posted 08-12-2002 12:19 PM            

September's clear skies showed contrails' climate impact.
© GettyImages

Clouds formed by the water vapour in the exhaust from jet planes have a small but significant effect on daily temperatures, a new study confirms.

The grounding of commercial flights for three days after last September's terrorist attacks in the United States gave David Travis at the University of Wisconsin-Whitewater and colleagues a chance they never thought they'd have: to study the true impact that contrails from jet engines have on our climate1.

"It was a tarnished golden opportunity," recalls Patrick Minnis, an atmospheric scientist at NASA'a Langley Research Center in Hampton, Virginia. Despite a wealth of experiments, it had been virtually impossible to gauge the effect of contrails because air traffic, particularly over regions such as Europe and North America, never stopped. Until 11 September 2001, that is.

Contrails left high in the atmosphere spread out into cirrus-like clouds under the right atmospheric conditions. Natural cirrus clouds - thin layers of wispy water vapour that often resemble fish scales - trap heat being reflected from the ground and, to a lesser extent, reflect some of the Sun's rays.

Travis's team compared the average daily high and low temperatures over North America from 11 to 14 September 2001, with climatic records from 1977 to 2000, matching the weather over those three days with similar weather in September from historical records.

They found that the difference between daily high and nightly low temperatures in the absence of contrails was more than 1 oC greater than in the presence of contrails. Comparing the three-day grounding period with the three days immediately before and after, the impact was even larger - about 1.8 oC.

"It's obviously a significant effect," says Andrew Carleton, an atmospheric scientist at Pennsylvania State University in University Park and a member of Travis's team.

The researchers suggest that in regions with crowded skies, contrails work just like artificial cirrus clouds, preventing days from getting too hot by reflecting the Sun's rays, and keeping nights warmer by trapping the Earth's heat.

Averaged over the globe, which is largely free of air traffic, the effect is negligible. "But locally, contrails are equally as significant as greenhouse gases," says Carleton.

posted 08-12-2002 07:08 PM            

Did this reporter, Tom Clarke, really talk to The "Esteemed" Dr. Patrick Minnis?

How could he possibly have been left with such uncertainty.

posted 08-12-2002 08:47 PM            

Scientists had suspected that the wispy condensation trails left by jumbo jets could behave like high altitude clouds and alter the climate. But constant air traffic had prevented them from testing the idea.

However, the three-day grounding of aeroplanes across North America after September 11, gave climatologist David Travis and his colleagues at the University of Wisconsin the jet-free skies they needed.

With the contrails absent, they found that daytime high temperatures were higher and night-time low temperatures were lower at 4000 weather stations across North America.

Hot and cold

The team analysed the average daily temperature range for the three-day periods before, during and after the aircraft grounding, and compared them with the same periods from 1971 to 2000.

The temperature range both before and after the grounding showed little difference from the average. But during the three day aircraft grounding the temperature range was 1.1 °C greater than average.

The loss of cloud cover is known to have this effect and therefore supports the team's argument that contrails behave as artificial cirrus clouds, insulating the Earth by reflecting sunlight from above and retaining heat from below.

The study noted that the regions that usually have the greatest number of contrails, such as the northeast, midwest and pacific northwest, showed the greatest increase in temperature range during the aircraft grounding.

Local weather

However, John Mitchell from the UK's Met Office warns that this could simply be due to variations in local weather during the study period. "Whilst it's an intriguing study I'm not sure how much one can deduce from it since the effect of contrails is not clearly separated from that of natural cloud coverage", he says.

"We are not saying the temperature changes were solely due to absent contrails. Weather patterns across the country obviously contributed to our results," Travis replies.

"But even so the range of temperatures measured during the aircraft grounding was the largest for at least 30 years, which suggests that contrails are in someway enhancing the blanketing effect of clouds," he told New Scientist.

West Nile virus

The increased insulation from contrails may seem to be good news in terms of heating and cooling bills, but according to Travis even small changes in climate can have widespread affects.

Insect populations, for example, are kept in check by extremes of temperature. Less variable climates could potentially result in larger mosquito populations, with increased risks of malaria and West Nile virus. The latter is currently sweeping across the US.

Many plants are also affected by temperature extremes. For example, maple syrup can only be tapped after nights when the temperature drops to —3 °C.