"Step into the confessional, child, and tell me your sins. There can be no redemption without a full accounting."
"Yes, Father. Forgive me, for I have sinned: I still take long hot showers. I sometimes don't use the power strip to turn off my computer. And I ... I ..."
"What is it, my child?"
"I flew to Vegas for the weekend. I'm so ashamed."
In a rapidly warming world, the new sin is the gratuitous production of carbon dioxide. If you're seeking to be shriven, a plethora of Internet-based carbon calculators will toll your climatic peccadilloes. The results are generally expressed in metric tons of CO2 per year, although some reckon your carbon footprint--the amount of land it would take either
to absorb all those emissions or to provide the natural resources you are scarfing up. Ideally, they also help you identify how best to lighten your carbon load.
Not all carbon calculators are created equal, however. Using different (and occasionally questionable) methodologies and assumptions, their results vary widely. After I entered the requested minutiae of my life, the nine calculators surveyed here yielded a pollution profile ranging from a relatively svelte 8.85 metric tons to a porcine 34. Same life, similar data, radically different outcomes.
Like a sinner choosing a confessor, then, you might want to shop around to find a compatible calculator. Simple versions hit the high points (home heating and cooling, auto use, and air transport) and rely heavily on "front of mind" information--that is, your guesses about your energy use. The better ones help you out with, for example, drop-down menus for the make and model of your car so that the miles-per-gallon figures are plugged in automatically. Lazier calculators use regional or even national averages to reach their conclusions. Opting for speed and simplicity guarantees diminishing returns; at some point, you might as well simply contemplate the 20 tons of CO2 the average American produces each year. (By comparison, Germans emit 10 tons, Mexicans 4.2, and Haitians 0.2. The world average is 4.5 tons, according to the United Nations Development Programme's 2007-2008 human development reports.)
Sophisticated calculators require substantial homework and the patience to submit to an exhaustive grilling. You may be asked to tally your electricity and gas bills over a year, count how many incandescent lightbulbs your home has, or remember how many airplane trips you've taken and how long each was. For the eco-obsessive, some get deep into the energy-efficiency weeds, giving extra credit for each double-paned window and low-flow showerhead. After determining what state you live in, a well-designed calculator will also factor in the source of your electric power: coal, hydro, natural gas, nuclear, or (lucky you) wind or solar. Consider it the environmental version of original sin. At the Nature Conservancy's Web site, for example, I mistakenly entered "Washington, D.C." instead of California and was dunned an extra 3.2 tons a year as a result. (Much of Washington's power comes from coal-fired generators, while half of California's comes from hydro, geothermal, nuclear, and alternative sources.)
Rule of thumb: The more narrowing questions you're asked up front--where you live, how large your house is, how much money you make--the better the results. (The income figures are a proxy for general consumer spending, a very difficult figure to quantify otherwise.) The more precise the information you enter--how many kilowatt-hours you use in a year, how many miles you drive--the more precise the results. Conversely, as with computers, so with carbon calculators: garbage in, garbage out.
Why are there so many calculators? The vast differences in design are not happenstance: Apart from a few academic models, most are tendentious. Some are clearly aimed at the well-heeled, busy, and guilty, offering a quick calculation and quicker absolution via PayPal in the form of carbon offsets, the financing of compensatory good works elsewhere. Al Gore's climatecrisis.net offered to offset my "average" total of 8.85 tons of CO2 per year for $108. (Watch out, though, if you settle for the Web site's default averages for home and automotive emissions: In my case, doing so doubled my tonnage, upping the amount needed to expunge my guilt to $216.) TerraPass, another site selling offsets, reckons only by household, not individual. While methodologically defensible (it is kind of silly to parse out one's personal fraction of the family car), doing so does yield a profitably larger number.
Other Web sites tote up your excesses and then offer to sell you the energy-efficient appliances you'll need to bring your score down. Nothing wrong with that as long as it serves the larger interest of reducing carbon emissions. But you'll never be able to buy enough superefficient toasters to cancel out those weekend flights to Las Vegas.
To satisfy the suspicious as well as the simply curious, the workings of a carbon calculator ought to be transparent. That is, you should be able to root around and find not only the data sources the calculator relies on but also the formulas and equations it employs to crunch the numbers. Here the academic and government models tend to excel; the EPA's calculator is available in a spreadsheet version, with all the calculations out in the open. At the other extreme, BP and TerraPass are mum on how they tote up their sums.
Even when a calculator's cards are all on the table, there's a lot to be dubious about. "I don't feel we're ready for carbon calculators," says Arpad Horvath, an associate professor of civil and environmental engineering at the University of California at Berkeley. "The science isn't there yet." What's missing, in Horvath's view, are the fundamental lifestyle assessments that would tell us how much carbon we're really creating. Calculators tally tailpipe emissions from cars pretty well, but what about the emissions required to manufacture the car? Or to bring the steel to the assembly plant? Some calculators take a stab at estimating these indirect emissions, and when they do, the results are very large: My score from the Berkeley Institute of the Environment was 11.1 tons per year from direct emissions, but 34 tons including indirect emissions. A true accounting of total emissions, Horvath suggests, would be far larger still.
Many of the calculators I surveyed promised new, snazzier versions in the near future. Those with snazz already in place tell you what lifestyle changes would do the most to lower your score and allow you to challenge your friends and associates to take public transit to work for a week, say, or swap out those last incandescent bulbs for compact fluorescents. For really big savings, though, all these Web sites make clear that the days of carefree air travel have to end. (Oil prices are pretty much taking care of that anyway.) So maybe instead of flying to Vegas for the weekend, you could just stay up all night, get drunk, and send all your money to the Sierra Club.
Now go, child, and sin no more.
Paul Rauber is a senior editor at Sierra. He promises to get rid of his minivan as soon as possible.
Illustration by Gilbert Ford; used with permission.