For decades Lake Chad, in the semi-desert region where Chad joins Niger, Nigeria and Cameroon, has been retreating into itself, a geographic backwater on the frontier between west and central Africa, where regional co-operation has so far achieved little in the attempt to manage a vital and shrinking resource.

On the map, the lake is clearly delineated, the biggest expanse of water for 1,000km in any direction. About 30m people live in its drainage basin, a vast area of 2.4m sq km reaching to Algeria and Sudan.