Dust in the Wind

The Sahara Desert and the Amazon rainforest are about as different as two places on Earth could be. But they’re tightly linked together in a way you could hardly imagine. It all started 7,000 years ago, when the Sahara wasn’t a desert at all, but a land of huge lakes. The largest was the mega-lake Chad, bigger than all of our Great Lakes combined. The mega-lake was home to a huge number of diatoms, algae-like plankton that form the base of the food chain. After algal blooms, billions of dead diatoms would sink to the lake floor. When natural climate change caused the lake to shrink dramatically, it left behind the massive Bodélé Depression, which is now called the “dustiest place on Earth.” This dust is rich with phosphorous from centuries of dead diatoms and iron from the lakebed. Each winter, winds blow down from the mountains of Saharan Africa and carry it high into the sky. The dust then blows across the Atlantic in huge plumes and over the Amazon. There, rising water vapor from the rainforest condenses on the dust particles and rains back down, rich in minerals, like fertilizer from the sky. The phosphorous and iron are essential to the vigorous plant health across the region. Amazingly, at about the same rate that the Saharan winds carry it in, floods wash previous years’ phosphorous out of the soil, down river, and into the sea. There, it feeds new algal blooms—completing a circle thousands of miles wide and thousands of years in the making.