German and UK scientists have challenged the idea that the climate was significantly influenced by the absence of contrails when the US FAA grounded flights after the events of 11 September 2001.

According to US scientists who studied US skies after the temporary grounding, the absence of contrails triggered variations in the Earth's temperature range by 1.1°C each day.

But follow-up work by a number of scientists working independently has shown that the observed change in the daily temperature range was more likely to be a statistical quirk associated with the weather, and that contrails by themselves are likely to have had only a minor effect.

"The theory is that contrails suppress daily temperature range by cooling daytime temperatures and warming nighttime temperatures, so in their absence the daily temperature range increases," says University of Leeds professor Piers Forster.

But the German and UK studies, which incorporated contrails into their climate models, actually found that contrails over the USA only suppress the range by a tiny amount.

The UK study, led by Leeds University which joined forces with the Met Office within the aviation research initiative Omega, found it would take 200 times as many flights over the USA as there are today to see daily temperature range changes approaching those suggested by the US work, conducted by David Travis of the University of Wisconsin.

Further US studies by research scientist Gang Hong and colleagues have re-examined the temperature data for the USA, looking not only at the 2001 data but going back to earlier Septembers.

They found that such 1°C changes in temperature range were not uncommon and that the 2001 change was most likely caused by changes in wind direction affecting low cloud cover.

Source: Air Transport Intelligence news