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The Hydrologic Cycle on the Tibetan Plateau: 1961-2000
Reference
Shenbin, C., Yunfeng, L. and Thomas, A. 2006. Climatic change on the Tibetan Plateau: Potential evapotranspiration trends from 1961-2000. Climatic Change 76: 291-319.

What was done
Monthly potential evapotranspiration estimates for the Tibetan Plateau (TP) were calculated by the Penman-Monteith equation using meteorological data from the Meteorology Center of the National Meteorology Bureau of the Peoples' Republic of China for 101 stations having records for the 40-year period 1961 to 2000.

What was learned
For the Tibetan Plateau as a whole, the authors report that "potential evapotranspiration (PET) has decreased in all seasons," such that "the average annual evapotranspiration rate decreased by 13.1 mm/decade," or 2.0% of the annual total, over the course of their four-decade study. Their analyses also revealed that wind speed was "the most important meteorological variable affecting changes in PET rates on the TP," and they write that "decreasing wind speeds have led to decreasing PET rates," which finding, in their words, is "in accordance with Barry (1992) who argues that wind is probably the most important factor controlling PET rates in all high altitude environments."

What it means
Shenbin et al. conclude that "decreased wind speeds as the primary cause of decreasing PET rates point to changes in the strength of the local circulation system (the monsoon) which in turn would affect a far larger region than the TP alone," noting that "decreases in the strength of the regional circulation system of the Asian monsoons which may lead to the observed reductions in wind speed on the TP could be responsible for observed lower PET rates in recent years." This finding, they state, "is in contrast [our italics] to predicted increased monsoonal activity and an increased hydrological cycle under global warming scenarios (IPCC, 2001)."

References
Barry, R.G. 1992. Mountain Weather and Climate. Methuen, London, United Kingdom.

IPCC. 2001. Climate Change 2001. The Scientific Basis. (Houghton, J.T., Ding, Y., Nogua, M., Griggs, D., Vander Linden, P. and Maskell, K., Eds.) Cambridge University Press, Cambridge, United Kingdom.

Reviewed 22 November 2006