[This piece was written for the Ann Arbor getDowntown Commuter Challenge blog. I will probably work up a revised version of it for EcoGeek in the next few days, as well. Although I talk about local things, the larger point of it is still relevant, even if the Ann Arbor Commuter Challenge doesn't matter to you.]

Right now, midway through the month, Workantile Exchange, the coworking space I am affiliated with, is leading the 50-100 employees segment of the 2011 Commuter Challenge and has avoided about 1/2 ton of CO2 emissions from the logged commutes. We use a ‘ton’ to mean a lot, but what does that mean for CO2?

A car weighs a ton or two (a current model Ford Focus has a curb weight of about a ton and a half), but it’s made from much heavier materials. But CO2 is a gas. It is much harder to have an image of what it means to have a ton of CO2.

A little research and some calculation shows that a ton of CO2 will fill a volume of about 17,850 cubic feet at standard atmospheric pressure. To help visualize it, this is roughly the volume of a two story house about 2,000 square feet in size. (25 feet deep x 40 feet wide x 17.86 feet tall) If you know of a house that is about that size, then think of that volume filled with pure carbon dioxide; that’s what one ton of CO2 is.

Add up all the participants and the amount of CO2 they are not putting into the atmosphere with their commutes this month, and there’s a subdivision’s worth of CO2 that has been offset already.

Another way to look at it is to think about how long it takes for you to put a ton of CO2 into the atmosphere. The EPA uses the figure of 19.4 pounds of CO2 per gallon of gas. So, roughly speaking, every 100 gallons of gas you use puts a ton of CO2 into the atmosphere.

And that’s just the carbon dioxide. If we are talking about automobile emissions, then there are all kinds of nitrogen oxides, uncombusted hydrocarbons, soot, and other pollutants that are being put into the atmosphere, as well.