Changing Climate and Overgrazing are Decimating Mongolian Steppes.

Satellite observations have identified the Mongolian steppes as a hotspot of global biomass reduction, the extent of which is comparable with tropical rainforest deforestation. To conserve or restore these grasslands, the relative contributions of climate and human activities to degradation need to be understood. Here we use a recently developed 21-year (1988-2008) record of satellite based vegetation optical depth (VOD, a proxy for vegetation water content and aboveground biomass), to show that nearly all steppe grasslands in Mongolia experienced significant decreases in VOD. Approximately 60% of the VOD declines can be directly explained by variations in rainfall and surface temperature. After removing these climate induced influences, a significant decreasing trend still persists in the VOD residuals across regions of Mongolia. Correlation in spatial patterns and temporal trends indicate that a marked increase in goat density, the associated grazing pressures, and wild fires are the most likely non-climatic factors behind grassland degradation.

Figure 3. Trends in VOD residuals, changes in goat density and fire hotspots. (A) Trends in VOD residuals after removing the influence of climatic factors. Only statistically significant trends (p,0.05) are shown. The analysis conducted on VOD was also applied on NDVI. No significant trends were observed in NDVI residuals and thus they are not shown here. (B) Differences in goat density (heads per square kilometer) between 2008 and 1990. (C) Annual average number of recorded fires for each 0.25u grid cell between 2001 and 2008, based on MODIS global monthly fire location product.