The Department has not made an assessment of the effects on greenhouse gas emissions of the temporary grounding of aircraft following the terrorist attacks in New York and Washington DC on 11 September 2001.
When all air traffic in the western world was grounded after the 9/11 incident, within three days the global temperature change was 1°, and when traffic resumed it went back by 1°. Is not that a very significant scientific statement? Will my hon. Friend ensure that the Department looks into why this happened, and the impact that it should have on our future policies? I asked this question of her predecessor five years ago, and I would appreciate an answer.
I am grateful to my hon. Friend—and it has not taken me five years to dream up an answer. First, this is not a question of greenhouse gases. The scientific interest that followed the grounding of the aircraft was do with the issue of contrails, which are the evaporation and condensation trails emitted by aircraft. We do not know the science of contrails very clearly, but there are two possible effects: first, that they reflect radiation back beyond the earth, and therefore have a cooling effect—or, secondly, that they become cirrus clouds and trap radiation, and therefore have a warming effect. The phenomenon that was observed was thought possibly to be due to the absence of contrails leading to a heating effect. However, the Department has followed subsequent studies, and we now believe that there is no evidence that contrails, or the lack of them, were responsible for the temperature rise observed at the time, and that it was a natural fluctuation.