Bill Holvey, of Halswell, asks :-

Mass jet travel introduce a lot of water vapour into the hitherto fairly dry stratosphere. Does this have any effect on climate?

Olaf Morgenstern, an atmospheric scientist with NIWA at Lauder, responded.

The answer, for the gaseous water vapour part, is no, not really. If the water vapour is injected into the troposphere (the lower atmosphere), its residence time is measured in days, and air traffic does not contribute measurably to the cycling of water through the lower atmosphere.

If it is injected into the stratosphere (above ca 10 km), it stays in the atmosphere for longer, maybe for months, and contributes to global warming. I have read numbers such as 10 milliWatts per square metre. This compares to man-made carbon dioxide which at present contributes perhaps 2 Watts per square metre, so water vapour from aircraft likely contributes less than one per cent to man-made global warming. Unlike carbon dioxide, water vapour emissions do not accumulate.

Aircraft also, under some conditions, produce contrails, high clouds that trap heat. The regional impact of these can be much larger in regions with a lot of air traffic, but are harder to quantify because they are so variable, short-lived, and also do not accumulate.

Rockets, which go through even higher altitudes in order to launch satellites and resupply the International Space Station, produce a variety of different exhaust products, depending on the type of fuel used. There is some concern about the soot (“black carbon”) produced by some rockets. At present, the impacts of these on climate do not compare with other human activities.

There is also some concern that particles produced by solid-fuel rockets may promote ozone depletion. However, both the climate impact and the ozone depletion would also require a considerable increase in the number of rocket launches (e.g. in the context of space tourism) to become a serious problem.