Satellite observed widespread decline in Mongolian grasslands largely due to overgrazing

The Mongolian Steppe is one of the largest remaining grassland ecosystems. Recent studies have reported widespread decline of vegetation across the steppe and about 70{\%} of this ecosystem is now considered degraded. Among the scientific community there has been an active debate about whether the observed degradation is related to climate, or over-grazing, or both. Here, we employ a new atmospheric correction and cloud screening algorithm (MAIAC) to investigate trends in satellite observed vegetation phenology. We relate these trends to changes in climate and domestic animal populations. A series of harmonic functions is fitted to Moderate Resolution Imaging Spectroradiometer (MODIS) observed phenological curves to quantify seasonal and inter-annual changes in vegetation. Our results show a widespread decline (of about 12{\%} on average) in MODIS observed normalized difference vegetation index (NDVI) across the country but particularly in the transition zone between grassland and the Gobi desert, where recent decline was as much as 40{\%} below the 2002 mean NDVI. While we found considerable regional differences in the causes of landscape degradation, about 80{\%} of the decline in NDVI could be attributed to increase in livestock. Changes in precipitation were able to explain about 30{\%} of degradation across the country as a whole but up to 50{\%} in areas with denser vegetation cover (P < 0.05). Temperature changes, while significant, played only a minor role (r(2) = 0.10, P < 0.05). Our results suggest that the cumulative effect of overgrazing is a primary contributor to the degradation of the Mongolian steppe and is at least partially responsible for desertification reported in previous studies