Supersonic jets, ozone and climate
Nothing to worry about?
My old friend Don Wuebbles and his group at the University of Illinois have just revisited a question he looked at years ago when he produced one of my all-time favourite plots during his time at Lawrence Livermore National Laboratory. At the time, there was discussion about a possible fleet of supersonic aircraft that would fly much higher than conventional jets, reaching altitudes of 20 km or more in the stratosphere. The question was: How much damage to the ozone layer would be caused by the oxides of nitrogen from their exhausts? His famous plot to answer that question is reproduced below. Wow, that ends in 1988. It must have been produced about 35 years ago! Miraculously - after all that time - I can still recall that we called it the “Wuebbles” plot.
At first sight, it looks like a time series showing an anti-correlation between emissions of effects on ozone of nitrogen oxides (NOx) and CFCs. But that’s not what it is. What the plot really shows is the effect of our changing knowledge in chemistry on the predicted outcome from two different perturbations:
The ozone impact due to a steady-state release of nitrogen oxides from supersonic aircraft, and
The ozone impact due to a steady-state release of CFCs
With our understanding of the chemistry in the late 1970s, the predicted effect of supersonic aircraft was an increase in ozone. For other periods, it predicts a decrease. There was something seriously wrong with the model chemistry over that late 1970s period. The predicted decrease on ozone from CFCs was also much larger at that time. A good thing too perhaps. That may have serendipitously added the urgency needed to solve the ozone problem that would arise soon after with the discovery of the Antarctic ozone hole. It’s a great example of how changes in our understanding can affect the bottom line in science.
Thirty years later, when I was writing ‘Saving our Skins’ (now also available on Substack), I asked Don if there was any update to that plot. He sent the version below, which shows that by 2006 at least the science still wasn’t settled (but at least the SIGNS of the changes had remained constant). Since the early 1980s, they’ve consistently predicted that both perturbations consistently lead to a decrease in ozone (as they did prior to the mid-1970s). But the sensitivity was still in question. The just-published work may have been motivated by my inquiry.
Their just-published work, with more realistic inputs from the jets into their model, predicts much smaller effects on ozone, as shown below. While it consistently predicts a decrease in ozone throughout the tropics and the northern hemisphere, the changes are much smaller in the southern hemisphere. There - where there’s much less air travel to contribute to the problem - it in fact predicts a slight increase in ozone in the summer months. That’d be good news for skin cancer in New Zealand.
They found is that their proposed fleet of supersonic aircraft would cause a reduction in global column ozone of less than 1 percent (around 2 Dobson Units in a global average of 300). Very small.
But the new study goes further. They now also calculate the effect of the proposed fleet on global warming. The results are shown below.
The effects of that small ozone depletion are the largest contributor (at 59.5 mW/m2), which is slightly offset by the absorption and scattering from clouds and aerosols formed by emissions of water vapour, black carbon, and sulphates in the fuel. Their calculated annual global average change in radiative forcing of climate from the projected fleet is 45.4 mW/m 2 (or 0.045W/m2). That’s tiny compared with the radiative forcing of about 2 W/m2 from CO2, and less than 2 percent of the total from all greenhouse gases.
But every bit counts. Avoiding that extra 1 or 2 percent of warming by dispensing with the fleet may make all the difference.
The authors point out that there are many untested assumptions in their calculations, and differences compared with other recent studies. There’s also the question of what would happen if these extra fights in the stratosphere resulted in fewer flights lower down? The consequent reduction in those irksome contrails that I discussed a few weeks back could have a big positive effect.
The bottom line in the paper is
“Atmospheric impacts of any proposed fleet of supersonic aircraft need to be fully examined and understood before putting these aircraft into operation”.
Good advice. In any case, shouldn’t we all be striving to reduce our air travel?