the Along Track Scanning Radiometer (ATSR) instrument on the European Remote Sensing (ERS) and ENVISAT satellites (Arino and J.M. Rosaz 1999). GLOBSCAR, a complimentary product to GBA-2000, maps the global distribution of burned area at 1-km spatial resolution and monthly time intervals using the ATSR-2 instrument on the ERS-2 satellite (Simon et al. 2004). These products have been used to compute the emissions of greenhouse gases and aerosols from biomass burning and to explore the impacts on tropical ozone levels (Schultz 2002, Duncan et al. 2003, Palacios-Orueta et al. 2004). Other global fire mapping studies include those of Dwyer et al. (2000), who determined the spatial and seasonal distributions of active fires at the global scale between April 1992 and December 1993, and Riaño et al. (2007), who identified global patterns of fire frequency, seasonality, and periodicity for different land-cover types using 20 years of AVHRR data and established correlations with environmental variables.
In the 1970s, reports of the southward advance of the Sahara Desert caused increased concern about human-induced desertification (Lamprey 1975, Desert Encroachment Control and Rehabilitation Programme 1976, Smith 1986, Lamprey 1988, Suliman 1988). Based on a survey of about 250 regional soil degradation experts, the Global Assessment of Human-Induced Soil Degradation also reported extensive worldwide desertification (Oldeman et al. 1991). Desertification became the dominant theme of an environmental convention, the United Nations Convention to Combat Desertification, which emerged from the Rio summit of 1992.
Satellite data sets have played a critical role in assessing the role of human activities in desertification. Using the long time series AVHRR record, a study by Tucker et al. (1991) discredited the widely held claims of desertification in the Sahel. The authors found that a satellite-derived vegetation index was highly correlated to measurements of rainfall over the 1980-1990 period, thereby suggesting that vegetation in the Sahel was simply responding to interannual rainfall changes rather than any human-driven causes. Another study by Prince et al. (1998) using AVHRR data for 1982-1990 also found that vegetation productivity was marching in lockstep with precipitation changes and found no evidence for a human hand. Indeed, the wetter conditions prevailing since 1994 seem to be associated with a gradual recovery in vegetation (Anyamba and Tucker 2005). Measuring and attributing desertification remains difficult because a wide variety of environmental changes are taking place at a range of spatial and temporal scales (Reynolds and Stafford-Smith 2002, Reynolds et al. 2007).