Why do they call it a ''carbon footprint?''

After all, there is nothing wrong with carbon. It's the building block of life.

But carbon dioxide -- the most ubiquitous of the greenhouse gases -- is given off as carbon changes form. When coal is burned to generate electricity for your home or gasoline is consumed to power your car, your ''carbon footprint'' expands accordingly.

The United States is a world leader in per capita emissions of these greenhouse gases. The Environmental Protection Agency estimates the average emissions for a household of two at 41,500 pounds per year.

That means you and your community can make a big difference by measuring your carbon footprint and taking steps to reduce it.

What do I need to know?

You'll need some hard data in hand before you can calculate your carbon footprint.

• How much electricity, gas and water does your household use?

• How many miles do you drive each year?

• How often do you travel by air and how far?

Invisible footprint

Beyond this obvious energy consumption, some of our carbon usage is hidden. Carbon-based fuels power the pumps that bring water to our taps and a lot of fuel is burned to give us our daily bread. The average food item on the American table has traveled 1,500 miles to get there.

Shrink your footprint

Once you've identified the major sources of your carbon footprint, what can you do to reduce it? Most online calculators give you a chance to lower your score by listing your recycling habits and then show you how much you can save with further reductions in consumption or other measures.

To make a difference, you can take big steps or baby ones. Here are some tips and some resources for doing that.

At home

Big step: Buy a new house. Modern construction techniques and proper materials can halve the amount of energy required to heat and cool an older home.

Baby step: Lower your water heater temperature and set your thermostat no higher than 70 degrees in winter and no lower than 78 degrees in summer.

On the road

Big steps: Change your work schedule to avoid stop-and-go traffic at rush hour. Or move closer to where you work or to a mass transit route that gets you there.

Baby steps: Carpool, walk, bike, take a bus or work from home at least one day a week. Keep your car tuned, your air filter clear and your tires inflated to recommended pressure.

At the tap

Big step: Fill in the swimming pool and tear out your lawn.

Baby step: Catch rainwater and use it for irrigation.

Food

Big step: Become a vegetarian. Feeding crops to cows and pigs requires seven times the energy input of eating those crops directly. Grow your own produce.

Baby steps: Eat fruits or vegetables in season.

If we all changed our lives, we could change the world. But experts say even smaller steps make a difference. And your chances of sticking to a reduced-carbon diet increase when you set realistic goals.

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Carbon offsets: Buyer beware

Q. What is carbon offsetting?
A. Carbon offsetting compensates for greenhouse gas emissions with an equivalent carbon dioxide reduction. Anything we do that consumes energy in turn produces these emissions. Driving a vehicle or heating or cooling a home are only the most obvious examples.

Q. How do I offset ?
A. First you figure out your ''carbon footprint'' using one of the many calculators available online. Then you can take steps to reduce your footprint or buy ??carbon offset' credits from environmental projects.

Q. What are the options for credits?
A. Carbon credits try to create a market for reducing greenhouse emissions by giving a monetary value to the cost of polluting the air. Politicians tout carbon emissions trading schemes as the best means of tackling climate change, arguing that by putting a price on carbon emissions firms have a financial incentive to reduce their carbon footprint. Many companies sell carbon credits to commercial and individual customers.

Q. How do I know credits aren't a scam?
A. Ask for evidence. Reputable credit programs will have an audit trail or similar documentation that they'll share, though they sometimes charge a fee to provide it. Any credit program should be fully traceable. But results are sometimes hard to prove.

Q. So do credits really make a difference?
A. Lack of verification makes it difficult for buyers to assess the true value of carbon credits. Critics say cheap credits cleanse the conscience but not the environment, as it is easier to buy credits and keep polluting than to invest in cleaner technology or change your lifestyle.

The only sure way to make a difference is to reduce your CO2 emissions directly by being cleaner and greener in your own household and travel habits.

Sources: Green Business News, Financial Times, CarbonFootprint.com