Over the last 50 or so years, pan evaporation has been carefully monitored.  For decades, pan evaporation measurements were not analyzed critically for long term trends. But in the 1990s scientists reported that the rate of evaporation was falling.  According to data, the downward trend had been observed all over the world except in a few places where it has increased.
It is currently theorized that, all other things being equal,  as the global climate warms evaporation would increase proportionately and as a result, the hydrological cycle in its most general sense is bound to accelerate. The downward trend of pan evaporation has since also been linked to a phenomenon called global dimming. In 2005 Wild et al. and Pinker et al. found that the "dimming" trend had reversed since about 1990.
Other theories suggest that measurements have not taken the local environment into account. Since the local moisture level has increased in the local terrain, less water evaporates from the pan. This leads to false measurements and must be compensated for in the data analysis. Models accounting for additional local terrain moisture match global estimates. 
In a different view, an analysis of pan trends in records from 154 instruments shows no coherency and pattern of statistically significant trends, with 38% decreasing, 42% no change and 20% increasing. Changes in the local environment are implicated, in which increasing tree density near the pans elevating surface friction and slowing local wind runs, reducing pan evaporation. The evaporation paradox is a result of ongoing changes in the nearby environments.