Researchers solve evaporation paradox

Many climate researchers have been unable to explain why evaporation figures have decreased in areas where rainfall and cloudiness have become more prevalent

December 4, 1998
Web posted at: 3:20 PM EST

By Environmental News Network staff

(ENN) -- Two university professors have a solution for the "evaporation paradox" that they believe explains why evaporation rates have been consistently lower at weather stations around the world even through all other data leads scientists to believe that global warming is occurring.

Indeed, recent temperature, precipitation, stream flow and cloud cover records indicate that global warming is occurring, but scientists have been vexed by lower evaporation rates recorded from metal pans at weather stations around the world.

Some climate researchers have asked how could there be less evaporation when more rain and snow are falling? This problem, dubbed the "evaporation paradox," caught the attention of Marc B. Parlange, a professor in the Johns Hopkins University department of Geology and Environmental Engineering, and Wilfried Brutsaert a professor of Civil and Environmental Engineering at Cornell University.

Parlange and Brutsaert say that evaporation pan records do support predictions made by the climate computer models that are used to simulate the impact of increased greenhouse gases. The paradox arose, they say, because many researchers have misinterpreted the evaporation measurements.

"The evaporation pans are a great tool to study climate," said Parlange. "But their data have to be used correctly."

To probe the paradox, the researchers focused on the Earth's hydrologic cycle, in which water falls to the ground as rain or snow, soaks into the soil or moves into bodies of water, then evaporates to form clouds that start the cycle once again.

Over the past two or three decades, data collected in parts of the United States, the former Soviet Union, India and Venezuela have indicated that the pace of this hydrologic cycle is picking up. In other words, it has been taking less time to complete each cycle of precipitation through evaporation.

Scientists disagree on the reason. It may be a natural phenomenon or one that humans have triggered by releasing more carbon dioxide. But the existence of this accelerated water cycle is now widely accepted.

Nevertheless, many climate researchers have been unable to explain why evaporation figures have decreased in areas where rainfall and cloudiness have become more prevalent. If less water vapor is moving into the atmosphere, how could more rain and clouds be forming?

To explain the paradox, Parlange and Brutsaert looked at the way evaporation is recorded. Weather stations throughout the world usually place water-filled metal pans, about a foot deep and three feet in diameter, outdoors on wooden platforms. Each day, a technician measures how much water has disappeared from the pan.

But Parlange and Brutsaert found that decreasing pan evaporation does not necessarily mean that less evaporation is occurring in the surrounding landscape. A key reason is that the interpretation of the pan measurements has not taken into account the role of humidity in the air or the moisture that is already present in or missing from the surrounding landscape.

For example, a pan of water placed outside in a hot, dry desert would evaporate very quickly. But put the same pan in a cool rain forest, and the water would evaporate much more slowly.

Thus, evaporation figures may drop in some areas merely because more rain and snow have saturated the terrain. This "regional land-surface moisture" must be factored in when climate researchers interpret raw measurements from evaporation pans, the researchers say.

When it is, the paradox disappears, and evaporation rates fall in line with other signs of climate change, the researchers say.

Parlange and Brutsaert published their research in the Nov. 5 issue of Nature. In their article, they assert that "in many situations, decreasing pan evaporation actually provides a strong indication of increasing terrestrial evaporation."

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