What was done
The authors derived five regional trends of pan evaporation from data obtained from 746 homogeneous reporting stations in the United States - (1) Eastern US, (2) Western US - and 190 such stations in the former Soviet Union - (3) European, (4) Middle Asian, (5) Siberia - for the last half of the 20th century.  The US data employed were averages of mean daily pan evaporation from May to September, while the data from the former Soviet Union were for months when there was no ice in the pans.

What was learned
All of the regions exhibited downward trends in pan evaporation over the last half of the 20th century, with four of them (all but the Middle Asian region) registering significance at the 99% level or better.

What it means
In the words of the authors, "the downward trend in pan evaporation over most of the United States and former Soviet Union implies that, for large regions of the globe, the terrestrial evaporation component of the hydrological cycle has been decreasing."  They also note that this observation "corresponds well with both decreases in maximum summer temperatures over these regions and a decrease in growing-season degree-days over the Siberian and European former Soviet Union."  Speculating on the cause of these related trends, the authors suggest "increases in cloud cover, especially low cloud cover," since "pan evaporation has been decreasing and is correlated negatively with cloud cover ... for the five regions."

Clearly, these related decreasing half-century trends in pan evaporation, maximum summer temperatures, and growing-season degree-days are all just the opposite of what would be expected in the climate alarmists' illusionary world of "unprecedented" global warming.  But, they are just what would be expected in the real world of observed contemporaneous reductions in solar radiation reaching the planet's surface (see our Journal Review Solar Radiation Reductions at the Earth's Surface).  Hence, we can only conclude, as the data always force us to conclude, that the unprecedented warming so highly hyped by those who yearn for adoption of the Kyoto Protocol is nothing more than, well, nothing.

Addendum
In response to the work of Brutsaert and Parlange (1998), who demonstrated that trends in pan evaporation and actual evaporation may well be opposite, Golubev et al. (2001) conducted a reassessment of the work of Peterson et al. (1995).  Using "parallel observations of actual and pan evaporation at six Russian, one Latvian, and one U.S. experimental sites," they recalibrated trends in pan evaporation "to make them more representative of actual evaporation changes."  When this was done, they found that "pan evaporation time series over southern Russia and most of the United States reveal an increasing trend in actual evaporation during the past forty years."

In contrast to what we originally concluded above, on the basis of the earlier work of Peterson et al., this new finding is not in conflict with the predictions of current climate models.  However, it does not exactly jibe with Peterson et al.'s comments about there being "decreases in maximum summer temperatures over these regions and a decrease in growing-season degree-days over the Siberian and European former Soviet Union."

So what's the bottom line with respect to this issue?  It's hard to say.  In all likelihood, the final word on the subject has yet to be written.

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.