Every year, the wind from the Sahara transports about 180 million tons of dust. And thousands of miles downwind, this fine dust shapes both the ecology of the places it lands and the climate. When it passes through Spain, most of the dust that falls is less than 20 microns in diameter, nor can it be seen by the human eye. Scientists have long known that breathing in these particles is not good for your lungs. And the dust, as it is pushed around, can cause significant damage to the health of communities.
But it can also be good in many ways. On the other side of the world, in the Amazon, plants grow strong thanks to this phenomenon. The haze fertilizes plant and ocean life. In addition, it could be a shield against climate change.
How does it work? The dry desert expanses in North Africa are the world’s largest sources of dust, there is no doubt about that. Under the right conditions, usually between spring and fall, large amounts of dust are blown into the “Saharan air layer,” a mass of hot, dry wind a kilometer above the Earth’s surface. Land.
Once the coldest air masses from the ocean push it into the atmosphere, the dust can float for days or even weeks, in a phenomenon that all Spaniards are already familiar with: haze. But even east-west winds sweep it across the Atlantic to the Caribbean or the US in a few days. As the plume of dust moves, some of it falls in a steady shower of particles.
A rain of fertilizer. A few years ago, a NASA satellite managed to quantify how much dust this transatlantic voyage makes. The scientists not only measured the volume of dust, but also calculated how much phosphorus and iron is transported through the ocean from one of the most desolate places on the planet to one of the most fertile: the Amazon. It turns out that phosphorus is an essential nutrient for protein and plant growth.
The phosphorus that reaches Amazonian soils from Saharan dust, some 22,000 tons per year, is about the same amount that is lost to rain and floods. Much of that land doesn’t have enough to support the abundance of life that grows on it, and a key feature of rainforest habitat, rain washes away unused phosphorus very quickly.
cyclone destroyer. Many scientists think that traveling dust plays another role in the Atlantic: it helps suppress the formation of tropical cyclones. Layers of dusty air are often bone dry, and that spells doom for tropical storms, which feed on moist heat. That is, if a developing storm hits the dust layer, the dry air helps to quench it. Think of a flame without the oxygen it needs to keep burning.
Dust layers are often driven by strong winds, so they can cross the ocean in just a few days. And a storm that develops into a towering whirlwind can be blown down by those winds, preventing it from getting bigger.
Climate Shield. It’s not entirely clear what effect an even dustier future would have on the climate, because dust can both heat and cool the planet. But it is true that plumes flowing over the ocean reflect incoming heat from the sun, which would otherwise be absorbed by the dark surface of the ocean. Dust also changes the way different types of clouds form. Sometimes it generates a bank of reflective clouds that can deflect additional heat.
And there are more elaborate ways in which dust interacts with the weather. It can even fertilize photosynthetic organisms which, under the right conditions, can stimulate population explosions that result in the extraction of carbon dioxide from the atmosphere. The fertilizing force of the dust may have caused at least a quarter of the change in carbon dioxide that brought Earth into the last ice age.
But let’s not forget that it harms health. Despite the haze’s positive factors, a study published in Nature Sustainability concluded that dust from depressions like Chad’s Bodèle, one of the world’s largest and most prominent sources, makes it difficult to breathe.
The scientists studied 15 years of records of the impact of dust on air quality in communities downwind on the African continent. They found that how densely laden the air was with dust was devastatingly related to whether a newborn baby would survive a year. In West Africa, if the dust thickened the air by 25%, the chance that that baby would survive was reduced by 18%.
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