Every year, millions of tons of dust from the Sahara desert cross the Atlantic Ocean, bringing vital phosphorus and other fertilizers to depleted Amazon soils. For the first time, scientists have quantified in 3D how much phosphorus makes this trans-Atlantic journey. Their study puts the number at about 22,000 tons per year, which roughly matches the amount that the Amazon loses from rain and flooding.
NASA’s CALIPSO satellite has quantified in three dimensions how much dust makes the trans-Atlantic journey from the Sahara desert to the Amazon rain forest; among this dust is phosphorus, an essential nutrient that acts like a fertilizer, which the Amazon depends on in order to flourish. Image credit: NASA’s Goddard Space Flight Center.
The Sahara Desert is a near-uninterrupted brown band of sand and scrub across the northern third of Africa. The Amazon rain forest is a dense green mass of humid jungle that covers northeast South America. But after strong winds sweep across the Sahara, a tan cloud rises in the air, stretches between the continents, and ties together the desert and the jungle.
This trans-continental journey of dust is important because of what is in the dust.
Specifically the dust picked up from the Bodélé Depression in Chad, an ancient lake bed where rock minerals composed of dead microorganisms are loaded with phosphorus – an essential nutrient for plant proteins and growth, which the Amazon rain forest depends on in order to flourish.
“We know that dust is very important in many ways. It is an essential component of the Earth system,” said study lead author Dr Hongbin Yu of the Earth System Science Interdisciplinary Center, a joint center of NASA’s Goddard Space Flight Center and the University of Maryland.
“Dust will affect climate and, at the same time, climate change will affect dust.”
“To understand what those effects may be, first we have to try to answer two basic questions: how much dust is transported? And what is the relationship between the amount of dust transport and climate indicators?”
The new estimates, published online in the journal Geophysical Research Letters, were derived from data collected by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite from 2007 though 2013.
The data show that wind and weather pick up on average 182 million tons of dust each year and carry it past the western edge of the Sahara at longitude 15W.
The dust then travels 2,575 km across the Atlantic Ocean, though some drops to the surface or is flushed from the sky by rain.
Near the eastern coast of South America, at longitude 35W, 132 million tons remain in the air, and 27.7 million tons fall to the surface over the Amazon basin.
About 43 million tons of dust travel farther to settle out over the Caribbean Sea, past longitude 75W.
The team estimated the phosphorus content of Saharan dust by studying samples from the Bodélé Depression and from ground stations on Barbados and in Miami.
They then used this estimate to calculate how much phosphorus gets deposited in the Amazon basin.
Although the 7-year data record is too short to make conclusions about long-term trends, it is an important step toward understanding how dust and other windborne particles behave as they move across the ocean.
Year by year, the pattern is highly variable. There was an 86 percent change between the highest amount of dust transported in 2007 and the lowest in 2011.
The scientists believe this variation is due to conditions in the Sahel, the long strip of semi-arid land on the southern border of the Sahara.
Years of high rainfall in the Sahel were typically followed by low dust transport in the next year.
Although the mechanism behind this correlation is unknown, the team has a few ideas: “increased rainfall could mean more vegetation and therefore less soil exposed to wind erosion in the Sahel.”
“A second, more likely explanation is that the amount of rainfall is related to the wind circulation patterns that sweep dust from both the Sahel and Sahara into the upper atmosphere, where it makes the long journey across the ocean.”
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Hongbin Yu et al. The Fertilizing Role of African Dust in the Amazon Rainforest: A First Multiyear Assessment Based on CALIPSO Lidar Observations. Geophysical Research Letters, published online February 24, 2015; doi: 10.1002/2015GL063040
