What the term means

It's a measure of the total amount of harmful greenhouse-gas emissions that people produce, either directly or indirectly. These gases warm the atmosphere by absorbing heat that's radiated by the earth, then releasing only a portion of that heat into space. The most dangerous, long-lasting greenhouse gases produced by humans are carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (aerosols containing this were banned in the U.S. nearly 30 years ago). Carbon dioxide, or CO2, is the most pervasive of the evil gases (a molecule stays airborne for more than a century), and since it's tough to separate and quantify each distinct greenhouse gas, scientists commonly use CO2 to measure the global-warming problem.

Why is it called a "carbon footprint" and not "carbon-dioxide footprint"?

All of the greenhouse gases we just mentioned have carbon in common. Analysts often convert these gases to "carbon equivalents" so they can compare apples to apples in terms of emissions.

Is carbon all bad?

Actually, it's not. Diamonds are pure carbon, and what's not to like about them? Our bodies are mostly made of carbon, as is just about everything we touch and see, from tree stumps to trash bags. But carbon is also oil, natural gas, and coal — hence, its polluting connotation. Match the carbon in the atmosphere with pairs of oxygen molecules that are created through the combustion of trees, dung, and fossil fuels, and it becomes CO2. The current global atmospheric concentration of carbon dioxide is as much as a whopping 40 percent higher than levels in the air before the industrial revolution.

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