Seesaw across the ocean

How the Sahara Desert greens the Amazon

Two of the world's largest and most fragile ecosystems face each other across the Atlantic. On one side is the Amazon rainforest; on the other the Sahara. They seem to be ecological opposites, and unconnected. The Sahara is rainless and largely empty of vegetation. The Amazon is one of the wettest places on Earth, and certainly the most biologically diverse, with perhaps half of the world's species beneath its canopy. But these two opposites are not so far apart. For one thing, the physical gap is surprisingly small. The Atlantic is narrow near the equator, and the two ecosystems are less than half as far apart as London and New York. For another, many believe they have a surprising symbiosis. Their fates may be intertwined in a rather unexpected way—and one that could have important consequences in the coming decades.

The key to the symbiosis lies in the remote heart of the Sahara, a region called Bodele, in northern Chad. Few people go here. It is littered with unexploded bombs and land mines left behind during Libya's invasion of Chad in the 1980s. And it is by far the dustiest place on Earth. Satellite images show year-round dust storms raging across Bodele and entering the atmospheric circulation. According to Richard Washington, of Oxford

University, two fifths of the dust in the global atmosphere comes from the Sahara, and half of that comes from Bodele.

Some of this dust stays local. But much of it is carried on the prevailing winds, which cross the desert wastes of Niger, Mali, and Mauritania before heading out across the Atlantic. The red dust clouds can grow almost 2 miles high as they approach America. They cause spectacular sunrises over Miami, before falling in the rains of the Caribbean and the Amazon. And there have been a lot of good sunrises in recent decades. The amount of dust crossing the Atlantic grew fivefold between the wet 1 960s and the dry 1 980s.

The Sahara dust has a series of unexpected effects on the Americas. According to hurricane forecasters in Florida, during dry, dusty years in the Sahara, there are fewer hurricanes on the other side of the Atlantic. It seems that dust in the air interrupts the critical updrafts of warm, moist air that fuel the storms. Equally surprisingly, desert bacteria caught up in the winds are being blamed for bringing new diseases to Caribbean coral reefs, and even for triggering asthma among Caribbean children.

And there is another important link. Saharan dust storms carry huge amounts of minerals and organic matter that enrich soils widely in the Americas. Bodele dust seems especially valuable. Its dunes are the dried-out remains of the bed of the vast Lake Megachad, which covered the central Sahara until its abrupt demise. Most of the dunes are made not of sand or broken rock but of the remains of trillions of diatoms, microscopic freshwater creatures that once lived in huge numbers in the lake. These fragments blow freely in the wind. That's why they make such plentiful dust storms. And they also make great fertilizer. If Bodele had any rain, the diatoms would make rich farmland. Instead, Chad's loss is the Americas'

gain, says Hans Joachim Schellnhuber, a German physicist turned Earth-system scientist, who, as director of Britain's Tyndall Climate Centre, in Norwich, has made a study of the unlikely connection. "Bizarre as it may seem, the arid, barren Sahara fertilizes the Amazon rainforest. This process has been going on for thousands of years, and is one reason why the Amazon basin teems with life."

The two unique habitats are on a kind of seesaw, he says. When the Sahara is dry, as it has been for much of the past quarter century, its dust crosses the Atlantic in huge quantities and fertilizes the Amazon, making the rainforest superabundant. When the Sahara is wet, the dust storms subside and the Amazon goes hungry. That the Sahara seems to have only two basic modes, wet and dry, suggests that there may be two distinct modes in the Amazon, too. The last big change in the Sahara came 5,500 years ago, when the region lurched from wet to dry, probably within a few decades. As yet we know little about how the Amazon changed at that time. But if Schellnhuber is right, the Sahara's loss at that time may have been the Amazon's gain. There may have been a major change for the better in the rainforests. In the twenty-first century, the seesaw could be on the move again. There are hints that the Sahara may become wetter, says Schellnhuber. And if the wetting turns to greening, and the vegetation feedback kicks in, the whole of North Africa could change dramatically. That would be good news for the Sahara, of course. But it might be bad news for the Amazon, which already seems to be close to its own tipping point, as the climate dries and rainforests give up their carbon. Could a wetter Sahara be the final nail in the Amazon's coffin? Schellnhuber believes it could.

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