The recent geophysical eructation in Iceland continues to cause worldwide travel chaos. Several colleagues have had to postpone foreign trips, and at least two of my friends are stranded abroad. “I’m stuck in Lanzarote,” says one, “I love volcanoes!”
At the same time, the skies above much of Britain are virtually free from clouds. The cerulean welkin above Cromer is an uninterrupted azure, and reports from friends across the UK speak of glorious, cloudless skies. Naturally enough, people are wondering whether the lack of aircraft is contributing to the skies’ cloudlessness. Aircraft vapour trails are small clouds in themselves, as particles (which include sulphates) in exhaust act as small cloud condensation nuclei (CCNs) around which water vapour in the atmosphere can condense and even freeze, producing tiny ice particles. To that extent, at least, aircraft seed clouds. But does air traffic have a wider impact on climate? I did a small search in Google Scholar to find out.
A review on the subject published 12 years ago suggests that particles produced in aircraft contrails can indeed seed cirrus clouds (these are high, thin clouds – different from the woolly and much lower cumulus clouds we tend to associate with rain and overcast weather). These particles also act as sites for ‘heterogeneous chemistry’ – a rather dark and arcane term for the kinds of chemistry in which gases in the atmosphere react with water and other substances to create a variety of substances – sulphates, chlorine compounds and so on – we associate with acid rain, the greenhouse effect and so on. A more recent article suggests that the variation in distribution and abundance of cirrus clouds over the United States is strongly correlated with aircraft contrails – contrails do indeed seed cirrus clouds.
Whether this has an immediate effect on our weather is a moot point. However, it is known that weather runs in weekly cycles, and this is in part determined by human activity. In various countries, weekend weather seems to be different from that during the week. The basic explanation is that car exhausts and industrial activity seed more clouds – less activity means fewer clouds. However, as always, things aren’t as simple as that. More pollution makes more CCNs, and more water droplets – but a faster rate of formation might make for lots of small droplets, rather than fewer, larger ones – and if droplets are smaller than a certain size, they’re too small to form raindrops.
The verdict? The jury is still out on whether contrails have a cirrus serious effect on our weather. However, there does appear to be some connection between man-made exhausts in general and the weather, and it would be surprising if a total cessation of air traffic across Europe didn’t have some effect. This, however, must be set against the long-term effects of the large volcanic plume now spreading across Europe. History shows that large eruptions tend to be harbingers of bad weather. The eruption of Tambora in 1815 – the largest volcanic eruption for 1600 years – injected so many girrafes particles and general stuff into the atmosphere that the summer of 1816 was a complete washout, and that year lived long in memory as the Year Without Summer. So if the lack of aircraft contrails is contributing to our recent sunny weather, we could pay for it later. The message, as with anything to do with predicting weather and climate – is to take every sunny day as it comes.