Sunday, February 3, 2013

High-flying low-hanging fruit

Airbus A380 takeoffA few months ago, I wrote about some of the efforts I'd been making to moderate our household energy consumption: improving insulation, buying a more efficient refrigerator, replacing lightbulbs, installing a real-time electricity monitoring system and so on.  The results are visible in the "home energy report" that now arrives regularly from the power company.  "Great! You used 34% less energy than your efficient neighbors!"  (Efficient neighbors are defined to be the top quintile of all our neighbors, so this is a good statistic indeed.)

When improving energy efficiency, we want to start with the things that are easy to do and make a big difference - the "low-hanging fruit".  The new refrigerator was the biggest "low hanging" item for us.

Suppose though that I move on from thinking about "what can I do around the house" and ask about my whole lifestyle.  Which elements of my life are greediest for energy, or (more or less equivalently, in our present world) contribute the most to greenhouse gas emissions?

There's an obvious answer: air travel.  

Academics are enthusiastic travelers (and with reason: in my own experience at least, new ideas often come from the chance to meet face-to-face with new people.)  I travel about 40,000 miles a year.  A ballpark figure for carbon dioxide emissions from flying is 200 grams per passenger-mile, so that's about 8 tonnes of CO2.  According to the European Union's EDGAR database, United States annual carbon dioxide emissions run about 17.3 tonnes per capita.  So, this particular "fruit" for me weighs about half of the whole tree!  It hangs low indeed.

Am I willing to harvest it?

Photo by Flickr user Ndecam, licensed under Creative Commons.


byron smith said...

You're basing your figures on 200g CO2 per passenger mile, or (since I use metric) 124g CO2 per passenger kilometre.

Both the Wikipedia source links for this figure are broken. I note that this figure is significantly lower than the numbers that I use when getting a ballpark estimate of GHG costs of aviation.

a. Do you travel economy or business class? Business class seats take up significantly more room and most nuanced calculations take this into consideration.

b. Have you included a multiplication factor for the increased impact of other (non-CO2) GHG forcings from aviation? There is raging debate over this, but many give an answer that is close to a multiplication factor of 2 over straight CO2 emissions.

c. How have you calculated distances travelled? By great circle distance or some other measure? Again, most flights include significant deviations from the most direct great circle route for various reasons (direction of runway, circling while waiting to land, avoiding politically sensitive airspaces, etc.).

According to DEFRA (the UK government Department for the Environment, Food and Rural Affairs), these modifications result in the following figures (working backwards). These are the official recommendations for UK govt departments who want to calculate carbon emissions. See pp. 44-55 for advice on flying.

The summary is this:

1. Calculate great circle distance of flight (Which can be calculated from various websites - generally just do a search for "distance X to Y" listing the start and end of your flight.

2. Add 9% to great circle distance for (c) above.

3. For economy domestic (within UK) flights, use 171.5g CO2e per passenger kilometre (this is higher due to larger portion of fuel used in take off and landing, as well as lower load factor (i.e. such flights are, on average, less full). See table on p. 49 for all these figures.
For economy short haul (within Europe), use 92.5g CO2e per passenger kilometre.
For business/first class short haul, use 138.7g CO2e per passenger kilometre.
For economy long haul (outside Europe), use 113.2g per passenger kilometre.
For business long haul, use 239.6g CO2e per passenger kilometre.
For first class long haul, use 330g CO2e per passenger kilometre.

4. Multiply this figure by 1.9 for non-CO2 forcings (contrails, other GHG emissions at high altitudes, etc.), due to (b) above.

Using this method, a one way flight (assuming a direct route exists) from here in Edinburgh to Pittsburgh business class would have the following associated emissions:

1. Distance = 5634.72 km.
2. x1.09 = 6141.84 km
3. x239.6 gCO2e/km = 1471568.01gCO2e = 1,472 kg CO2e
4. x1.9 = 2,796 kg
Or the same trip flying economy would be = 1,321 kg.

Of course, all this assumes an 100-year timeframe for CO2e calculations. On any shorter timeframes, the figure would be considerably higher.

John Roe said...

Hi Byron, thanks for your questions, and thanks for the link to the DEFRA paper. I'll have to take a look at that. For what it's worth, the 200g/mile figure is consistent with the results from the offset calculator at, which is the other source I checked. To answer the rest of your questions: economy class, actual flight miles (based on the airline's figures), and yes, this is purely CO2 figures without taking into account the "equivalent" effect from other sources. (The NYT article I link to got this wrong at first and had to issue a correction.)

It puzzles me that *short* haul European flights are the most economical according to the figures you quote. Most other sources have long haul more economical overall, as one would expect. Or are European flights extra jampacked and uncomfortable these days? It is a while since I took one.