Not too long ago, the concept of a Carbon Footprint was a foreign one. But as global climate change theory has gained traction in the general public, carbon footprint has become common lingo. Online calculators which account for ones personal carbon emissions have emerged as a popular tool for the climate-minded to track and reduce their greenhouse gas emissions. These calculators are so prevalent that if you search Google, you'll find a wide selection waiting for your personal energy and transportation data. Here's the problem though: not all carbon footprint calculators are created equal. So to help you parse out the good from the bad, and assess the accuracy of a quick and dirty approach, I tested nine different calculators.

The Calculators
The calculators were provided by a smattering of nonprofits, energy companies and government agencies. The organizations ran the gamut from mainstream to fringe. Overall, though, the calculators were quite similar. They largely concentrated on two categories: home energy use and transportation energy use. Home energy use broke down into heating and electricity, while transportation divided into ground and air use. These are the nine which I tested:

• BeGreen
• Bonneville Environmental Foundation
• Conservation International
• Cool It
• Environmental Defense
• Environmental Protection Agency
• National Wildlife Federation
• Terrapass
• World Resource Institute

Before you use one of these calculators, you'll need to gather some information. Generally, you'll need the following data:

• your monthly electricity consumption in kilowatt-hours (kWh)
• your monthly home energy consumption (natural gas, oil or otherwise often measured in therms)
• the amount of driving you do each year (in miles)
• the miles-per-gallon (MPG) of your car(s)
• the number and length of flights you've taken
• additional information, such as waste, diet or public transit may be included as well

While bringing together my own personal energy information, I realized that I didn't have accurate numbers for my personal transportation, but I did have all of my home energy data, which I got from my power company. In the end, none of these calculators will be able to provide a finely detailed assessment, but the more accurate the data you can feed them, the closer they will come to your actual footprint.

Over the course of the year, I averaged 256 kWh per month and 56.6 therms from natural gas. For the transportation data, I estimated that I drove around 15,000 miles over the last year and that I went on four round-trip medium haul flights.

The Results
As based on the estimates from nine calculators, I've determined my carbon footprint is between 11 and 19 tons with an average of 14.8 tons of carbon dioxide per year. I found that it was easier for the calculators to estimate home-energy carbon emissions than it was for transportation emissions. The area of greatest variation within the results was air travel.

Nine different personal carbon calculators, U.S. tons of CO₂ per year.

One possible reason for the variation within travel emissions estimates is that, while home energy use is fairly straightforward to estimate (which are based on known, standardized equations), travel is a bit more complex. For instance, the level of detail which you factor into your estimate can change the outcome a great deal. A flight from Salt Lake City to Seattle could be described as a short range haul (less than two hours). It could also be described in terms of air miles (687 miles one way, 1374 roundtrip). The latter is more accurate, but even that level of information could be improved upon. The type of aircraft, direction of flight, and even the weather conditions can affect the carbon emissions. The calculators aren't built to capture that fine a level of detail, and their methodology vary considerably.

Which is the best?
Of the nine which I evaluated, three stood out for various reasons. The first, the Bonneville Environmental Foundation, was visually appealing, and it had the option to either approximate your footprint by using averages, or to enter in your own data.

The second, from TerraPass, had a simple and user friendly interface, but it still incorporated a significant level of detail. For example, you can enter individual flights with specific destinations. This calculator however, used your combined electric/gas bill to determine home energy use, rather than building an estimate using kWh and therms, which is a more accurate method.

The final calculator which I recommend is the World Resource Institute's. What distinguishes WRI's calculator from most of the other calculators is that they offer comprehensive methodology for how they derive their estimates. As with the others, it was simple and thorough. All three broke down the carbon footprint into driving, air travel and home energy use.

A Footprint in Context
The per capita emissions for the U.S. was about 20.5 metric tons in 2005. If I convert my carbon footprint from U.S. tons to metric tons, it's roughly 13.5 metric tons, which is about 65% of average. I'm feeling pretty good about my energy use. Now let's look at the rest of the world.

Top ten countries CO₂ emissions, metric tons per capita for 2005. ^a

Alas, my elation is short lived. While my carbon footprint is small for an American, it is still nearly three times the world per capita average of 5 metric tons per year. And the per capita ranking is somewhat misleading; all those countries ranked ahead of the U.S. on the chart are actually quite small in terms of population. In terms of total emissions, the U.S. dominates the world.

Top ten countries CO₂ emissions, total metric tons for 2005. ^b

Over at WorldMapper.org, they put these numbers into a visual context which illustrates just how big an impact the world's large, industrial countries have on GHG emissions versus the rest of the world.

Territory size shows the proportion, by their global warming potential, of all greenhouse gas emissions that come from there.^c

Treading Lightly
Knowing the personal impact on climate change can be useful in curbing energy use. However, these sorts of general footprint calculators are just a start. In order for people to make better decisions - in terms of how they get from point a to point b, which refrigerator to buy or how they power their home - they will need more detailed information in an easily digestible format. Because there is no silver bullet for the emissions problem, the carbon emissions implications for everyday, mundane choices must be made clear, so that all of those decisions add up to the type of national behavior change which could precipitate real climate change mitigation.

Here are a few links to some organizations which are trying to help people make better decisions and achieve that sort of change.

   Consumer Reports Greener Choices
   EcoLabelling.org
   Travel Analytics

^a World Resource Institute: Climate Analysis Indicator Tool — CO₂ Emissions: CO₂ emissions per capita (source: IEA) Units: Metric tons of CO₂ per person
^bWorld Resource Institute: Climate Analysis Indicator Tool — CO₂ Emissions: Total CO₂ emissions (source: IEA) Units: Million metric tons of CO₂
^c© Copyright 2006 SASI Group (University of Sheffield) and Mark Newman (University of Michigan).
Footprint image from Footprint Designs
Beach image from Flickr