Sahara's dust flies to the rescue

Nasa's photo shows a brown swathe of dust being blown across the ocean from the barren Sahara. Photo / AP

Huge dust clouds swirling across the Atlantic from northern Africa to South America are shown in stunning new photographs from the US space agency Nasa that illustrate how Earth's largest tropical rainforest relies on its biggest, hottest desert to flourish.

Scientists have calculated how much dust makes this transatlantic journey from the Sahara to the Amazon basin where it fertilises depleted soils with life-sustaining nutrients.

Each year, 27.7 million tonnes of Saharan dust reach the Amazon basin according to analysis of three-dimensional imagery supplied by a Nasa satellite of the enormous brown plumes that can be seen from space.

The scientists have also calculated how much phosphorus - a remnant of the desert's past as a lake bed - makes that journey across the ocean from one of the planet's most desolate places to one of its most fertile.

About 22,000 tonnes of Saharan phosphorus, an essential nutrient for plant life, settles on the Amazon each year, replacing the same amount that is washed away by rain and flooding.

"This is a small world and we're all connected together," said Hongbin Yu, an atmospheric scientist at the University of Maryland who works at Nasa Goddard Space Flight Centre.

Yu and his colleagues published their work in the journal Geophysical Research Letters as part of a research project about the role of dust in the environment and its effects on local and global climate.

"Dust is an essential component of the Earth system," he said. "Dust will affect climate and, at the same time, climate change will affect dust."

While the Amazon ecosystem depends on the dust to bring life by replenishing its losses, an unusual Saharan dust storm in Britain last year produced health warnings for the elderly and people with asthma about dangerous air pollution.

But the dust - about 182 million tonnes a year on average - is usually carried west across the Atlantic from the Sahara by the prevailing winds and weather.

Much is flushed into the sea by rain, but on average nearly 28 million tonnes falls on the Amazon and 43 million tonnes across the Caribbean.

The figures vary markedly each year, depending on the previous year's rainfall in the Sahara. When rainfall increased, the next year's dust fall was lower, so bad meteorological news for the arid swathes of North Africa is good news for the Amazon.