What was done
The authors calculated seasonal (May-September) values of daily mean temperature, maximum temperature, minimum temperature, water vapor pressure, cloud cover, relative humidity, vapor pressure deficit and solar irradiance, as well as seasonal total precipitation and pan evaporation, for 85 stations scattered across the eight climatic regions of China, after which they (1) determined trends of pan evaporation over the last half of the 20th century in the eight regions and for China as a whole, and (2) looked for causes of the trends in contemporaneous trends of the other meteorological parameters and various combinations of them.

What was learned
Lui et al. report that "for China as a whole, pan evaporation decreased over the past half century," noting that "the decline is statistically significant (at the 99% level)," and that "the downward trend of pan evaporation in China was -29.3 mm/decade, similar to the findings reported in several other countries." As for the likely cause of the phenomenon, the researchers concluded that "the widespread decline of pan evaporation is mainly caused by decreasing solar irradiance," but that "other parameters influence or mediate its effect." With respect to the alternative concept that "increased terrestrial evaporation will increase moist air over the evaporation pan, thus reducing evaporation from the pan (Brutsaert and Parlange, 1998; Lawrimore and Peterson, 2000; Golubev et al., 2001)," they report that it "cannot explain the decline in pan evaporation seen in China," stating that "to the contrary, our analysis of China as a whole supports the previous report that the decline in pan evaporation results from a decrease in solar radiation (Roderick and Farquhar, 2002)." This conclusion is important, for as described in detail by Roderick and Farquhar (2004), it suggests that earth's terrestrial environments are becoming "effectively wetter."

What it means
The findings of Lui et al. support the conclusions of Roderick and Farquhar (2004), i.e., that because of the long-term decline in pan evaporation driven by the concomitant decline in solar radiation, "it is now clear that many places in the Northern Hemisphere, and in Australia, have become less arid," and that "a good analogy to describe the changes in these places is that the terrestrial surface is literally becoming more like a gardener's 'greenhouse'," conducive to more prolific terrestrial plant growth.

References
Brutsaert, W. and Parlange, M.B. 1998. Hydrological cycle explains the evaporation paradox. Nature 396: 30.

Golubev, V.S., Lawrimore, J.H., Groisman, P.Ya., Speranskaya, N.A., Zhuravin, S.A., Menne, M.J., Peterson, T.C. and Malone, R.W. 2001. Evaporation changes over the contiguous United States and the former USSR: A reassessment. Geophysical Research Letters 28: 2665-2668.

Lawrimore, J.H. and Peterson, T.C. 2000. Pan evaporation trends in dry and humid regions of the United States. Journal of Hydrometeorology 1: 543-546.

Roderick, M.L. and Farquhar, G.D. 2002. The cause of decreased pan evaporation over the past 50 years. Science 298: 1410-1411.

Roderick, M.L. and Farquhar, G.D. 2004. Changes in Australian pan evaporation from 1970 to 2002. International Journal of Climatology 24: 1077-1090.