has been steadily increasing not only due to human activities such
as excessive husbandry over-cultivation of land, deforestation,
the cutting of firewood and salinisation caused by indiscriminate
irrigation, but also because of global climatic changes. Around
one-third of the Earth's surface is threatened, and the subsistence
of more than 100 million people is directly concerned.
The combination of aerial photos and satellite
images allows one to monitor the long-term evolution of the vegetal
coverage in an endangered region. However, the use of satellite
images is even more important for warning systems which make possible
long-term planning for the threatened zones. Thanks to the high
frequency of their global images, sensors such as NOAA AVHRR, Meteosat
and SPOT Vegetation permit one to follow closely changes in vegetal
coverage and note in time any deviations from the normal pattern.