If there’s one choice we make every day that has the greatest single impact on our carbon footprint, for most of us it’s how we move ourselves and our gear from Point A to Point B. According to the 2007 Report of the Intergovernmental Panel on Climate Change, “Transport” is responsible for 13.1 percent of all human-generated greenhouse gas emissions worldwide. But in the United States, where no one actually walks that ribbon of highway, “personal travel” accounts for 34 percent of the 25.9 metric tons of carbon released into the atmosphere per individual per year.
Since driving is a daily event for most Americans, small changes that shave a few miles off each week can add up to a significant change in CO2 emissions over the course of a year. (To figure out just how significant, the U.S. EPA provides this Personal Emissions Calculator. For a more personalized estimate of your car’s emissions, check out this site from the Clean Air Conservancy.)
What’s Your Output, Joe?
Let’s consider the carbon output of a hypothetical American couple with one car. According to the EPA, the average two-person household in the U.S. emits 29,400 pounds of carbon into the atmosphere each year, not counting transportation. For each average American car (traveling an average of 231 miles/week and getting 20 miles per gallon), we pump another 12,100 pounds of carbon into the sky. So our couple’s total carbon output is 41,500 pounds per year, more than a quarter of which comes from car exhaust.
Now let’s see what happens if they cut back on driving. What if this couple reduced the distance they drove by a modest 10 miles per week? That would keep 531 pounds of CO2 out of the atmosphere each year, reducing their car emissions by 4.4 percent and total household emissions by 1.3 percent. A recent National Geographic article takes this calculation further; if everyone drove their cars 20 fewer miles a week, it would reduce CO2 output by 107 million tons, or 9 percent of all auto emissions.
Avoid The Airport, Joe!
So far so good, but I’ve ignored one big slice of the transportation pie: air travel. Modern aircraft emit less than half as much CO2 per passenger as a car driving the same distance; but since we fly much longer distances than we drive, even a single flight can have a huge impact on our carbon output. Also, exhaust from an airplane in flight is thought to have a greater impact on the atmosphere than an equivalent amount of CO2 released at ground level. (Here is an easy way to calculate how much CO2 you produce on any given flight.)
Urban smog settles in and around central Los Angeles. Image credit: NASA/Terry Lathem.
Back to our average American couple. Let’s say they fly 4,000 miles, the equivalent of a round trip from Newark, New Jersey to Tampa, Florida, twice a year. This would increase their annual CO2 output by 0.36 metric tons per passenger per trip, for a total of 3,175 pounds of CO2, roughly what they emit in three months of driving. (It could be worse; a single trip to London would generate over 5,000 pounds of CO2 for the pair.) Cutting out one plane trip would reduce this pair’s transportation-related CO2 emissions for the year by 10 percent.
What does all this mean for us? For one thing, we can achieve a significant reduction in auto-related emissions without buying a new car or making major lifestyle changes. Planning car trips to combine errands, taking public transportation whenever possible, and switching to a clean machine for short trips can be combined in varying measures to produce a meaningful impact, even if it only reduces the miles we drive by five or ten percent.
Cutting down on flying may be more problematic in a country where families live thousands of miles apart and where many jobs demand it. As a lifelong traveler (at nineteen, I spent my then-life savings on a ticket to Australia), I will not renounce air travel altogether, but I’m flying less. This year we’re driving to Maine for our summer vacation, which should be cleaner (and saner, with two young children) than the cross-country flights we’ve made in the past. And if we manage to swing a trans-Atlantic trip in another year or two, I’ll figure out some way to balance the cost in emissions, perhaps involving a combination of car-free days, carbon offsets, and other measures.
I’d love to hear your comments on whether you try to compensate for your CO2 output, and if so, how. It will be the topic of an upcoming blog. You can post your comments below, or feel free to email me directly.
Elsewhere on the Web:
A report from the Environmental Defense Fund, Reinventing Transit, describes how many American communities are finding “smarter, cleaner, faster transportation solutions.”
Green Cities, Brown Suburbs – If you’re fortunate enough to live in a city with a temperate climate and good public transportation then your contribution to the transportation slice of the pie is probably way below the U.S. per capita average. This article lists the annual CO2 emissions per household in 15 U.S. cities and compares emissions between cities and suburbs.
The Clean Air Conservancy provides emissions calculators for travel by car, plane, and boat; explains how CO2 emissions are calculated; and sells emissions reduction credits for CO2, SO2, and nitrous oxides.
Carol Berkower is a mathematician and molecular biologist living in Baltimore.
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