Hairyplanes

By stoat on February 17, 2008.

A little while ago, I ventured into aircraft CO2, and as good as said that the climate impact of aircraft fuel use should be weighted up by a factor of 2-3 because of various side effects: I thought of water vapour being dumped in the stratosphere. It turns out I'm wrong on that: at the altitudes planes fly at that effect is small, and the extra radiative forcing is dominated by ozone (via NOx) and contrails. There is lots of uncertainty, but a figure of about *3 for the radiative forcing is plausible. The govt website http://actonco2.direct.gov.uk/index.html says it uses a factor of two.

But.

Thats wrong.

I claim credit for realising that you have to worry about how long the effects last, but someone else has to point out to me more explicitly that its not the radiative forcing that matters but, effectively, the global warming potential, which is to say the time-integrated radiative forcing. And since contrails are short lived, and ozone not long-lived, those effects are small-ish. So I now think that the weighting factor to be used should be much smaller - perhaps so small as to be indistinuishable from 1.

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If the multiplier is indeed 1 then that means that we've already solved the "problem" with aircraft travel in the UK. The passenger levy is at the correct level for a Pigouvian Tax (as outlined in Stern and to the levels Stern thinks right, $85 per tonne CO2). Thus we need to do nothing more with that sector.
Good news, no?

The IPCC special report section 6.2.2 kicks the whole GWP issue into the long grass on the grounds that it's too complex (the efficiency of NOx to Ozone conversion varies from season to season, height of plane, etc. etc.), and uses RFI instead.

According to the UK Commission for Integrated Transport here

"... Some studies have attempted to express these upper-atmosphere impacts of aviation as a multiple of its CO2 emissions, using an index of radiative forcing (RFI). On this basis, IPCC (1999) estimated that the total climate change impact of aviation emissions to 2050 would be 2.7 times the carbon dioxide impact, though more recent research has reduced this figure to a central estimate of 1.9 (Sausen et al., 2005). Scientific uncertainty places a wide band around both estimates. But, just as important, the short-term nature of the non-CO2 global warming impacts means that these multipliers are very sensitive to the time period considered. Forster, Shine and Stuber (2005) show that the IPCC central estimate of a multiplier of 2.7 could increase to 3.7 if a 20-year time horizon was considered, but would fall to 1.2 using a 500-year time horizon."

[Thanks for finding the quote. The RFI is the wrong measure for total impact; this is exactly the point I was making. RFI is the same measure as RF. You need to use a GWP-like one -W]

William,

If the net GWP of aviation is more or less just a function of CO2 emissions, this would make a pretty influential paper. Tons of people (myself included) are currently using higher values for aviation in commercial carbon calculation and offset products.

[On what basis? -W]

This gives a tentative attempt (pg. 25) to weigh up the relative AGWP of CO2 versus the rest, and as you intuit, the ratio of the non-CO2 effects vs. the CO2 effects tends to 1.0 in the long term (although still significant on timescales of a few decades in this calculation).

However, I think the deeper point is that because calculating the size of CH4 and O3 AGWP is highly dependent on assumptions about plane height, track, season, etc., we can't calculate the integrated AGWP well enough in the first place to come up with a reliable multiplier.

Also, using AGWP integrated over a future time period to determine present day values for offsets inevitably opens up question of discount rates, which opens up a whole new can of worms. If "not long-lived" turns out to be 30+ years, I'm not sure we can get away with ignoring it...

We might be better off sticking with a 2x multiplier, however unrigorous.

[A little while ago, I would have gone for 2-3 as a good rough estimate. Now, I think 1 is the best rough estimate. That people haven't realised that its GWP not RF that matters is a bit of an indictment of the carbon offset industry (not that they need inditing :-) -W]

someone else has to point out to me more explicitly that its not the radiative forcing that matters but, effectively, the global warming potential

I think ECI at Oxford University have done a good job on that.

First, look at the press coverage they had for their 'Predict and Decide: Aviation, climate change and UK policy' report back on 17 October 2006.

Within that report commissioned by ECI, on page 17 in Chapter 2: What is Aviation's Contribution to Climate Change? They explained:

Although the IPCC's calculations are accepted as robust, there are growing concerns about the way that other people are now applying the RFI multiplier and its use as a means of estimating the future impact of non-CO2 emissions from aviation. This is partly because it was developed as a means of assessing the impacts to date from historic emissions, rather than as a way of assessing future impacts. More details of these concerns are given in Appendix A. Consequently, there is increasing interest in developing an alternative metric, based on global warming potential (GWP) or global temperature change potential (GTP). However, neither of these measures is considered sufficiently fully developed or, in the case of GTP, tested in relation to non-CO2 aviation emissions to be used reliably at present.

Again, in a paper by ECI and commissioned by ClimateCare, it is not the RFI, nor the GWP that would in theory be the best choice of metric for aviation emissions, but the GTP instead. Take a look at page 8 in Aviation Emissions & Offsets, which reminds us of the 'infancy caveat' that prevents GTP being used by carbon offset markets today.

[Thanks for finding that - definitely relevant. 1.7x on a 100-y timeframe looks on the high side to me, as does 36x instantaneously. But these things need to be pondered. They lose credibility points for then referring to runaway Cl Ch, though -W]