If you’re like me, you need a visual reference. CO2 is a gas, and I think of gases as volumes, not weights. A book weighs something. A person weighs some amount (usually too much). A car weighs one to three tons, depending on size. That ton is 2,000 pounds, or 10 adult 200 pound guys.
But gases fill things up.
When I inflate a balloon, I can see how big it gets; the more air I blow into it, the bigger it gets. But I don’t think about how much it weighs. Weight seems irrelevant. And yet, when we speak of carbon, we talk about metric tonnes.Â Metric tonnes are size, and tons are weight.Â An 30 foot high weather balloon doesn’t weigh much, but takes up a lot of space.
At an Advisory Committee meeting at Philadelphiaâ€™s Franklin Institute some time ago, I posed the question of what is a metric tonne of CO2 to a group of very smart climate scientists. They immediately started spewing numbers and concepts and approaches with machine-gun speed and precision and after a few moments of talking about moles and scfâ€™s (standard cubic feet) and pound moles, they had the answer: one ton of CO2 is about the size of 35 Toyota Camrys, which about 57 tons in weight. (2011 manual Camry = 3263 lbs x 35 /2,000).
Hereâ€™s a short version of how they got there.
- One ton equal 2,000 pounds.
- The density of CO2 is 0.1234 pounds per cubic foot.
- So one pound of CO2 fills about 8.1 cubic feet of space. Thatâ€™s a cube thatâ€™s 2 feet on each side. That may seem like a small volume to weight one pound, but remember that one pound is what a regular can of corn or two cans of tomato sauce weigh. (Thatâ€™s two 8 ounce cans, of course.)
- 2,000 pounds of CO2 fill about 16,200 cubic feet.
- That 16,200 cubic feet is about 25.3 feet on each side.
- One way to visualize one ton of CO2 is as four standard (not the new stretch buses) New York City transit buses, which are about 4,016 cubic feet (8.5 X 13.5 X 35). Thatâ€™s typical for city buses or long distance or charter buses.
- Or 35 Toyota Camry’s.
So there you have it: one metric tonne of CO2 is approximately 35 Toyota Camrys:
But there’s also weight.
Not to be too confusing, when you have driven 8,000 miles, getting 20 miles per gallon, you have emitted four tons of CO2 by weight.Â Which means youâ€™ve emitted about as much carbon as your SUV weighs, which in turn equals the volume of 140 Camrys (4 tons of CO2 = equivalent in metric tonnes of 35 Camrys x 4). The US average miles driven per car was 13,785 in 2001.
Hopefully this has clarified what a metric tonne is, and the significance of efforts that remove actual–or the equivalent of–vehicles from the road.By Gelvin Stevenson, Ph.D., Director of the NY Energy and Environmental Funders breakfast program of the Center for Economic and Environmental Partnership, Inc. and Adjunct Professor of Environmental Economics at Pratt Institute.