The Secret Greenhouse Gas
29.8.06
GWA Science Correspondent

It’s not as sexy or scary as the greenhouse gases we read about every day in the papers such as carbon dioxide, methane or nitrous oxide. Barely a day passes without some pronouncement on what has to be done to reduce these products which we are all producing in too great an abundance for our future wellbeing. But what about poor old water vapour? Why is it being ignored?
After all it is by far the most plentiful comprising between 80 or even 98% of the total of greenhouse gasses, depending on which study you particularly fancy.

Could the reason be that because there is not much we can do about it, or even worse, that the implications of what is occurring in the stratosphere with respect to water vapour content, make too unpalatable reading?

Well, what is happening up there in that layer above the troposphere? The stratosphere starts at 10km (about 33000 ft,a popular level for aircraft to cruise at), and extends up to 50 km. As a vehicle climbs through the stratosphere, the temperature gradually rises although at a much lower rate and in an inverse sense, to the troposphere.

At the top of the layer the temperature is virtually the same as at sea level, around 15 degrees Celsius. Just as the boundary layer at the top of the troposphere is called the tropopause, so consistently is named the stratopause defining the boundary at the top of the stratosphere.

So what is the big deal do I hear you ask? H2O in whatever form is essentially boring stuff. However, what appears to be happening in the stratosphere has big implications for us all and perhaps it deserves a lot more attention.

At the NOAA (National Oceanic and Atmospheric Agency) base in Boulder City in Colorado USA scientists have been measuring the water vapour content of the stratosphere and have concluded that it is increasing at a rate of 1% per year when measured over the time scale going back to 1981.As you can see from the graph which is courtesy of NOAA, at the lower levels of the stratosphere the rate is even higher approaching 3%. Scientists have reason to believe that the effect may be similar globally.

Why could it be so? From basic physics it’s certainly no secret that the warmer the atmosphere the more water vapour it will hold. If the stratosphere is increasing its water vapour content then it seems reasonable that the whole layer is warming up.

There are esoteric arguments between scientists as to whether water vapour should be included as a ‘forcing’ as it is called or a feedback. This is because the residual time for water vapour in the atmosphere is relatively short compared to the other GHGs. When water vapour reaches Dew Point,‘dew’ forms coming back down to earth as rain (a feedback). In the troposphere the residual time is about 10 days, which is quite small compared to the time CO2 remains there (decades). But up in the stable stratosphere it is a different story and water vapour remains there for several years, long enough to add to the greenhouse effect.

A warming stratosphere may not be the only explanation for the increase in the water vapour level. Aircraft jet engine efflux also adds water vapour, at least in the lower limits.
The jury is still out but it all adds to the general picture that we’re building a warmer world