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.