Background
Significant declining trends in pan evaporation have been detected in the United States and Russia from the 1950s to 1990s (Peterson et al., 1995; Golubev et al., 2001), in China for the period 1955-2000 (Liu et al., 2004), in India from 1961 to 1992 (Chattopadhyay and Hulme, 1997), in Thailand between 1982 and 2000 (Tebakari et al., 2005), and in Australia from the 1970s to 2002 (Roderick and Farquhar, 2004). Typical values of the rate of decline are in the range of 2-4 mm per annum per annum (mm a^-2), with the exception of India, where values as great as 12 mm a^-2 have been observed in many places.

What was done
Since the only regional set of declining pan evaporation data in the Southern Hemisphere was what Roderick and Farquhar (2004) had put together for Australia, the two scientists decided to do the same for New Zealand, to see if the phenomenon was indeed expressed throughout most of the planet's major land areas.

What was learned
Roderick and Farquhar (2005) report that averaged across the 19 New Zealand sites they studied, "the decline in pan evaporation was roughly 2 mm a^-2 since the 1970s." Hence, they concluded that "the trend for decreasing evaporative demand previously reported throughout the Northern Hemisphere terrestrial surface may also be widespread in the Southern Hemisphere."

What it means
Because of the essentially global expression of this intriguing phenomenon, the two Australian researchers speculate that "some of the decrease in pan evaporation that has been observed both here and elsewhere may be an effect of greenhouse warming, arising from, for example, an increase in cloud cover (Dai et al., 1997)." If this is indeed the case, it is a most interesting development, for it means, as they wrote in their 2004 paper, that the terrestrial surface is getting "both warmer and effectively wetter," stating that "a good analogy to describe the changes ... is that the terrestrial surface is literally becoming more like a gardener's 'greenhouse'," which sounds to us like a great trajectory for earth's plant life to be on.

References
Chattopadhyay, N. and Hulme, M. 1997. Evaporation and potential evapotranspiration in India under conditions of recent and future climate change. Agricultural and Forest Meteorology 87: 55-73.

Dai, A., DelGenio, A.D. and Fung, I.Y. 1997. Clouds, precipitation and diurnal temperature range. Nature 386: 665-666.

Golubev, V.S., Lawrimore, J.H., Groisman, P.Y., 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.

Liu, B., Xu, M., Henderson, M. and Gong, W. 2004. A spatial analysis of pan evaporation trends in China, 1955-2000. Journal of Geophysical Research 109: 10.1029/2004JD004511.

Peterson, T.C., Golubev, V.S. and Groisman, P.Y. 1995. Evaporation losing its strength. Nature 377: 687-688.

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

Tebakari, T., Yoshitani, J. and Suvanpimol, C. 2005. Time-space trend analysis in pan evaporation over kingdom of Thailand. Journal of Hydrologic Engineering 10: 205-215.