Will climate change increase or decrease evaporation? The simple view is that as temperature increases, so will evaporation. However when researchers have actually looked at measurements of pan evaporation, it is decreasing 1.
It turns out there are multiple competing effects. Temperature is increasing but so is the amount of CO2 in the atmosphere. Carbon dioxide is a great plant fertilizer so we would expect more plant growth and in fact that is happening. The Earth is getting greener 2, and in particular, the leaf area is increasing as measured by Leaf Area Index (LAI)3. LAI is leaf area divided by land area.
More leaves means more evapotranspiration from plants right? Not necessarily. With higher atmospheric CO2 plants use water more efficiently4. The stomata on the leaves don’t have to open so much which decreases evapotranspiration. The increased CO2 in the atmosphere works to both increase evapotranspiration – by increasing leaf area – and decrease evapotranspiration – by increasing the efficiency with with plants use water.
Recent work has quantified all these effects5. It is now clear that plant water use efficiency is increasing but this is outweighed by the increase in leaf area. Evapotranspiration from plants is increasing. Also, the increase in the leaf area means more rainfall is intercepted before it reaches the ground and this intercepted water is also evaporated.
More evaporation from plants means more water in the air and therefore less evaporation from meteorological instruments such as pans. This could explain the measured decrease in pan evaporation in recent decades. However climate change influence on evaporation is likely to be specific to location. The ocean contributes most evaporation and recent work suggests the water cycle is intensifying i.e. more rainfall and more evaporation. This also suggests that wet areas are become wetter and dry areas drier6. Whether there is increased or decreased evaporation as measured by a pan depends on where the pan is.
References and further reading
1. Roderick, M. L. and G. D. Farquhar (2004) Changes in Australian pan evaporation from 1970 to 2002. International Journal of Climatology 24(9): 1077-1090. (link)
2. Zhu, Z et al. (2016) Greening of the Earth and its drivers. Nature Climate Change doi:10.1038/nclimate3004
3. Curry, J. (2016) Rise in CO2 has greened planet Earth (link).
4. Frank, D. C. et al. (2015) Water-use efficiency and transpiration across European forests during the Anthropocene. Nature Climate Change doi:10.1038/nclimate2614 (link).
5. Zhang, Y. et al. (2016) Multi-decadal trends in global terrestrial evapotranspiration and its components. Scientific Reports 6:19124 doi:10.1038/srep19124
6. Durack, P. J., Wijffels, S.E. and Matear, R. J. (2012) Ocean salinities reveal strong global water cycle intensification during 1950 to 2000. Science 2012 April 27 336(6080):455-8. doi:10.1126/science.1212222