean life. Should Reader decide to stay for a quick intro to Dust basics, bear in mind its derived from the work of Attish Kanhai (Research Officer,Institute of Marine Affairs, Trinidad and Tobago). The second section [The Wet Sahara] is transcribed verbatim from a research paper for which credit is appropriately placed. Intellectual property rights and therefore every credit ensuing is attributable to these authors, respective employers or affiliate organisations and institutions.
Figure 1: Image courtesy IMA webpage
Sahara dust is the largest transport of dust on the planet. Plumes from the Sahara Desert are big enough to be seen from the Space Station. The largest pulses occur from February to October to coincide with north-easterly Harmattan wind.
On average, 182 million tons of dust - an equivalent of 689,290 dump trucks drift past the western edge of the Sahara each year. Though some of it falls into the ocean, flushed from the sky by rainwater, by the time it reaches the east cost of Southern America approximately 132 million tons still remains in the air. And as it moves over the Amazon Basin, an estimated further 27.7 million tons will settle - to initiate a critical natural function.
Saharan dust is rich in phosphorus, essential in energy transformations in plant. Amazon soils lose as much as 90% of its phosphorus as rivers wash the Basin’s soil cover out to sea each year. Decomposing leaves and plants help recycle some of the phosphorus already in the soil but the dust provides a key outside source of the nutrient. Without this continuous source of phosphorus input from the Sahara the biodiversity of the Amazon will likely experience adverse effects.
About 43 million tons of the dust goes on to settle over the Caribbean Basin and is a major irritant to the human population. The iron that gives Saharan dust its rich red colour feeds the phytoplankton in the Caribbean and also along the south east coast of the United States. Oxygen produced by phytoplankton photosynthesis is responsible for at least half of the world’s oxygen supply. It also accounts for a similar percentage of Big Blue’s carbon dioxide uptake.
Saharan dust also plays a key role in the suppression of hurricane formation in the Atlantic Ocean. Every three to five days during the summer months of the northern hemisphere dust storms leave the African coast forming a layer of hot dusty air known as the Saharan Layer. s it transits it pulls moisture up into the atmosphere - which can influence rainfall levels as far west as California. These dust storms have three components that can suppress a hurricane.
One is dry air. Non-humid conditions in the middle parts of the atmosphere make hurricane formation unlikely. Another feature is a strong surge of air embedded within Saharan layer. Known as vertical wind shear, this will typically hamper storm development.
The third feature of the Saharan layer’s storm fighting powers is just - dust. Researchers believe that the drifting body of dust suppresses cloud formation, which in turn prevents tropical waves from intensification.
Sahara dust is not recent phenomena.
This section of this post from this point foward can be viewed in its original form here
“There was also extensive human settlement throughout the Sahara, with lifestyles that would never be possible today,” McGee says. “Researchers at archaeological sites have found fish hooks and spears in the middle of the Sahara, in places that would be completely uninhabitable today. So there was clearly much more water and precipitation over the Sahara.”
This evidence of wet conditions shows that the region experienced regular monsoon rains during the early Holocene. This was primarily due to the slow wobbling of Earth’s axis, which exposed the Northern Hemisphere to more sunlight during summer; this, in turn, warmed the land and ocean and drew more water vapor — and precipitation — over North Africa. Increased vegetation in the Sahara may have also played a role, absorbing sunlight and heating the surface, drawing more moisture over the land.
“The mysterious thing is, if you try to simulate all these changes in these early and mid-Holocene climates, the models intensify the monsoons, but nowhere near the amounts suggested by the paleodata,” McGee says. “One of the things not factored into these simulations is changes in windblown dust.”
Tracking a dust plume
In their results published today, McGee and colleagues propose a reduction in African dust may indeed have contributed to increasing monsoon rains in the region. The researchers came to their conclusion after estimating the amount of long-range windblown dust emitted from Africa over the last 23,000 years, from the end of the last ice age to today.
They focused on dust transported long distances, as these particles are small and light enough to be lifted and carried through the atmosphere for days before settling thousands of kilometers away from their source. This fine-grained dust scatters incoming solar radiation, cooling the ocean’s surface and potentially affecting precipitation patterns, depending on how much dust is in the air.
To estimate how the African dust plume has changed over thousands of years, the team looked for places where dust should accumulate rapidly. Dust can sink to the floor of open ocean, but there layers of sediment build up very slowly, at a rate of 1 centimeter every 1,000 years.
Places like the Bahamas, by contrast, accumulate sediment much more quickly, making it easier for scientists to determine the ages of particular sediment layers. What’s more, it’s been shown that most of the windblown dust that has accumulated in the Bahamas originated not from local regions such as the U.S., but from the Sahara.
Dust’s climate role
McGee and his colleagues obtained sediment core samples from the Bahamas that were collected in the 1980s by scientists from the Woods Hole Oceanographic Institution. They brought the samples back to the lab and analyzed their chemical composition, including isotopes of thorium — an element that exists in windblown dust worldwide, at known concentrations.
They determined how much dust was in each sediment layer by measuring the primary isotope of thorium, and determined how fast it was accumulating by measuring the amount of a rare thorium isotope in each layer.
In this way, the team analyzed sediment layers from the last 23,000 years, and showed that around 16,000 years ago, toward the end of the last ice age, the dust plume was at its highest, lofting at least twice the amount of dust over the Atlantic, compared to today. However, between 5,000 and 11,000 years ago, this plume weakened significantly, with just half the amount of today’s windblown dust.
Colleagues at Yale University then plugged their estimates into a climate model to see how such changes in the African dust plume would affect both ocean temperatures in the North Atlantic and overall climate in North Africa. The simulations showed that a drop in long-range windblown dust would raise sea surface temperatures by 0.15 degrees Celsius, drawing more water vapor over the Sahara, which would have helped to drive more intense monsoon rains in the region.
“The modeling showed that if dust had even relatively small impacts on sea surface temperatures, this could have pronounced impacts on precipitation and winds both in the north Atlantic and over North Africa,” McGee says. Noting that the next key step is to reduce uncertainties in the modeling of dust’s climate impacts, he adds: “We’re not saying, the expansion of monsoon rains into the Sahara was caused solely by dust impacts. We’re saying we need to figure out how big those dust impacts are, to understand both past and future climates.”
Ina Tegen, a professor at the Leibniz Institute for Tropospheric Research in Germany, says the group’s results suggest that “dust effects today may be considerable as well.”
“Dust loads vary with changing climate, and due to the effects of dust on [solar] radiation, ice formation in clouds, and the carbon cycle, this may cause important climate feedbacks,” says Tegen, who was not involved in the research. “The changing climate since the last ice age can be considered a ‘natural laboratory’ to study such effects. Understanding the past is the basis for predicting future changes with any confidence.”
This research was supported, in part, by the National Science Foundation.
Related Links
Paper: “Glacial to Holocene changes in trans-Atlantic Saharan dust transport and dust-climate feedbacks”
David McGee [McGee Lab
Department of Earth, Atmospheric and Planetary Sciences
School of Science]