

We need to talk about your ''carbon footprint,'' a concept you may have learned from Al Gore. If you've seen ''An Inconvenient Truth'' or read the best-selling book, you know how strongly he feels about everyone's duty to reduce carbon dioxide emissions. He advises you to change your light bulbs, insulate your home, and cut back on driving and air travel. If you must make a trip, he notes helpfully, ''buses provide the cheapest and most energy-efficient transportation for long distances.''

Fine advice, and it would be even better if he journeyed to his lectures exclusively on Greyhound. But he seems to prefer cars and planes. When you tally up his international travel to inspect melting glaciers and the domestic trips between his homes -- one in Washington and another in Nashville, not to mention the family farm in rural Tennessee featured in the movie -- you're looking at a Godzilla-sized carbon footprint.

No matter how many fluorescent light bulbs you install in your second home's basement, you could save a lot more energy by eliminating the whole place. Even if you dutifully shut down each home when you leave it -- turning off the electricity, draining the pipes and turning off the heat, etc. -- you're still expending extra energy commuting between your homes. A trip to a weekend house can easily burn more gasoline than a commuter uses all week.

Yet somehow, in all the years I've been reading lists of energy-saving tips, I've never noticed, ''Sell second home.'' A cynic might attribute this oversight to a high correlation between fervent environmentalism and second-home ownership -- Robert Redford and his place at Sundance, the Kennedys and their compound on the Cape, Laurie David and her home on Martha's Vineyard, John Kerry's seaside and mountainside manses.

Granted, some environmentalists deal publicly with their carbon footprints. Gore and David say they offset their energy usage by sponsoring reductions in greenhouse gases through alternative forms of power and energy conservation (like building wind farms and paying farmers to turn methane into electricity). But are ''carbon offsets'' sufficient compensation? Not to activists like Charles Komanoff, an economic consultant to environmental groups.

He argues in Grist, an environmental magazine, that paying a penny or so per mile to offset the carbon from driving your car isn't the moral equivalent of riding your bike instead. It's more like the Catholic Church's old system of selling indulgences so the rich could avoid something scarier than global warming: purgatory. Quoting Gandhi -- ''Be the change you want to see in the world'' -- Komanoff says his fellow environmentalists should stop offering ''get out of purgatory free'' cards to the rich and instead insist that everyone personally reduce energy use.

I'm not such a purist myself -- I'd let the average person salve his conscience with a carbon indulgence. But I'd hold environmentalist preachers like Gore to higher standards, especially when they're engaging in unnecessary energy use. And since I cannot afford a second home, I can objectively determine it to be unnecessary.

If you're going to own a second home while ordering everyone else to carpool, you must atone for your excesses, and it's not enough just to offset the carbon. Gaseous emissions aren't the only externalities of your home. By owning it, you're also inducing envy in your neighbors. You're contributing to the competitive urge that the economist Robert Frank calls ''luxury fever.'' When you go off for the weekend, those of us left sweltering in the city start lusting for our own second homes. We start dreaming of cutting down carbon-dioxide-absorbing forests to make room for neo-Adirondack cabins with central air and heated pools.

The best way to tamp our enthusiasm -- or, I as prefer to put it, to reward our virtue -- would be with money. Besides paying farmers not to waste methane, you should be paying us not to build second homes. You could make the payment directly to your neighbors. Or, if you prefer, mail it to me, and I'll distribute it among the worthiest of my fellow single-home owners.

If you're short on cash, you could still atone with an in-kind payment: let me stay in your country home while you perform your energy-saving penance back in the city. It wouldn't have to be a long penance. By my calculations, the month of August would just about wipe out your sins.