IT'S A RELIEF TO BE ON THE GREEN ISLE, a CO2-safe zone where, for once, my everyday habits won't add to climate change. I've come to Denmark determined to be an ultra-low-carbon traveler, but I spent my first few days in Copenhagen, where I really had to work at it. I rode a Dahon folding bike all over the place and calculated the CO2 output of every mile of public transport so I'd know how many trees to plant or clean-energy credits to buy when I got back home, to make up for my carbon transgressions. In the biggest sacrifice, I spent three nights sleeping in one room with 33 other people at the Sleep-in Green hostel in Copenhagen. I took to calling it the Can't-Sleep-in Green. So what if the bunk beds squeaked and the girls from Texas donned their boots at five in the morning (and left large canisters of hair products that simply would not sort into any of the dazzling array of recycling bins)? It was great for my planetary balance sheet: solar electricity, organic juices, energy-efficient lightbulbs, and minimal waste.

To get with the program even more, I sought tips from a local carbon guru, Bente Hessellund Andersen, who's known for her polite climate harangues directed at non-green European leaders. A longtime environmentalist, she's neat and pretty, a soccer mom without the SUV. She doesn't even own a car. "I feel sad when young people tell me they need to have a car—very sad—since it's their kids who'll suffer from global warming," she told me during a torrential rainstorm at a bar near Tivoli Gardens. We were both sopping wet, having arrived, naturally, by bicycle.

Each Dane produces on average 13 tons of carbon dioxide per year, Andersen said. (She herself produces less than that.) Each American produces about 20. "We call it 'the American lifestyle' when we eat a lot of meat," she said, and proceeded to explain the golden rules of a low-carbon life, ticking off a list that made McDonald's look like the seventh ring of hell: Eat vegetarian, locally grown foods, preferably organic, because they require less gasoline to transport and fewer fossil-fuel-laden pesticides and fertilizers. If you're really feeling zealous, eat your food cold, or, if you need it hot, keep a lid on the pot. "And as a tourist, buy locally made goods," she added. I don't think she had in mind my latest souvenir: a gel-filled plastic pen with a floating Viking ship inside.

But the biggest carbon spewers, Andersen went on, are transportation and housing. Together they contribute 60 to 80 percent of a person's CO2 footprint. I was off the hook with my carrot slaw consumption and with the hostel and the bike, but the planes and trains I'd taken had done some damage. Andersen referred me to some Web-based carbon calculators to figure out my transportation debt and told me to consult a report about food-associated CO2 emissions, appetizingly called "Eating Oil." It might help me assess how many greenhouse gases were released to make my $12 lime-drenched mojito the other night at a restaurant called Pussy Galore's Flying Circus. I should have had an aquavit.

On Samsø, though, I could stop tallying and relax. Some islands feel magical because of healing waters or succulent fruits. Samsø's aura comes from the fetching way the islanders crunch their numbers to achieve the great Kyoto-driven concept known as offsets. Here's how it works: If you use dirty power, which most of us must, you just have to make up for it. Islands, nations, or citizens can take steps like planting thousands of trees to cancel out the carbon dioxide they've added to the atmosphere. They can also produce clean energy for others to use, thereby preventing additional emissions of greenhouse gases. With enough of these measures, it's possible—on paper, anyway—to become carbon neutral or even carbon negative. Samsø's wind turbines, for example, produce about 105 million kilowatt-hours of electricity per year, but the island needs only a quarter of that, leaving a surplus of 77 million kilowatt-hours, which enters Denmark's main power grid. Meanwhile, Samsø's petroleum-fueled transportation sector uses the equivalent of just 53 million kilowatt-hours per year, so the islanders figure they're still in the cosmic clear with room to spare.

The bottom line: Samsø is 140 percent carbon negative, while virtually the rest of the universe—except for off-the-grid pockets in a few communities and the solar-powered International Space Station—is carbon positive to the tune of adding 27 billion metric tons of CO2 to the atmosphere each year. (Afghanistan and Chad are among the nations with the lowest per-capita carbon emissions; the U.S. releases the most overall—nearly one-quarter of the planet's total.)

Someday, Samsø hopes to use its surplus power to make hydrogen or charge lithium-ion batteries to run its cars. In the meantime, I can thumb a teeny car ride (three miles) in the rain. It's offset! I eat a thick slice of beef at a smorgasbord. Even though the meat probably originated in Argentina, I am guilt-free. Samsø's clean power flowing into the Denmark grid means some coal plant doesn't have to produce the wattage. Sorry, Bente. Please pass the roast.