Abstract: This paper synthesizes and updates the analyses on precipitation change in Northwest China previously reported by Chinese researchers,and discusses about the possible causes for the observed trends in precipitation.Annual precipitation in the region has been found to increase over the past half a century,with the increase in western part of the region more significant.The last decade witnesses a further rise in annual precipitation compared to 1990s.Analyses using tree-ring data from a few sites show that the increase of precipitation since the beginning of 1980s might have been unprecedented in terms of the last 2 centuries; however,more reconstructions using tree-ring data from across the region indicate that the present precipitation might not have been so abnormal for the past centuries because lager than present precipitation ever appeared in a few of decades in history.Considering the observed and proxy data in combination,it is not unlikely that the evident increase in annual or seasonal precipitation over the last decades might have been the consequence of natural climate variability on decadal to multi-decadal time scales.Updated analysis of the net moisture budget and precipitable water using NCEP and ERA reanalysis data shows that no significant increase in moisture transport into the region across the western,southern and northern boundaries,and the significant increase in net moisture flux are mainly due to the large decline of moisture transport out of the region across the eastern boundary,during the time period 1979-2012.This implies that the observed increase in precipitation in the region might not have been explained as a response to global warming,in spite of the fact that almost all climate models driven by the increased CO₂ concentration in atmosphere for both past and future have produced the upward precipitation trends consistent with observations,indicating that the probability that the increased precipitation in the region is a response to anthropogenic global warming is larger as compared to the other regions of the country including North China.On the other hand,evidences are accumulating that the expanded irrigation areas in Oasis and cities in Northwest China have caused the increase in actual evaporation,resulting in the increase in atmospheric moisture and precipitation observed at the meteorological stations.Accelerated melting of the mountainous glacial due to regional climate warming and the black carbon deposition on ice and snow surface,and theresulting rise of lake water levels might have provided extra moisture into local atmosphere.The paper also makes a first-order projection of precipitation in 2-3 decades to come.It is estimated that annual and summer precipitation in Oasis and cities of Northwest China will be likely to increase assuming the enhanced human activities and urbanization.This will not rule out the possibility that natural climate variability on multi-decadal scales will dominate the precipitation trends for the coming 2-3 decades,and will make the locally anthropogenic increase of precipitation less evident for certain places and time periods.

Key words: climate change, precipitation, tree-ring, moisture flux, human activity, natural variability, projection, arid region, Northwest China