That's a surprisingly complex question to answer. When I recently ran my own carbon numbers using two different online calculators, I got wildly different answers (neither of which was as good as I'd hoped -- a disappointment since I'm currently car-free.)

Organizations which "inventory" greenhouse gas emissions (such as Econergy International, which did Boulder's emissions inventory in 2004) must account for several factors.

The obvious contributors to carbon dioxide emissions are: what your local utility burns in its power plants (the "fuel mix"), your vehicle's fuel efficiency, how much you drive, the number of people in your household, and your home's energy sources and consumption.

Less-obvious but still important factors are: how often and how far you travel by airplane; your workplace energy consumption; how much trash you recycle, compost, or landfill; whether you release ozone-depleting chemicals into the atmosphere; whether and how much you use recreational vehicles; and the energy required to manufacture and transport everything you buy or use (including food).

By now you're probably starting to see that there are no easy answers when it comes to measuring your personal carbon footprint. That's why it's important to be skeptical about online tools.

Since climate change has gained widespread media attention, carbon footprint calculators have become all the rage online. Sites that offer them include Al Gore's Inconvenient Truth, the US Environmental Protection Agency, CarbonFund.org, BP Global, the Nature Conservancy, and ZeroFootprint. There are many, many more.

The trouble with all of these calculators that I've seen is that they make a lot of assumptions in order to make them easy and fast to use. Unfortunately it's not always easy (or even possible) to find out what those assumptions are and whether they reflect your circumstances. Also, for simplicity's sake many of these calculators neglect potentially major carbon emission sources.

If you use carbon footprint calculators, it's better to run your numbers through several of them rather than just one. You'll probably get very different answers, but after looking as several results you'll start to get a feel for what's realistic in your case.

Choose carbon calculators that best reflect your major energy usages. For instance, I often fly for business, so it's important that I use calculators that asks about my air travel habits.

Try it! Go to several carbon calculators, run your numbers, and post your diverse results in a comment below. What did you learn from this process?

... This process should help you realize that as the city of Boulder proceeds with its Climate Action Plan, it's important to understand how those emission estimates were calculated. Without that context, we have know way of knowing whether any efforts are making a real difference.

(Photo courtesy of Stevie-B, via Flickr, Creative Commons license.)