I was surprised that the impact of aircraft vapour trails, or contrails as they are known, did not feature in the GGWS programme. After all, (if I understood last year's BBC2 Horizon programme correctly) a plane's vapour trail apparently reflects more solar heat than is kept in the earth's atmosphere by the CO2 released by the burn of the aircraft engines.

You can get the full transcript from the bbc Website, but I've pasted below the key bit of the Horizon programme as it relates to vapour trails. It suggests that contrails are a key factor in Global Dimming - the term for the decrease of 22% in the latter half of the last century in the amount of solar radiation reaching the surface of the earth. Horizon reported that when all planes in the US were grounded, as happened in the US after Sept 11, surface temperature jumped. Given the proportion of CO2 which is man-made, and the proportion of man-made CO2 which is released by planes, it seems to me that, on balance we should keep planes flying (and stop moaning at people who take holidays or fly anywhere)as planes are actually helping to cool the planet which is, we are all agreed, heating up for one reason or another?

From Horizon:
DR DAVID TRAVIS: As we began to look at the climate data and the evidence began to grow I got more and more excited. The actual results were much larger than I expected. So here we see for the 3 days preceding September 11th a slightly negative value of temperature range with lots of contrails as normal. Then we have this sudden spike right here of the 3 day period. This reflects lack of clouds, lack of contrails, warmer days cooler nights, exactly what we expected but even larger than what we expected. So what this indicates is that during this 3 day period we had a sudden drop in Global Dimming contributed from airplanes.

NARRATOR: During the grounding the temperature range jumped by over a degree Celsius. Travis had never seen anything like it before.

DR DAVID TRAVIS: This was the largest temperature swing of this magnitude in the last thirty years.