Every year, wind and weather systems lift an average of 182 million tons of dust from Africa's Sahara desert and blow it westward. According to a report from NASA about the phenomenon, that's the amount that could be carried in 689,290 semi trucks. After traveling 1,600 miles over the Atlantic, about 27.7 million tons of the stuff falls to the ground in the Amazon basin. The dust migration is so massive that it can been seen from space -- and who better to see stuff from space than NASA?
From 2007 to 2013, the space agency observed the dust transport using its Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, or Calipso. The satellite is equipped with lidar, which uses laser pulses from space to measure objects on Earth, like forests or, in this case, drifting plumes of dust. According to the report, Calipso collects "curtains of data" that help researchers see the dust in multiple dimensions, including its height. "Knowing the height at which dust travels is important for understanding, and eventually using computers to model, where that dust will go and how the dust will interact with Earth's heat balance and clouds, now and in future climate scenarios," says the report.
In addition to measuring the volume of the dust migration, scientists also determined how much phosphorus the drifting dust contains. According to NASA, this is the first time a satellite-based estimate of the phosphorus transfer over multiple years has been determined. The findings were published in a paper in Geophysical Research Letters on February 24.
The phosphorus comes from rocks composed of dead microorganisms in an ancient lakebed in Chad called the Bodélé Depression. When the phosphorus lands in the Amazon after its wind-based journey from the lakebed, it provides the plant life there valuable nutrients.
"Nutrients -- the same ones found in commercial fertilizers -- are in short supply in Amazonian soils," says NASA. "Instead they are locked up in the plants themselves. Fallen, decomposing leaves and organic matter provide the majority of nutrients, which are rapidly absorbed by plants and trees after entering the soil. But some nutrients, including phosphorus, are washed away by rainfall into streams and rivers, draining from the Amazon basin like a slowly leaking bathtub."
Hongbin Yu, lead author of the study and a University of Maryland atmospheric scientist who works at NASA's Goddard Space Flight Center, says that the amount of phosphorus deposited by the wind migration is roughly equal to that which is lost through rain and flooding in the Amazon -- about 22,000 tons per year.
Despite this helpful general equivalency, researchers also noted that the volume of dust involved in the migration changes from year to year. Yu said there was an 86 percent change in the dust's volume from its highest year in 2007 to its lowest year in 2011. Researchers think that a strip of land known as the Sahel, which stretches along the southern edge of the Sahara, is responsible for the variation. When that area is moist from rainfall, the dust migration is lower the next year, and when it's dry in the Sahel, the dust migration is greater. Although that seems like a pretty straightforward relationship, Yu says that it's not quite clear why the Sahel rainfall is so closely linked to the dust migration.
"One possibility is that increased rainfall means more vegetation and less soil exposed to wind erosion in the Sahel," says the report. "A second, more likely explanation is that the amount of rainfall is related to the circulation of winds, which are what ultimately sweep dust from both the Sahel and Sahara into the upper atmosphere where it can survive the long journey across the ocean."
The high-quality video above details even more information about the dust migration and truly makes you feel like the Earth is a giant organism taking good care of itself by moving nutrients from one place that has little need for them to another where they act as valuable plant food. Or, as Yu says: "This is a small world, and we're all connected together."