Has the recent global warming caused increased drying over land?

To assess potential changes in surface moisture conditions over global land from 1948 to 2002 associated with surface warming, the Palmer Drought Severity Index (PDSI) and land surface model-simulated soil water content were analyzed. The PDSI was derived using observed precipitation and temperature and a simple land surface model (Dai et al. 2004), while the model simulations were done by running the Community Land Model (CLM), a comprehensive land surface model designed for climate simulations, in an offline mode using observation-based precipitation, surface air temperature, radiation and other atmospheric forcing. The precipitation and temperature forcing data were derived by combining 6-hourly synoptic variations in the NCEP/NCAR reanalysis with monthly data derived from station records, while observed cloudiness and the ISCCP-based satellite solar radiation data were used to adjust the year-to-year variations and mean biases in the reanalysis surface downward solar radiation. Both the PDSI and CLM-simulated soil water content show a large drying trend over northern hemispheric land since the middle 1950s, with widespread drying over much of Eurasia, northern Africa, Canada and Alaska. In the-Southern Hemisphere, the land surface was wet in the 1970s and relatively dry in the 1960s and 1990s; and there was a large drying trend from 1974 to 1998 although trends over the entire 1948-2002 period are small. Decreases in land precipitation in recent decades are main cause for the drying trends, although large surface warming during the last 2-3 decades also contributed to the drying significantly.