For many years, scientists have believed that the southern expansion of the Sahara has been due to human activity. However, results from the AVHRR instrument and its measurements of vegetation suggest a different explanation: rainfall patterns. In drier years (1984 was one of the driest summers in recorded history in Northern Africa), the Sahara expands south, but in wetter years (such as 1994), vegetation moves back and there is no net expansion of the Sahara as had been previously suggested. Note: An animation of vegetation changes in North Africa from September 1984 to September 1994, from AVHRR data. Animator: Jesse Allen (Raytheon). Scientist: Compton Tucker (NASA/GSFC), Sharon Nicholson (Florida State University). Platforms/Sensors/Data Sets: NOAA-11/AVHRR, NOAA-12/AVHRR, NOAA-7/AVHRR, NOAA-8/AVHRR.