[AR] Re: Ozone loss at high flight rates
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 29 Nov 2016 14:52:13 -0500 (EST)
On Wed, 23 Nov 2016, Craig Fink wrote:
Short answer: the damage to the ozone is not a showstopper.
But, might initiate a New Ice Age due to global cooling caused by 20%
increased cloudiness at high latitudes?
Nope. The 20% increase is in clouds in the stratosphere and mesosphere,
i.e. at high altitudes, which are not major contributors to cloud cover
overall. (In fact, tha paper notes that polar stratospheric clouds
generally are rare, seen only in unusually cold years.) There conceivably
might be some significant indirect effect on something, but it would be
tricky to find and confirm this.
In fact, the net effect on global temperature looks to be a slight warming
("effective radiative forcing" positive, by a fraction of a watt per
square meter, depending on flight volume), mostly because of the extra
water vapor in the stratosphere. Water vapor is a strong greenhouse gas.
The "long pole in the tent" is the ozone depletion. At 10^5 flights/yr,
it's about 10% of the peak CFC-caused depletion, seen in the 1990s (it
took a while for CFCs emitted earlier to work their way up). That's
detectable but not a problem. At 10^6 flights it would be comparable to
the CFC depletion peak, which starts to be a mild concern but isn't a big
deal if it stops there. (And anybody who's flying something a million
times a year -- that's a takeoff about every 30 seconds 24x7 -- is going
to be seriously exploring alternatives like space elevators, to get ops
costs down even if it means a big up-front investment in R&D and
construction.)
Mind you, all of this comes with some big caveats, notably that the NOx
formation rates during airbreathing ascent and reentry are poorly known,
and so the results in the paper are first guesses only. There is a marked
shortage of data on NOx formation in supersonic LH2-burning jet engines,
especially ones with unusual cycles and as-yet-poorly-known combustion
temperatures. And what little information we have on NOx production
during reentry is mostly for the shuttle, which is not representative of
truly reusable vehicles (especially SSTOs) that bring their tanks back
with them and hence make milder reentries at higher altitudes.
Henry
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