Scientists have conclusively demonstrated the extent of the link between the Sahara desert and the Amazon rainforests.

It might sound unlikely, but their work has shown that the Amazon rainforest depends on dust from one tiny area of the Sahara desert to restock its soil with nutrients and minerals. Analysis of images from NASA's MODIS satellite have revealed the Bodélé, a region of the Sahara not far from Lake Chad, as the source of more than half the material that fertilises the rainforest.

The Bodélé depression was already known as one of the largest sources of dust in the world, but the scientists involved in the research say no one had any idea of the scale of the region's importance to the Amazon. It transpires that if the Bodélé was not there, the Amazon would be a mere wet desert.

Dr Ilan Koren, lead author of the paper said: "Until now no one had any idea how much dust [The Bodélé] emits and what portion arrives in the Amazon. Using satellite data, we have calculated that it provides on average more than 0.7 million tons of dust on each day that it is actively emitting dust."

The dust is swept into the atmosphere by the surface winds in the Sahara. The Bodélé region loses most of its dust during the spring and winter months, unlike the rest of the Sahara, because of its unique geography.

The Bodélé depression is located downwind (in the winter, when the Harmattan winds blow) of a huge crater-like valley between the Tibesti and Ennedi mountains. This crater narrows to a cone-shaped pass which focuses the winds, and they speed up towards the Bodélé. This is how the region, which is just half a per cent of the size of the Amazon, can produce as much dust as it does.

Dr Koren explains the process: "In the early morning on an emission day the winds speed up to the critical velocity for lifting and transporting dust when they reach the Bodélé.

"By using data from two satellites that take images of the same areas three hours apart, we can estimate the wind speed and calculate the size of the 'dust parcels' that are produced at the Bodélé. We are then able to track the progress of the parcel the next day after it has left the Bodélé and watch it progress across the desert."

The research team use the MODIS satellite to watch the dust, and the MISR instrument, which only covers a small area, to find out more about the quantity of dust in each parcel.

The work has prompted more questions, however. The team wants to know how long the Bodélé depression has been 'sending' dust to the Amazon, and how long it will continue to do so.

The research is reported in the first edition of the Institute of Physics open-access journal, Environmental Research Letters. ®