This is incredible: NASA scientists recently studied in detail how dust from the seemingly barren Sahara Desert gets picked up by strong winds and carried more than 3,000 miles across the Atlantic Ocean, where it fertilizes the lush Amazon rainforest. The hypnotic video below gives an overview of the process:
So how does desert dust manage to fertilize a rainforest thousands of miles away? The answer is that this dust contains phosphorus, a key ingredient for plant growth. Phosphorus is in surprisingly short supply in the Amazon. But there's a lot of it in the Sahara — because the Sahara once had a vast lake.
That prehistoric lake, referred to as Lake Mega-Chad, used to be filled with algae and other microorganisms. But more than 7,000 years ago, a changing climate caused the lake to dry out and recede (it is now the much smaller present-day Lake Chad), leaving a lot of nutrient-rich dust behind.
Much of this dust now lies in the Bodélé Depression in Chad, which, because of its geography, is particularly prone to severe dust storms. The wind comes whipping from the east through a wind tunnel formed by two nearby mountain ranges and picks up phosphorus-rich dust in the region.
Once aloft the air, that dust can travel thousands of miles across the ocean:
This dust turns out to be crucial to the Amazon rainforest. Although plants need phosphorus to grow — it's a key ingredient of modern fertilizer — there's surprisingly little phosphorus available in the Amazon soil. NASA's press release explains why:
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.
In any case, scientists have known about the Sahara-Amazon connection since at least 2006. But this newest study in Geophysical Research Letters, led by Hongbin Yu of NASA's Goddard Space Flight Center, gives us a better understanding of how much dust is actually traveling through the air — and how important it is.
Using NASA satellites, the authors find that winds carry, on average, about 27.7 million tons of dust from the Sahara to the Amazon basin each year, containing 22,000 tons of phosphorus. As it happens, that's about the same amount of phosphorus that washes away from the Amazon soil.
The study itself is fascinating — the researchers used a "lidar" instrument on NASA's CALIPSO satellite to study the dust. The satellite would send off little pulses of light and track how the pulses bounce off particles in the atmosphere. The researchers then painstakingly distinguished between dust and other particles based on their unique properties.
It's a great demonstration of how far-flung parts of the globe can be interconnected in totally unexpected ways. But it also raises a lot of questions. Scientists are still, for instance, trying to figure out how long the Sahara has been fertilizing the Amazon — and how far into the future it will last.
Further reading: Here's a great look at how the Sahara went from a lush savannah (complete with crocodiles) to barren desert in just a few thousand years.