He points out that computer models are not yet good enough to tell whether global warming from the greenhouse effect will make the Sahel wetter or drier. Even short-term predictions are shaky. In June, on the basis of changes in the ocean currents and temperatures in the Pacific and Atlantic oceans, the office revised its prediction to a very wet season. The second forecast was correct, as the region experienced the wettest rainy season for at least 20 years. An assessment of exactly how much land turns into desert each year hinges largely on definitions.
Satellite images are an obvious source of data.
But there are problems. One report by the UNEP of desertification in Bangladesh mistakenly included all the paddy fields as desert because they were dry at the time when the pictures were taken. And the spread of thorn bushes in East Africa is destroying feeding grounds for nomadic cattle herds. Yet a satellite will register increased biological productivity. Critics of the orthodox idea of spreading deserts say it is still implanted in the minds of policy-makers.