The idea that the absence of contrails caused by jet aircraft had a significant influence on the climate post-9/11 has been challenged by a team of UK and German scientists.
According to US scientists who studied US skies after the September 11 terrorist attacks in 2001, the absence of artificial clouds caused by the grounding of all civil aircraft triggered variations in the earth's temperature range by 1.1 degrees Celsius (2 degrees Fahrenheit) each day.
But follow-up work by a number of scientists working independently have shown that the observed change in the daily temperature range or DTR was likely a statistical quirk associated with the weather and that contrails by themselves are likely only to have a very minor effect on DTR.
After all, they reckon, such a 1 degree Celsius change in climate change terms would be a very large effect - for comparison, global warming over the last 100 years is pegged at around 0.7 Celsius.
"The theory is that contrails suppress DTR by cooling daytime temperatures and warming night-time temperatures, so in their absence DTR increases," Professor Piers Forster from the University of Leeds tells Future Proof.
The UK and German studies which incorporated contrails into their respective state-of-the-art climate models actually found that contrails over the US do indeed suppress DTR, but only by a tiny amount.
The UK study led by Leeds University within the aviation research initiative Omega which joined forces with the UK Met Office found that it would take 200 times as many flights over the US as there are today to see DTR changes approaching those seen in the US work conducted by David Travis of the University of Wisconsin.
A further US study by Hong and colleagues has re-examined the temperature data for the US, not only looking at the 2001 data but going back to earlier Septembers. They found that such 1 degree Celsius 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.
And for those non-believers, here are the academic references:
Dietmuller, S., Ponater, M., Sausen, R., Hoinka, K.P., and Pechtl, S., 2008: Contrails, natural clouds, and diurnal temperature range. Journal of Climate, 21(19), 5061-5075.
Forster and Rap, OMEGA study http://www.omega.mmu.ac.uk/Studies/Adding%20to%20a%20climate%20model.pdf
Hong, G., Yang, P., Minnis, P., Hu, Y.X., and North, G., 2008: Do contrails significantly reduce daily temperature range?. Geophysical Research Letters, 35(23), L23815.
Travis, D.J., Carleton, A.M., and Lauritsen, R.G.,2002:Climatology: Contrails reduce daily temperature range - A brief interval when the skies were clear of jets unmasked an effect on climate. Nature, 418(6898), 601-601.
