Volume 9, Number 24: 14 June 2006
In an insightful article that lay undiscovered in a pile of papers on one of our desks for over a year, Roderick and Farquhar (2004) say "there has long been an expectation, e.g. by the Intergovernmental Panel on Climate Change and others, that potential evaporation will increase as the average air temperature near the surface increases." This unsettling hypothesis can readily be evaluated via long-term measurements of pan evaporation that yield a measure of the atmosphere's demand for moisture; and, in fact, such evaluations have been carried out successfully in recent years at a number of sites around the world.
The first published report on the subject revealed just the opposite of what the IPPC had suggested, indicating, in the words of Roderick and Farquhar, that "on average, pan evaporation had decreased over the USA, Former Soviet Union and Eurasia for the period 1950 until the early 1990s (Peterson et al., 1995)." In addition, they say that "subsequent reports have confirmed this to be a general trend throughout the Northern Hemisphere," citing in support of this statement the studies of Chattopadhyay and Hulme (1997) with respect to India, Thomas (2000) pertaining to China, and Moonen et al. (2002) for Italy.
In their own investigation of the subject in the much-less-studied Southern Hemisphere, Roderick and Farquhar used data for the period 1970-2002 from 31 sites in Australia, plus data for the period 1975-2002 from 61 Australian sites, to look for trends in pan evaporation and annual rainfall. In doing so, they could find no statistically significant change in precipitation, but they detected a statistically significant "decrease in pan evaporation rate over the last 30 years across Australia of the same magnitude as the Northern Hemisphere trends." Of even more significance, however, was Roderick and Farquhar's exposition of the implications of these and other researchers' findings.
The two scientists began by noting there are two major types of terrestrial surfaces: water-limited, where precipitation is less than potential evaporation, and energy-limited, where precipitation is more than potential evaporation. At the latter sites, they state that "a decrease in pan evaporation, at constant rainfall, implies that actual evaporation will decrease and runoff and/or soil moisture will increase," which is generally considered a positive outcome in most parts of the world; and they report that these predicted changes "have been observed in Russia over the last 50 years (Robock et al., 2000; Golubev et al., 2001; Peterson et al., 2002)." In water-limited environments, on the other hand, they note that "changes in actual evaporation are dominated by changes in rainfall," but they point out that "a general trend for water-limited sites is that, at constant rainfall, a decline in pan evaporation will result in an overall increase in biological productivity because of a reduction in the moisture deficit (i.e. the supply of water is more capable of meeting the atmospheric demand)," which is also a positive outcome.
Consequently, and in spite of a century or more of global warming and the IPCC's "consensus wisdom," there is ever-accumulating evidence of worldwide long-term increases in both soil moisture content (Robock et al., 2000, 2005) and biological productivity (see Biospheric Productivity (Global) in our Subject Index). In describing this happy situation, Roderick and Farquhar say "it is now clear that many places in the Northern Hemisphere, and in Australia, have become less arid," and that "in these places, the terrestrial surface is both warmer and effectively wetter." In fact, they say in their concluding sentence that "a good analogy to describe the changes in these places is that the terrestrial surface is literally becoming more like a gardener's 'greenhouse'."
Yes, the greening of planet earth continues, aided not only by the aerial fertilization and anti-transpirant effects of atmospheric CO2 enrichment, but by the atmosphere's changing temperature and moisture characteristics as well.
Sherwood, Keith and Craig Idso
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.
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.
Moonen, A.C., Ercoli, L., Mariotti, M. and Masoni, A. 2002. Climate change in Italy indicated by agrometeorological indices over 122 years. Agricultural and Forest Meteorology 111: 13-27.
Peterson, B.J., Holmes, R.M., McClelland, J.W., Vorosmarty, C.J., Lammers, R.B., Shiklomanov, A.I., Shiklomanov, I.A. and Rahmstorf, S. 2002. Increasing river discharge to the Arctic Ocean. Science 298: 2171-2173.
Peterson, T.C., Golubev, V.S. and Groisman, P.Y. 1995. Evaporation losing its strength. Nature 377: 687-688.
Robock, A., Mu, M., Vinnikov, K., Trofimova, I.V. and Adamenko, T.I. 2005. Forty-five years of observed soil moisture in the Ukraine: No summer desiccation (yet). Geophysical Research Letters 32: 10.1029/2004GL021914.
Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.
Roderick, M.L. and Farquhar, G.D. 2004. Changes in Australian pan evaporation from 1970 to 2002. International Journal of Climatology 24: 1077-1090.
Thomas, A. 2000. Spatial and temporal characteristics of potential evapotranspiration trends over China. International Journal of Climatology 20: 381-396.