On driving versus flying

09 Jul 2008

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Dear Umbra,

My husband asked me this one the other day and I didn't know the answer, so I thought I'd ask an expert. Which is the more environmentally friendly method of travel: 100 people driving their own cars (let's assume non-hybrid vehicles) to a city three hours away, or 100 people flying in a plane to the same city?

Natalie W.
North East, Pa.

Dearest Natalie,

Carpooling is one way to be efficient; here on Floor 2B we also like to questionpool. Let's piggyback on Helen's earlier question about the efficiency of trains vs. planes and keep practicing our carbon math.

Planes, ~~trains~~ calculators, and automobiles.

Most carbon emission measurements are in distance rather than time, so to be super-efficient we will decide the city is 300 miles from our starting destination. Let's say the journey is from Philadelphia to Boston.

To recap what we learned about train rides vs. short-haul (300 miles or under) flights: planes are up to 10 times worse than trains. Short-haul flights are worse than long-haul flights per passenger-mile, because takeoff and landing use the most fuel in a flight. Flying 300 miles in the face of other, better transit choices is an egregious release of carbon.

Solo driving, though, is also fairly egregious. What, then, about this theoretical situation you pose?

Let's say a group of 100 wedding guests charters a plane from Philadelphia to Boston. Or the wedding guests all drive, but they have so many giant gifts for the couple (organic futons, recycled-plastic fleece coats, cast iron crock pots, bicycles) that there is no room to carpool, and each must drive their own car.

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For British Helen, we used emissions factors from the U.K. Department of Environment, Food, and Rural Affairs. A factor is one of the numbers in a multiplication equation; a transport emissions factor basically mathematically represents volume of fuel burned and, multiplied by a distance factor, results in an amount of carbon emitted. To compare single occupancy cars and a plane, we'll look at the DEFRA numbers again. DEFRA's emissions factor for medium-sized cars is 0.3442 kilograms CO2 per mile; for a short-haul plane it's 0.1753 kg CO2 per passenger-kilometer, multiplied by two, then multiplied by 109 percent. (These last two seemingly random multipliers account for radiative forcing and uplift factor, which are plane-specific attributes that affect CO2 pollution.)

According to our DEFRA-based calculations, each wedding-bound car is emitting 104 kg CO2 over the trip; that's a total of 10,400 kg CO2 for 100 cars. If the guests all flew, they would each emit 184 kg -- times 100 passengers would be 18,400 kg CO2. I can further tell you that the Boeing 737-300 that United flies on its Philadelphia-Boston route holds around 128 passengers, so if the plane were full (wedding crashers!), the total emissions would be a bit higher.

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Given that these are enough numbers and that if I add any more we'll all stop reading, what can we conclude? Per person, this journey isn't dramatically different from a carbon angle, with the 100 cars coming out slightly ahead. But more important, the math reminds us that no man is an island. Whether one person or four drives in the generic mid-size car, it emits about 104 kg CO2 total. So should the 100 guests choose to carpool, driving fewer cars to the wedding, their group carbon footprint can be lowered.

Dearest Natalie, I have no idea why your husband asked this question, and I bet he regrets it now. I hasten to add that I am not an expert. Still, I hope that my inexpert math, or at least the resources for doing one's own math, will scratch whatever itch struck him, and convince us all to fly a little less often -- and to carpool more.

Factor-y,
Umbra

Yours is to wonder why, hers is to answer (or try). Please send Umbra any nagging question pertaining to the environment -- but first check out her FAQs!

The claims made in this column may not reflect the views of this magazine. Neither the magazine nor the author guarantees that any advice contained in this column is wise or safe. Please use this column at your own risk.

Umbra Fisk is Grist Research Associate II, Hardcover and Periodicals Unit, floors 2B-4B.

Comments: (10 comments)

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flying vs driving

okay, so clear this bit for me, please.

if "one person or four drives in the generic mid-size car, it emits about 104 kg CO2 total."
how does that compare with one person or 4 or 40 or 140 flying in the plane? surely the weight of that 1 or 4 should make no difference to emissions. 40, maybe. 100, probably.

any info on that ready at your fingertips would be most appreciated.

thanks

by mohoshe at 6:50 AM on 09 Jul 2008

all that math

Ah, but my husband is a mathematician, so all that math will make him happy. Thanks for answering one of our random questions!

Natalie W. in North East

by nwaddellr at 10:40 AM on 09 Jul 2008

flying vs driving

But!!!

there's always a but...

if the plane is flying anyway, a commercial flight on a regular schedule with an empty seat, then isn't it less carbon to hop on and not drive?

by dave9778 at 12:40 PM on 09 Jul 2008

"if it's there anyway"

Hi, dave9778
While I certainly see what you're saying, this is basically a twist on the argument, "Well, the product has already been made and is in the store - I can't help the production impact, so I may as well buy it!"
If fewer people fly a given route, choosing instead to take alternate transport X, then the regularly scheduled commercial flight will stop being run, or will run less often.

by sairen42 at 1:42 PM on 09 Jul 2008

Slightly ahead?

