A decades-long warming and drying trend on the Tibetan Plateau has drawn increasing interest in recent decades as the plateau becomes more desert-like. At an average altitude of more than 4000 meters, the plateau covers most of Tibet and parts of China and India. In spite of recent records showing a hiatus in global warming since 2000, the plateau’s complex climate has shown a consistent rate of warming. Most rain-gauge records and station measurements show upward trends in precipitation, lake areas and water levels. To reconcile conflicting observations, a team, including a scientist from the Department of Energy’s Pacific Northwest National Laboratory, quantified changes in aridity on the plateau and attributed these changes to several climatic factors.

The team used the ratio of precipitation to potential evapotranspiration (P/PET) as an aridity index and used meteorological records at 83 stations on the plateau to calculate PET. They analyzed spatial and temporal changes in P/PET from 1979 to 2011 and found stations  on the arid and semi-arid northwestern plateau became significantly wetter, while half the stations on the semi-humid eastern plateau became drier—but not significantly. Both spatial patterns and temporal variations of aridity correlate significantly with changes in precipitation, sunshine duration, and diurnal temperature range. Precipitation is the dominant driver of P/PET changes at an inter-annual timescale. Temperature changes do not directly alter aridity, but combined with thermal wind variations, they may lead to precipitation variations that are amplified by circulation feedback. In the warm season, there is also a significant correlation between aridity and PET when snow melts. Although climate monitoring over a large, diverse area is challenging, the trends identified in this study are consistent between point measurements and gridded data.