ملف:Sahel seasonality iotd.jpg

English: A aspect of a NASA attempt to "map out Sahara desertification in the Sahel sub-Saharan savanna was time. If land is not considered desertified unless the change is permanent, you need to track change over a long period—ten to twenty years at least—to see if vegetation is permanently altered. In 2006, Goddard’s Global Inventory Modeling and Mapping Studies (GIMMS) group, led by Tucker, released a twenty-four-year-long data set, the longest satellite-based vegetation record available. “The data set provides an essential 24-year record of vegetation dynamics that enables us to detect areas where degradation is taking place and areas of desertification,”...

• http://earthobservatory.nasa.gov/Features/Desertification/desertification2.php