Slightly ahead? The cars come out almost half the CO2!

by christophersj at 8:17 PM on 09 Jul 2008

Flying or Driving

The answer is of course to take the train.

R. Brian Gardiner

by briangardiner at 8:54 PM on 09 Jul 2008

still in a fog

I'm with mohoshe, how come 1-4 people in a car makes no difference in carbon emissions, but each additional person in a plane raises the carbon footprint for the flight? I know my car uses more gas when I drive from Portland to Seattle with 4 people in it, albeit far less than 4 cars with one person would, and I am sure a full plane would require more fuel than an empty one, but it seems like the change between one additional plane passenger would be a negligible increase in the overall weight of the flight. Also, it seems like this is also a question of infrastructure as well, if everyone suddenly quit flying from any city within a 300 mile radius of Boston, the resulting traffic jams would considerably increase the caron footprint of the drive. So a few of us can feel better about our own carbon footprints but it doesn't seem to be a real solution. It would seem that the train is by far the best option overall.

by marcus goodfellow at 8:47 AM on 10 Jul 2008

Co2 comparison articles

I disapprove of the way this article, and many others, are written. Two things specifically annoy me, first is the confusion about the primitive unit of co2 production, and the second concerns using unrigorous, untransparent physics, with difficult to interpret numerical terms.

Concerning my first issue, consider the following:

"I can further tell you that the Boeing 737-300 that United flies on
its Philadelphia-Boston route holds around 128 passengers, so if the
plane were full (wedding crashers!), the total emissions would be a
bit higher."

Obviously the plane will emit only slightly more CO2 with another 28 people onboard - in fact it will emit far less CO2 per person. Why does the author take the per person carbon as the primitive, when obviously you can only get per person carbon by dividing out of a single total carbon emission - that of the entire plane. It is in no way true that the plane produces X kg of carbon because each person contributes their little amount. Rather, the plane produces X and because we like to calculate everything, we attribute X/N where N is the number of paying passengers on the plane KG to each passenger.

Concerning my second worry, what is 0.3442 kgCO2/mile? Certainly a number from some website somewhere, but how do I know it in any way connects with reality? I suppose if its about 2.3 kg from 1 liter,
that means they are calculating for 6.7 miles to the liter, which is
25 miles to the gallon. There are not very many "mid sized cars" built
today which get 25mpg on the highway. I'll give you five dollars if you can find one rated so poorly on fuel economy.gov which qualifies as "mid size". I turned that figure exactly on
the trip to portland, which was not all highway, and which had 6
people and luggage in a 20 year old, full size car.

Aside from these two issues, the conclusion is bonkers: "Per person, this journey
isn't dramatically different from a carbon angle, with the 100 cars
coming out slightly ahead."

Slightly ahead? 10 000kg versus 18 000kg? How is that slightly ahead?
Also, the cars were all packed with organic footons, try to get those
on the plane!

Another thing, it's not the same to release co2 at ground level versus tens of
thousands of feet in the air. Also, for your not flying to make a
difference, you have to fly enough less that flights get canceled, and
that means people lose jobs (which for some reason upsets people). Also, planes put a lot of water in the air, which forms clouds, and might be responsible for global cooling.

It is, however, absurd to do these calculations at all - it relies on individual desire to save the world to, save the world. People cannot, and should not, be expected to do this. Put a price on carbon, and then people will be green by just trying to save money.

by tarl at 8:17 PM on 10 Jul 2008

driving vs flying

You should also calculate in the driveing you have to do when flying. Kind of hard to get an average but the extra transportation needed when flying can be considerable.

by Hillberg at 6:58 AM on 15 Jul 2008

Re: Co2 comparison articles

Tarl,
Done: 2008 BMW M5, mid-size, 17mpg highway. One of many new cars still with that bad a rating.
$5 please.

For the car: 1US gal gas = ~20lb CO2 emitted
So at 25mpg (a conservative number we now see) that's 240lbs CO2 = 108kg CO2 per car. So pretty close to the 104 kg CO2 per car stated above for a 300 mile trip.
If we took the ole' BMW at 17mpg, we're up to almost 350 lbs CO2 for the same trip per person. So we're up to 35,000 lbs CO2 for 100 BMW's.

For a plane, I agree, it's hard to break it down by person and expect that there is a linear relationship between the CO2 emitted and number of people. 1 passenger flying from Boston to Philly will technically emit more than 100 passengers flying as the flight will have taken place either way. (same as bus and train theory) It's essentially plane-pooling right?

Boeing states that the average 747 burns 5 gallons of fuel per mile. I'm sure this is a low-ball estimate as it doesn't take into account take-off and taxiing and all that. But whatever, for simplicity a 300 mile trip would equal 1500 gallons burned. Assuming jet fuel emits 21 lbs of CO2 per gallon, that's 31,500 lbs CO2 flat out.

So which is more environmental? Probably the train and carpooling. And at least they still have food to offer unlike the airlines.

Wanna get more detailed? Check out this paper for more info on the variables used in calculating per person CO2 emissions on planes:

http://www.carbonplanet.com/downloads/Flight_Calculator_I ...

by gc at 11:00 AM on 15 Jul 2008

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On driving versus flying