I had just written a blog on how great it was that a wall of trees spanning the width of Africa would help halt desertification and improve water retention for millions. But what about the others who depend on its rainfall who will now shed a tear?

The dust whipped up by desert storms absorb sunlight, darkening the skies and providing the right surface for water vapour to build and fall as rain elsewhere.

40 million tonnes of this dust is carried to the Amazon each year (1), fertilising half of its basin and explaining how its forests and biodiversity can survive when its soils are notoriously poor.

Scientists have shown that the nutrients present in wildlife there had been transported from Chad's Bodele depression.

Without deserts, there could be no forests.

So how can we judge to what extent this balance should be tipped? Do those building the Great Green Wall have a right to do so?

To what extent should we take this environmental subsididy into account economically, for climate change negotiations between North and South based on REDD for example?

It could be a little more complicated than first thought.

1- The Guardian newspaper, Nature's Choreography http://www.guardian.co.uk/commentisfree/2010/aug/09/editorial-environmental-research-amazon-sahara