Carbon “Footprint” and the Human Energy Reference Day (HERD)

A new ad campaign by BP (British Petroleum, or, if they prefer, beyond petroleum) asks people whether they know their "carbon footprint". The first ad was all people saying, essentially,"huh?". The second has people guessing, correctly, that it refers to our individual use of fossil carbon fuels and contribution to greenhouse gases, especially carbon dioxide (CO2). This could be a wonderful example of corporate responsibility. At least it might get more of us thinking about our collective and individual impact on the earth.

How should we measure our impact? What's a meaningful measure? Tons of CO2? Barrels of oil? These measures are pretty abstract. Let's do some calculations and see if we can derive a more useful unit of measure.

We each use about 2,000 k calories each day just living and breathing. A serious athlete might use twice as many while a comatose patient might use half as many. These calories all come from carbon, which we consume as food and exhale as CO2. This is our absolute minimum "footprint". I propose we use this as our measuring unit, a human energy reference day (1 HERD).

Here are a couple examples. If you use a 2 kilowatt hours of electricity in a day, that's about the same as your body burns, so 1 HERD. If you use 10 gallons of gas per week, that amounts to 45,000 kcal per day, or 20 HERDs. Looks like carbon "tire tread" is more appropriate than carbon "footprint".

By the way, eating more food than you need produces fat but not CO2 as long as you're alive. America's epidemic of obesity is not all bad. Taken together, these unburned calories represent an enormous carbon sink that should help mitigate global warming! Can you say carbon credit?