"Put it on my carbon tab." (damaging effects of carbon release on the environment)

New people notice it, but an "I.O.U." is attached to each of our paychecks. Accountants don't record it on their ledgers, tax collection agencies don't pursue it, and no bank will honor it. Yet this debt is tabulated each day, a subtle component of the work we do, the entertainment we enjoy, the objects we consume.

What is this pervasive lifestyle tab? It's the carbon dioxide we "spend" each day. The world economy is fueled by burning oil, coal, and gas, which releases almost 6 billion tons of carbon into the atmosphere every year. Initially, that carbon is released free of charge. But if scientists are correct, then the bill for our carbon spending will come due down the road as its environmental repercussions are debited. Climate change caused by unprecedented concentrations of carbon dioxide in the atmosphere will disrupt the world's economy, they say. Sea levels will rise, crop yields will drop, forests will migrate, and energy needs will climb. We will have spent away the security that a balanced atmosphere and stable climate provide.

William Cline, senior fellow at the Institute for International Economics in Washington, D.C., estimates those future costs, for the U.S. economy alone, at about $60 billion a year toward the middle of the next century if the atmospheric levels of carbon dioxide double, and at about $335 billion a year in the distant future, assuming temperature continues to rise. That is a bigger bill than most people expect when they are sitting in their cars in five o'clock rush-hour traffic.

On average, for every $3.19 of world economic output, one kilogram (2.2 pounds) of carbon is released into the air. That means the average person sends the equivalent of his or her body weight of carbon into the atmosphere for about every $200 that he or she spends.

All the clothes that we wear, the food we eat, and the houses we live in are made with processes that burn energy. When we look at those components of our lives, we may not see the energy that goes into them, but it is there in their manufacture, transport, maintenance, marketing, use, and disposal.

Of course, the amount of carbon people "emit" varies, usually depending on where they live. Though it contains less than 5 percent of the world's population, the United States emits 22 percent of all carbon. All developing countries together - which account for almost 80 percent of the world's population - produce not much more than that. The average American sends 15,000 pounds of carbon into the air every year. If you spent $20,000 last year (and you are of average size), then your spending dispersed 100 times your own weight of carbon into the environment.

Different sectors of the global economy emit carbon at different rates as well. Manufacturing emits more carbon per dollar earned than service industries do. Making and using a manufactured product typically releases more carbon than buying a service like a haircut, going to a play, or paying for education. Likewise, transportation produces a lot of carbon. In industrial countries, travel accounts for more than 20 percent of energy use-automobiles alone use about 17 percent. Each gallon of gasoline used to fuel our cars weighs almost seven pounds, and about six of those pounds are carbon. When it is burned, most of that carbon ends up as exhaust.

The problem is that people don't pay for their carbon emissions, and so they make little effort to spend them wisely. If they had to sign an I.O.U. for each kilogram of carbon released, to be paid from their future salaries, then they would suddenly have reason to tune their cars and recycle their newspapers. But the popular conception is that people have a right to use the atmosphere for free, and the popular system for collecting revenues does not bill people based on carbon.

Maybe it should. Imagine a meter on the outside of every house, like the gas meters that we have now. People could be charged for the carbon they emit. That would integrate the future costs of climate change right into household decisions. It might sound impossible, but the carbon content of electricity and gas can be calculated. Carbon metering would, in a sense, allow people to think with their pocketbooks when it comes to the environment and the future.

A tax on carbon would also go a long way toward linking emissions to economics. Since a carbon tax would be a levy on energy, it would stimulate efficiency and cut energy consumption. It also would shift the choice of fuels toward those with less carbon. Coal contains 80 percent more carbon per unit of energy than natural gas and 30 percent more than oil. And solar, wind, hydroelectric, and some other renewable sources of energy produce no carbon. So a tax on carbon would apply less to oil than to coal, less to gas than to oil, and not at all to most renewables.

Policy solutions are important, but lifestyle changes can make an impact too. The National Audubon Society says that when it comes to showering, shaving, or washing dishes, running the hot water only when actually washing and rinsing can eliminate between 325 and 1,000 pounds of carbon emissions a year per house. But many people don't feel any real pressure to make those changes - it's easy to see the value in it, but actually changing one's habits early on a Monday morning is another matter.

Keeping a car well tuned saves 300 pounds of carbon a year, assuming it travels 10,000 miles a year. Getting more miles per gallon through better efficiency and taking other anti-pollution measures would have an even bigger effect.

Replacing a 100-watt incandescent bulb used six hours a day with a compact fluorescent bulb can save 70 pounds of carbon a year, says National Audubon. Recycling paper or glass cuts emissions associated with their manufacture by 30 percent; recycling eight aluminum cans cuts emissions by a pound of carbon.

But the costs of emitting carbon are delayed. The bill won't have to be paid until well into the future-and when it is paid, today's user of light bulbs, paper, and aluminum cans will not necessarily be among the people who pay the most. The harm that may come to a tropical beach or low-lying country like Bangladesh originated with carbon released far from there. So it's easy for those invisible grams of carbon to slip to the back of one's mind.

Indeed, not only are carbon emissions unpaid for - they are unreported and, hence, unconsidered. Every day, newspapers recount retail sales and housing starts - but not the emissions associated with those trends. Journals report the prices of a barrel of oil, or a ton of coal, but not the carbon content of either fuel. Energy analysts pore carefully over up-to-the-minute tallies of supplies of Middle Eastern or North Sea crude oil, but they ignore the atmospheric effects of producing and burning their commodity.

The first step toward solving the problem is gathering information and trying to understand it. Changes in carbon emissions may have a far greater effect on our lives than changes in indicators like the Dow Jones stock index, so they should be tracked carefully. Even though they are measured in kilograms instead of dollars, carbon emissions are nevertheless an economic indicator and a measure of society's prospects for the future.

Fortunately, more efficient new technology and a shift toward service jobs instead of manufacturing have made many national economies less energy intensive as they have grown. In the industrial countries, energy use rose only one-fifth as much as economic growth between the first oil shock of 1973 and 1989.

Those changes came about because the higher cost of energy forced industry to find ways to use it more efficiently. The challenge now is to further encourage such changes, sometimes by using the same market forces. By incorporating the costs of pollution into the price for fuels, instead of just the costs of extraction, decisions over how carefully to use energy can reflect its real consequences.

Ultimately, reducing carbon emissions will depend on whether we incorporate their costs into the way we think about economics and the way we think about our lifestyles. By failing to do so, emissions will continue to rise as if the atmosphere could absorb them for free.