Calculating footprint often uses fuzzy math

Results vary, but give idea of environmental impact

By Tim De Chant | Chicago Tribune reporter

August 10, 2008

Frank Bentley's life changed seven years ago when he took an Internet survey that estimated his annual share of greenhouse gas emissions.

Bentley, then a college student, entered information about his lifestyle into an online calculator, which spit back a number showing how much he might be contributing to climate change—his "carbon footprint." Now the Palatine resident constantly tailors his habits to minimize that number, including living close to work, biking and using public transportation when possible, and eliminating meat from his diet.

But he also encountered a common problem that is seldom advertised—there's no single, universally accepted way of calculating someone's carbon footprint. Dozens of carbon calculators have sprung up on the Internet in the last few years, but they use different assumptions and can yield wildly varying estimates.

Many factors to calculate

The calculators help raise awareness, but "the downside is that the methodology is being worked on as we speak," said Daniel Kammen, a professor in the energy and resources group at the University of California at Berkeley who is senior energy and environment adviser to Sen. Barack Obama's presidential campaign.

Consumers try to cut through the confusion the best they can. When Bentley steps on various carbon scales with the goal of trimming more emissions from his lifestyle, the results can vary by as much as 10 tons of carbon output. Usually he averages several results to assess his true footprint.

"You can play with those numbers and see how it affects the outcome," said Bentley, a senior researcher for Motorola. "I'll take a couple and just see what the average is."

Understanding how the footprint estimates are reached may be the best way for people to make good choices about modifying their behavior, experts say. An accurate estimate includes a wide range of factors because nearly every aspect of modern life contributes to a person's carbon footprint. Carbon dioxide from transportation and the generation of electricity is the most prevalent greenhouse gas, but calculators also account for methane, nitrous oxide, fluorocarbon refrigerants and others. 

The calculators' disparate results stem from their conflicting assumptions. Some surveys determine a footprint from a few general questions, like household size or number of plane flights per year, while others delve deeper.

"One that had 100 questions, that was my favorite," Bentley said. 

Flying leaves big print

Much of the variation lies in the assessment of air travel, which is one of the biggest sources of carbon emissions for a typical middle-class American. While many calculators tally miles flown, some do not include the number of legs for each flight, which can throw off the results.  "Shorter flights have higher emissions per flight because most of the emissions occur during takeoff," said Christopher Jones, developer of the Berkeley Institute of the Environment's carbon calculator.  Jet engines also produce water vapor, which at high altitudes is a short-lived but potent greenhouse gas. Including water vapor in calculations can "more than double emissions," Jones said.

Despite the differing approaches, scientists still find carbon calculators useful. After all, even if the tool is not perfectly accurate, people who consistently use it to track and modify their behavior will see their footprint shrink.   Michele Hallett of Chicago said when she first stumbled upon carbon calculators a year ago, "the thing that stood out to me most was that airplane travel was a huge carbon dumper."  The tools inspired Hallett to take a couple of trips by train instead. When she couldn't fit a train ride into her schedule, she flew the first leg and returned by train to cut the carbon in half.

Hallett's brother and sister-in-law, Mark Hallett and Carmen Vidal-Hallett, have altered many of their daily routines to minimize their footprints. The three, who live in the same household, commute by bike when possible and grow tomatoes and herbs in their backyard garden, which reduces their reliance on produce that's been transported in carbon dioxide-emitting trucks.

Farm to freezer

Food can play a large role in a person's carbon footprint. Cows are notorious for their methane emissions, but feeding them also requires fuel and energy. A University of Chicago study found switching from a diet laden with red meat to a vegetarian one can lower a carbon footprint by nearly 1.5 tons.  Thorough calculators like Jones' can help identify hidden carbon sources like those in the food supply chain or the transportation system. They take a cradle-to-grave approach, tallying everything related to an individual's consumption.

For example, Jones' calculator will count not only tailpipe emissions from driving, but also those from a vehicle's manufacture. Most calculators are comparatively simple, counting only driving emissions.  Carbon calculators have grown in popularity as awareness about climate change swells. Many appear on Web sites that offer to offset visitors' carbon emissions for a price. Americans paid $54 million last year to assuage their global warming guilt, and Kammen expects that amount to surge past $250 million this year.

Buying smaller print

Carbon offset programs work by investing money in enterprises that will either reduce the amount of carbon released to the atmosphere or capture and store existing carbon, but the efficacy of the various options is hotly debated.   In addition, the less scrupulous offset brokers will sell carbon credits multiple times, defeating the purpose. Users should read the company's Web site to see if its programs undergo independent audits. 

U.S.-based carbon offset companies charge anywhere from $5 to $29 per metric ton (about 1.1 tons) of carbon. While Kammen said he expects the price to rise over time, his lab found $30 per metric ton to be an appropriate amount.  The other way to trim tons is through lifestyle changes. Carbon calculators can help consumers identify the small steps that are the easiest—and cheapest—to take, such as turning down water heaters and installing compact fluorescent light bulbs. Driving less is also a straightforward and increasingly cost-effective measure.

Taking it to the next level, however, can be daunting. Despite Bentley's efforts, his footprint remains a few tons higher than the American average of 22 tons—thanks mostly to air travel, which now represents 70 percent of his carbon emissions.  "I've done a lot to reduce carbon in every other area of my life," Bentley said. "But air transportation is one thing I really can't cut back on."