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August 26, 2016

Park your ass in economy class!

Don MacKenzie

Don MacKenzie

Bottom line up front
Air travel, especially internationally, generates a lot of greenhouse gas emissions, but business class is much worse than economy:

air travel oil barrels chart2

Read on for details:
Air travel is a big part of most academics’ carbon footprints, and a frequent source of hypocrisy charges for those of us who work on sustainability and climate issues. Conducting, supervising, and sharing research is an academic’s stock in trade, and presenting our work at conferences and as guest speakers at other institutions is seen as essential to establishing our reputations and building strong tenure cases (i.e., not losing our jobs). From a career standpoint, avoiding all travel would be pretty reckless, given prevailing norms. At the same time, frequent long distance travel also seems pretty reckless given its effects on the climate. This creates a tension, especially for those of us working on climate change and sustainability, since it turns out that climate researchers are seen as more credible if they practice what they preach.

Consider me: over the past 12 months, I have flown for business once to the East Coast (4,600 miles return), once to Chicago (3,400 miles return) four times to the Bay Area (1,300 miles return on each trip), and once in-state (500 miles return). That’s just about 14,000 miles. In this respect, I appear to be pretty average among my colleagues (although interestingly I feel like I travel less than my colleagues – I wonder if this is just my perception, or if transportation engineering faculty travel more than average…). I mostly fly Alaska Airlines, which is the most fuel efficient air carrier in the U.S., averaging 76 seat-miles per gallon. This implies that my business air travel consumed 13,700 / 76 = 180 gallons of jet fuel last year, generating something like 2 tonnes of CO2 emissions.

As significant as this is, international travel can blow it up. Next month I will be flying to Japan to give a talk at Tsukuba Global Science Week at the invitation of colleagues at the University of Tsukuba. It’s almost 4,800 miles each way between Seattle and Narita, so this one trip alone represents 70% of all my air travel last year.

But what about business class?
This upcoming trip has introduced a new wrinkle: my hosts in Japan very generously offered to fly me business class rather than the economy class I invariably choose when traveling on my own dime (or my own research budget). I have long wondered how much more carbon intensive business class travel is than economy. To take a cut at figuring this out, we can look at the numbers for a Boeing 747-400, which is the aircraft Delta Airlines uses to serve this route.

Below is an image of the seating layout on a Delta 747-400, from SeatGuru.

Delta_Airlines_Boeing_747-400_C

Delta offers three classes: Economy, “Comfort+” (economy with 3 inches of extra legroom), and Delta One (business class). The seat pitch (distance from the seat ahead) and width from SeatGuru are shown below:

Screen Shot 2016-08-24 at 10.46.09 PM

We can multiply the pitch and width to estimate how much real estate each seat takes up: 3.8 square feet for Economy, 4.2 square feet for Comfort+, and 11.7 square feet for Delta One. Every Delta One seat takes up as much space as three economy class seats. Inspecting the seating chart above suggests an even starker difference: rows 9-13, accommodating 20 Delta One passengers, take up as much space in the fuselage as rows 54-62, which accommodate 90 Economy passengers!

Based on the above, we can estimate that the fuel consumption (and GHG emissions) attributable to flying business class in Delta One is about 3-4 times as large as flying Economy. On a flight of this length, that means that if I fly economy, I’ll burn about 140 gallons of fuel. If I choose Comfort+, I’ll burn about 15 gallons more. But if I choose Delta One, I’ll burn more than 470 gallons of jet fuel, generating nearly 4 tonnes more CO2 emissions than if I chose Economy.

Wait a minute…
Perhaps you object to the above approach. You might say, “Hang on, if you replace those business class seats with economy class, you’ll be squeezing more people in, which will increase weight and fuel burn.” So let’s consider two extreme cases: one in which an entire aircraft is business class, and another in which it is entirely economy class. Using the seating chart numbers above, we’ll conservatively assume that those 48 Delta One seats could each be replaced by 3 Economy class seats, for a total of 472 economy class seats. Or, the 328 Economy and Comfort+ seats could be replaced by 109 Delta One seats, for a total of 157 Delta One seats. To keep it simple, we’ll assume the operating empty weight is the same in both cases: 178,756 kg.

Now, let’s assume a 120 kg combined mass for each passenger and his/her luggage. The landing weight for a 4800 mile (7700 km) flight would be the operating empty weight plus the weight of passengers and luggage. Based on Figure 5.2 in Schafer et al., we will assume a lift/drag (L/D) ratio of 15 and a cruise specific fuel consumption of 18 mg/s/N for a Boeing 747-400. We’ll further assume a cruise speed of 913 km/h (254 m/s). Applying the Breguet range equation, we can calculate the ratio of takeoff weight to landing weight:

ln(Wtakeoff/ Wlanding) = (7.7 x 106 m) x (9.81 N/kg) x (18 x 10-6 kg/s/N) / (254 m/s) / (15) = 0.357

Wtakeoff/ Wlanding = 1.43

Therefore, the takeoff weight is about 43% greater than the landing weight, and this additional weight is the amount of fuel needed for the flight (technically, for the cruise portion of the flight, excluding landing and takeoff operations, which will be relatively small on a flight of this length).

The table below summarizes these calculations, along with the average emissions per passenger based on emissions intensities inferred from Tables 6.1 and 6.2 in Schafer et al.

Economy Business
passengers 472 157
weight per passenger 120 120 kg
weight of passengers and luggage 56640 18840 kg
OEW 178756 178756 kg
landing weight 235396 197596 kg
Fuel weight / landing weight 43% 43%
Fuel weight 100949 84738 kg
Fuel volume 32886 27605 gallons
Fuel per passenger (each way) 70 176 gallons
Fuel per passenger (return) 139 352 gallons
GHG emissions per passenger (return) 1.6 4.0 tonnes

Accounting for the reduced weight of passengers and luggage does lead to a somewhat smaller impact from business class travel, but business class still appears to be at least 2.5 times as emissions intensive as economy class in this case.

This brings us back to the figure at the top of this post, which summarizes these numbers in terms of barrels of oil equivalent, and compares them with all of my domestic air travel from the past year. One business-class round trip from Seattle to Tokyo would consume almost twice as much fuel — and generate nearly twice the greenhouse gas emissions — as all of my air travel in the previous year.

air travel oil barrels chart2

My Decision:

Extra comfort and space are nice, but for this flight, I’ll be choosing Comfort+ rather than Delta One in order to reduce my carbon footprint.

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