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Fifty Years of Pan Evaporation and Solar Radiation Data: What Do They Tell Us About Climate Change?
Reference
Roderick, M.L. and Farquhar, G.D. 2002. The cause of decreased pan evaporation over the past 50 years. Science 298: 1410-1411.

Background
"As the average global temperature increases," say the authors, "it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase." However, as they continue, "terrestrial observations over the past 50 years [when they assume the planet warmed by ~0.15C] show the reverse." Hence, they set out to explain this seeming "paradox."

What was done
Based on measurements of total solar irradiance (RS) from the northwest part of the former Soviet Union (49-67N) for the period 1960-1990, which revealed RS to have decreased by 2 to 4% per decade, the authors used an energy balance approach to calculate the expected change in annual pan evaporation, comparing their result with the observed change in this parameter over the same region and time period.

What was learned
The authors ended up demonstrating that the observed decrease in pan evaporation is both qualitatively and quantitatively consistent with "what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration."

What it means
Based on this finding and assuming the data used to derive it are correct, either (1) the general expectation that global warming should lead to significantly increased pan evaporation is wrong or (2) there was little to no net warming over the period 1960-1990.

Each of these alternatives tends to undercut the conceptual foundation for climate-alarmist claims of significant CO2-induced global warming over this period; for they suggest the existence of fundamental deficiencies in our understanding of either (1) basic meteorological phenomena, as in the first case, or (2) the temperature history of the earth, as in the second case.


Reviewed 8 January 2003