Grounded jets solve a mystery
Date: November 18 2002


By Deborah Smith, Science Writer

The unique atmospheric conditions following last year's terrorist attacks on the United States have helped Australian scientists solve a 50-year-old mystery: why bodies of water, like backyard swimming pools, are evaporating much more slowly than decades ago, despite global warming.

In the aftermath of September11, more than 10,000 aircraft were suddenly grounded for three days.

"It was like a big experiment you can't normally do," said Michael Roderick of the Australian National University and Co-operative Research Centre for Greenhouse Accounting.

People across the US reportedly remarked at how clear the skies became.

Dr Roderick and Professor Graham Farquhar used data collected during these three days to show that increased atmospheric pollution and cloud cover blocking direct sunlight from reaching the Earth has been responsible for the "pan evaporation paradox".

They said their results, published in the journal Science, showed much more attention needed to be given to reduced sunlight when trying to understand global climate change.

Compared with 50 years ago, about 15 per cent less water, on average, is evaporating now from terrestrial bodies of water, according to measurements around the globe by scientists monitoring special evaporation pans.

Yet in the same period the world's temperature has been rising by an average of 0.15 degrees each decade.

High-flying jets had been suspected of affecting climate because their contrails, or water vapour exhaust trails, can turn into cirrus-like clouds, which can act as insulators, reflecting the heat of the sun, as well as trapping heat below.

A US team confirmed this effect earlier this year. During the three days of clear skies in September the difference between the average day-time maximum temperature and night-time minimum across the US was more than one degree greater than when contrails were present.

Until this discovery it had been thought that air near the pans was getting more humid because of increased evaporation from surrounding areas.


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