Thursday, February 26, 2015

How the Sahara Keeps the Amazon Green


The Sahara Desert is huge, hot, and full of sand and dust. The Amazon basin is huge, warm, but full of greenery and wildlife. And one can’t live without the other. The Amazon, it seems, depends on the Sahara for its very survival.
The link: Dust. Specifically, phosphorous and other nutrients, kicked up by the wind and spread half a world away. “Even tens of millions of years after South America separated from Africa, the two continents are still inextricably linked, like an older brother and a younger brother,” says Charlie Zender, a climate scientist at the University of California, Irvine. Researchers have known for decades that wind could transport dust transcontinentally, but a new analysis of satellite data shows just how important that process is.
It’s a question of volume. By measuring dust at different altitudes, a NASA satellite called CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) reveals that the Sahara exports 182 million tons every year, enough to fill 689,290 semi trucks. Much of that falls into the ocean. Some makes it to the Caribbean islands. And some—about 27.7 million tons—settles over the Amazon River basin.
For the Amazon, this dust is like manna from heaven. Even though the Amazon has plenty of nutrients, rain and flooding washes away a lot of the phosphorous in the soil, a fertilizing nutrient critical to plant growth. The new data suggests that the winds deliver about 22,000 more tons of phosphorous from the Sahara every year—just enough to make up for what’s lost. “If you don’t have this African dust transport to the Amazon, in 10 years, or in 100 years, the Amazon will have lost a lot of phosphorous,” says Hongbin Yu, an atmospheric scientist at the University of Maryland, who led the study. “That’s a big problem for the plants.”
And where do those plants owe their specific gratitude? An ancient lakebed in Chad called the Bodélé Depression, where airflow and topography combine in just the right way. “Without that dust, one of the most biodiverse places on the planet wouldn’t be as rich,” Zender says. Best penpal ever. By Marcus Woo (wired.com)

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