The idea that the absence of airliner contrails during the brief US grounding in the wake of the 9/11 attacks had a significant influence on the climate has been challenged by a team of UK and German scientists.

According to US scientists who studied US skies after civil aircraft were temporarily grounded in 2001, the absence of artificial clouds triggered variations in the Earth's temperature range by 1.1°C (2°F) each day.

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

Sunset contrails
 © Andy Drysdale/REX Features

The claim was that such a 1°C change would have a very large effect in climate change terms because global warming over the past 100 years has been pegged at an increase of about 0.7°C.

"The theory is that contrails suppress DTR by cooling daytime temperatures and warming night-time temperatures, so in their absence DTR increases," says Prof Piers Forster of the UK's University of Leeds.

The UK and German studies that incorporated contrails into their state-of-the-art climate models actually found that contrails over the USA do suppress DTR, but only 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 DTR changes approaching those suggested by the US work conducted by David Travis of the University of Wisconsin.

A further US study by research scientist Dr Gang Hong and colleagues has re-examined the temperature data for the USA, not only looking at the 2001 data but going back to earlier Septembers. They found that such 1°C changes in DTR were not uncommon and that the 2001 DTR change was most likely caused by changes in wind direction affecting low cloud cover.

Source: Flight International