Large-scale monitoring effort indicates a decline in the global land evapotranspiration trend

More than half of the solar energy that reaches the Earth and is absorbed by land srfaces is used for evaporation of water. Evapotranspiration is a joint process of moisture evaporation from the land and transpiration from plants. Recently a survey (Nature 2010, 467, 951) was completed taht analyzed and generalized data from 253 globally distributed water vapor flux measurement and monitoring stations over a time span of 1982 to 2008. The results indicate that during 1982 to 1997 there has been an increase of the global annual evapotranspiration by 7.1 ± 1.0 mm. After that period increase of the global evapotranspiration seems to have ceased. The prime reason for this is the decrease of the average global soil moisture level.

A question of utmost importance is whether these changes reflect the natural climate variability or indicate a trend in the global water cycle, possibly being a consequence of a global climate change.

The key to answering this question is the quality of the measurement data provided by the monitoring stations. Two aspects are critical: (1) The temporal stability of the measurement systems, so that measurement data obtained over a long time period would be comparable and (2) comparability of data between stations operating in different locations around the globe.

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