by Megan Stoddard

Created on: March 09, 2010

A carbon footprint is the estimated amount of greenhouse gas each of us produces as we go about our lives. Greenhouse gas, for those not completely familiar with the term, is a form of pollution that contributes to depleting the protective ozone layer in the earth's atmosphere. The diminishing of the ozone layer results in more of the sun's rays being trapped in the atmosphere, causing climate change. Many greenhouse gases come from the burning of fuel, especially fossil fuels, to power vehicles, heat buildings and generate electricity.

Each of us produces some of these gases directly, such as emissions from our cars if we drive. Others are produced on our behalf. The food and goods we buy, for example, are part of our carbon footprints because of the greenhouse gases produced by growing or manufacturing them and transporting them to market.

The part of our carbon footprint that is under our direct control is called the primary footprint. Car emissions are part of your primary footprint, because you can choose whether and how much to drive.

So is the fuel that heats your home and the electricity you use. While you may not be able to completely avoid using any of these, you do have the ability to use them or not, and to increase or decrease your use of them.

Also included in a carbon footprint is the secondary footprint. It consists of greenhouse gases produced on your behalf, but not under your direct control. If you buy a shirt, the greenhouse gas that was produced by the factory where it was made is part of your carbon footprint.

So is the fossil fuel that was burned shipping it from the factory to the warehouse to the store. Ultimately, so are greenhouse gases that were produced making the raw material: growing the cotton, making the polyester in the lab and so on.

With the secondary part of your carbon footprint, you do not have the option of changing how much greenhouse gas is produced, at least not directly. Unlike the light switch in your home, you cannot control the electricity used in a faraway factory. If the farmer who grew the cotton to make your shirt used a tractor, its emissions were not under your control.

However, you can affect your secondary carbon footprint to some extent by choosing what to buy. If you get vegetables from the farmers market, for instance, they have traveled a much shorter distance to reach you than most grocery store vegetables.

If you buy a shirt from a thrift shop instead of buying a brand new one, you are not adding

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