Aridity trend over northern China is not an isolated phenomenon, but with significant worldwide linkage. The Palmer Drought Severity Index (PDSI) is the most prominent index of meteorological drought used in the United States (Heim 2002). The PDSI was created by Palmer (1965) with the intent to measure the cumulative departure (relative to local mean conditions) in atmospheric moisture supply and demand at the surface. A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived by Dai et al. (2004) using historical precipitation and temperature data for global land areas on a 2.5°grid.
Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r=0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snow-melt plays an important role. Basin-averaged annual PDSI covary closely (r= 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Nino - Southern Oscillation (ENSO)-induced mode of mostly interan-nual variations as the two leading patterns. The global very dry areas, defined as PDSI<-3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI>+3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ~20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.
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