Q:
If a family of four flew on a jet from Boston to Orlando, Florida, and a family of four drove from Boston to Orlando, which trip would put more CO2 into the atmosphere? Which would put more in the atmosphere per person?

Patricia
Salisbury, Massachusetts, USA

A:
An answer to this question is a bit complicated because it depends on both the type of car and the type of aircraft used, plus the details of the trip. However, we'll make some simple assumptions to come up with a general idea of how much carbon dioxide each trip will produce.

In general, automobile engines release about 19.6 pounds of carbon dioxide per gallon of gasoline. Thus, the amount of carbon dioxide released for a given trip depends upon the gasoline mileage of the car. Assuming the family is driving a new Ford Taurus, which gets 23 miles per gallon, and the trip from Boston to Orlando is 1280 miles, the car trip will release

(19.6(lb/gal) * .453(kg/lb) * 1280(miles) ) / 23(mpg) = 494.1 kg CO2 into the air, or 494.1kg/4 = 123.5 kg CO2 per person.

Airplane engines generally release 3.15 kilograms carbon dioxide for each kilogram of fuel burned. We will assume the family is flying in a full Boeing 737-400 aircraft, which can carry 146 people, and flies at about 440 miles per hour. Therefore, a trip to Orlando from Boston would take about 2.9 hours. The amount of fuel that a plane uses per hour depends on what the plane is doing. For example, it takes 6869 kilograms of fuel per hour for a Boeing 737 to climb from the ground to its cruising altitude, while it only takes about 893 kilograms of fuel per hour for the same plane to fly at cruising altitude. We will assume that the plane spends 15 minutes, or .25 hours climbing, and the rest of the time (2.65 hours) at cruising altitude. We will ignore any carbon dioxide that might be emitted from the plane during taxiing or idling at the airport, although these can be significant sources as well.

So the amount of fuel used by the Boeing 737 during the flight is:

6869(kg/hour)*.25(hours) + 893(kg/hr)*2.65(hours) = 4083.7 kg fuel

And, therefore, the total carbon dioxide emitted is:

4083.7 (kg fuel) * 3.15 (kg CO2/kg fuel) = 12863.65 kg CO2

Obviously the airplane emits more carbon dioxide than the car during the trip, about 26 times as much. However, if the airplane is filled to capacity (146 people for the B737-400), then the plane emits 88.1 kilogram of carbon dioxide per person, which is less than that emitted by the car.

Keep in mind, however, that this is a pretty rough estimate of carbon dioxide emitted by the airplane, as we don't really know how long the airplane spends at different fuel consumption levels, and we are ignoring any emissions while at the ground at the airport. Also, this is assuming the airplane is at full capacity. If the airplane were only at about 70 percent capacity (102 people), than the carbon dioxide emissions per person would be 125.9 kilograms of carbon dioxide per person, or about the same as the automobile emissions.