Our CO2 from flying - in case you were wondering (cragcentral message#85)

john ackers

website team CRAGger Islington

I think the CRAGs ought to review the aviation numbers they are using.

There is a lot of variation between the different models, you can get a 50% change in CO2 when you switch between models e.g climate care to choose climate to CRAG.

I have looked at two algorithms: Choose Climate algorithm and the Climate Care algorithm algorithm in detail. The differences are:

1. Takeoff fuel must be separately itemised as a fixed amount of fuel. Both algorithms use similar numbers for fuel take off and landing (based on this report). The CRAG spreadsheets that that have the 0.51 Kg/Km and 0.82 Kg/mile metrics don’t separately itemise takeoff fuel ; short flights are underestimated and long haul flights are wildly overestimated.
2. RFI. choose climate use a radiation forcing index (effect of other gasses apart from co2) of 2.7. ECI say that 2.0 is OK in a document written by ECI but commissioned by climate care. This is the controversial aspect of the calculation.
3. Larger planes are preferred (if not required) for long haul journeys. Smaller planes appear to use less cruising fuel per passenger per Km. Climate Care assume that passengers travel on B737 400 for distances under 3500 Km and B747 or A340 for everything else. The Choose Climate model is weak in that all flights are assumed to be by 747 which uses a lot of fuel per person at takeoff. (I have emailed website author but not had a response. But he would probably say just plug in the other aircraft).
4. the number of seats on the plane seems to vary between operator. Climate care assume that a B737-400 has 180 seats which according to the the 737 Wiki is about right as it seems to vary from 170 to 190 depending on the variant. According to the 747 Wiki A typical three-class layout accommodates 416 passengers, while a two-class layout accommodates a maximum of 524 passengers. Choose climate use 380. Climate care assume 400 but that is, I think, the average of a 747 and an Airbus A340.
5. Occupancy. Climate care and choose climate 80% and 100% respectively for seat occupancy. I think the likes of EasyJet and RyanAir do achieve 80% and above.
6. Freight. Climate care assume that 10% of fuel is used to carry freight.

I think we need to agree on what numbers we should be using and use those numbers in our calculations, both in spreadsheets and online. Ideally, we should use different formulas for different models of plane.

Also, Chit Chong (partner of Lucy in Islington CRAG) also emailed me with similar issues which I’ll leave him to post.

John

p.s. The Islington group in Jan 07 meeting decided to use Climate Care model but that was largely based on my comments.

Re-edited to improve clarity on 13-Apr-07.

david

website team CRAGger Sevenoaks

The original source for aviation forcings was the IPCC’s Aviation and the Global Atmosphere (1999) ^{http://www.grida.no/climate/ipcc/aviation/071.htm#623 (accessed 12/2/2006)”>1}. The relevant section is 6.2.3. Alternative Indexing of Aviation’s Climate Impact-RF Index which says:

RFI is a measure of the importance of aircraft-induced climate change other than that from the release of fossil carbon alone. RFI ranges between 2.2 and 3.4 for the various E- and F-type scenarios for subsonic aviation and technical options considered here

Jardine (2005) ^{http://www.climatecare.org/_media/documents/pdf/Aviation_Emissions_&_Off… (accessed 12/2/2006)”>2} (the ECI report commissioned by Climate Care) suggests 1.9 as the better figure, based on the TRADEOFF project . On page 7:

The IPCC calculate the change in radiative forcing of aviation emissions since pre-aviation times to be 0.049 W/m2 (See Figure 5).. This corresponds to a radiative forcing index of 2.7 as the total radiative forcing of 2.7 times that of CO2 alone (0.018 W/m2). However, a recent study (TRADEOFF) has updated this figure and a value of 1.9 is now the best-quantified estimate of radiative forcing index of aviation emissions.

Sausen et al. (2005) ^{http://folk.uio.no/gunnarmy/paper/sausen_mz05.pdf “>3} compare the IPCC (1999) and TRADEOFF results in more detail, and their Fig. 1 gives a good breakdown of the components of the aviation forcing and the level of uncertainty and understanding. The estimates for O3, CH4 and (especially) contrail forcings have fallen, meaning the total radiative forcing (0.048 W/m2) is 1.9 times that of the CO2-only forcing (0.025 W/m2), rather than 2.9 according to the IPCC (1999). Cirrus cloud forcing is highly uncertain and not included in these totals. Its likely range is 0.01 to 0.08 W/m2.

Although these effects do undoubtedly scale up the climatic effects of aviation emissions, Forster et al. (2006) ⁴ note that using the RFI method to quantify it for emissions trading is rather suspect. Normally, you’d use Global Warming Potentials which define the effect of a unit greenhouse gas over a defined timescale. Instead, the RFI measures the cumulative globally-averaged forcing by aviation emissions up to a certain year. It doesn’t define the timescale of the climatic effect, which is important where non-CO2 forcings operate over variable timescales. And to be fair, we should also include non-CO2 forcings of other sectors, like heating or electricity.

Unfortunately GWPs are unsuitable for most aviation forcings too (Forster, 2006; IPCC, 1999; Jardine, 2005). So it’s very much a case of “watch this space” for a reliable metric to scale up aviation emissions. Until then, we have to fudge it.

There’s a good overview of this area in the Workshop on the Impacts of Aviation on Climate Change: A Report of Findings and Recommendations (Section 2.3.2, p.25)

john ackers

website team CRAGger Islington

"Don't take my freedom away, Don't take my holidays..When it's cheaper to fly than to park at the airport..."

Have a listen to this gorgeous song by Shannon from Seize the Day. The lyrics could have been written for CRAGgers. They played it at the Green Party Conference on Friday night in Swansea.

Before they played it they asked people to pledge to never travel by air again. Two people did. Of course the audience thought that was a very noble gesture. But I suspect that they were members of a CRAG and the prospect of not having to ever calculate aviation emissions again was too great an opportunity to miss.