Why do we care about carbon (actually carbon dioxide)?
Let's start at the beginning. As you may already know, natural gas, oil, and coal are sometimes referred to as fossil fuels – but do you remember why? Every living thing contains carbon. In fact, plants are almost HALF carbon. What we now use to power our cars and run our heaters are the chemical remnants of long ago plants and animals (fossils). Doesn't sound so bad, right?
Unfortunately, when we burn these materials, we get a negative output – carbon dioxide, a "greenhouse gas." Carbon dioxide and other greenhouse gases are necessary in our atmosphere, trapping some of the warmth from the sun to keep the earth at a livable (for humans) temperature. The overabundance of these gases makes the greenhouse effect too strong, leading to the earth becoming too warm.
What is a carbon footprint?
Now that we've discussed what carbon is, let's talk about the term "carbon footprint." Whenever you walk through mud or sand, you leave a footprint – you leave an impact. The same thing happens when you consume fossil fuels. When you drive instead of walk or blast the AC instead of opening a window, the surplus carbon dioxide created leaves an impact on the environment.
The term carbon footprint, therefore, refers to a measure of your (or your company's or a product's) impact on the environment. Carbon dioxide isn't the only greenhouse gas: water vapor, methane and nitrous oxide are also included. For ease of calculations the other gases are converted to carbon dioxide so all gases may be added together to yield a single carbon dioxide equivalent.
Direct versus indirect emissions
Much of the confusion around footprints comes down to the distinction between 'direct' and 'indirect' emissions. The true carbon footprint of a plastic toy, for example, includes not only direct emissions resulting from the manufacturing process and the transportation of the toy to the shop: it also includes a whole host of indirect emissions, such as those caused by the extraction and processing of the oil used to make the plastic in the first place.
To give another example, the true carbon footprint of driving a car includes not only the emissions that come out of the exhaust pipe, but also all the emissions that take place when oil is extracted, shipped, refined into fuel and transported to the petrol station, not to mention the substantial emissions caused by producing and maintaining the car.
Courtesy of Michael Berners-Lee and Duncan Clark