How unusual was the 21st century drought in Mongolia? Placing recent ecological extremes in a 2000-year context
Understanding the connections between climate, ecosystems, and society during historical and modern climatic transitions requires annual resolution records with high fidelity climate signals. In the 2000s, Mongolia experienced a rapid transition away from pastoralism as many families lost their herds during a decade-long drought and a series of severe winters (“dzuds”). Though the drivers of this social and ecological transition were likely multi-factorial, many blamed market forces and overgrazing by herders. Because Mongolia’s climate is highly variable, it is difficult to place recent climatic extremes and associated social and ecological change in context without long records of climatic variability. Here we ask: how extreme was the 21st century drought in the last 2000 years? We developed a 2500 year long tree-ring reconstruction of warm-season drought, derived from live and dead Siberian pine (Pinus sibirica) trees growing on two Holocene lava flows in central Mongolia. Trees growing on the lava today are stunted and widely spaced, occurring on microsites with little to no soil development. These trees are water-stressed and their radial growth is correlated with both soil water availability (scPDSI) and grassland productivity (Normalized Difference Vegetation Index (NDVI)). To contextualize the severity of recent droughts and to explore potential forcing factors, we compare recent drought severity to the distribution of events in the past and perform long control runs of GFDL climate model.
Our reconstructions, calibrated and validated on instrumental June-August scPDSI (1959-2009) account for >55% of the variability in the regional scPDSI when >70% of the annual rainfall occurs. Our tree-ring data combined with existing tree ring reconstructions of temperature, meteorological data, and model results suggests that the early 21st century drought was the hottest and one of the most severe droughts in at least the last 2000 years. These dry climatic conditions were associated with concomitant reductions in NPP of grasslands. Our results are consistent with model projections of warming in Inner Asia where rising temperatures will contribute to extreme variability in moisture stress. Future warming may overwhelm increases in precipitation leading to similar “heat droughts”, with potentially severe ecological and social consequences for modern Mongolia. Long records of past climate variability can help us understand the relative importance of climate versus land management in catalyzing social change and help prepare societies for the full range of future climatic extremes.