Do aircraft contrails negate global warming?

28 March 2010

Question

Hi

Just listened to your last Sunday's broadcast and heard at the end that you're talking about planes vapour (or con) trails as cloud formers.

Some time ago, I heard on one of the World Service's broadcasts that, thanks to the 9/11 attacks (when they grounded all flights) people were able to work out that con trails during daylight hours were actually an assistance to prohibiting global warming whereas con trails during the hours of darkness were an aggrovator of same.

I hope you are going to be able to confirm, or otherwise, what I've previously heard.

Many thanks.

Regards
Neil

Answer

We put this to Dr Jim Haywood, from the Met Office: Ben - Does it matter whether it's day or night?

Jim - Yes, it does indeed. It's exactly what that question is about. Really, we're talking about a cooling, if you like from the reflection of sunlight and a warming during the night time which can cause a reduction in what's called the diurnal temperature range. After 9/11, there was certainly evidence of a reduction in the diurnal temperature range from a particular study. But it's quite difficult to disentangle that signal, if you like from the natural meteorological events that can occur. And when they looked at it in a little bit more detail, it became almost impossible to distinguish from other events that had nothing to do with 9/11. You still could see this signal in the diurnal temperature range, just due to the natural variability of the atmospheric system.

Ben - So it may have an impact that we can't see because it's no bigger than noise.

Jim - It's just difficult to detect. That's right.

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