To avoid setting too-high hopes for voluntary carbon offsets - the system by which a traveler can give money to a wind farm project in Minnesota, for example, to minimize the environmental impact of a flight to Tuscany - it helps to stay grounded.

For that, turn briefly to northern Norway and to Cecilie Hansen, farmer and vice mayor of an Arctic region along the Russian border. Hansen lives rooted in a natural place with cows in the fields and bears in the forest. Yet she has flown through crowded European hubs en route to work and play.

"We cannot travel in the future like we do today," Hansen said, sitting at her kitchen table one summer evening of endless sun. "Vacations in Spain. Flying here, there, in jet planes. It is not possible."

But we do keep flying, more and more each year, from Boston to New York for business, from Montreal to Marrakech for escape. So as Earth Day approaches and the planet it celebrates turns toward climatic peril, it is worth another look at offsets.

The idea seems simple enough. Where conscience and capitalism intersect, a market has grown quickly in recent years for individuals to pay money for an eco-friendly project to counter the damage modern living does to the atmosphere. You can, with a few clicks on the Internet, help finance projects to make up for carbon emitted by everything from the house you heat to the car you drive. Air travel is a relatively small contributor, with far more carbon emitted by road transport, or electricity generation, for example. But many individual offset buyers start with a single step, something as specific as a round-trip flight from Boston to Washington, D.C.

The seeming simplicity of the system dissolves quickly in details. Consider three examples encountered when trying to offset a flight.

At nativeenergy.com, a website run by a nonprofit offset provider in Vermont, the round trip to Washington measures 788.71 miles, is said to emit 0.315 tons (or 630 pounds) of carbon dioxide into the atmosphere per person, and would require a $12 payment to an alternative project. The project options offered by NativeEnergy include helping with methane reduction on the Brubaker family farm in Mount Joy, Pa., and providing wind turbines for farms such as that owned by Dean Tofteland in Luverne, Minn.

At Terrapass.com, a for-profit provider that handles offset programs for Expedia, Alamo, and Enterprise car rental agencies, and other leading travel providers, the round trip measures 796 miles, contributes 509 pounds of carbon dioxide per person, and would require a payment of $9.90. Customers can choose to contribute to a methane-burning project in Arkansas, for example, or, to the Holsum Irish Dairy in Hilbert, Wis., to support a pond system that turns methane gas from cow manure into power for the farm.

Things are different yet at my-climate.com, the site of nonprofit Sustainable Travel International. The Boston-Washington trip is not measured in miles, but is said to create 627 pounds of carbon dioxide per person and to take $11.29 to offset. That money can go, for example, to a project to replace diesel generators with biomass power at a village in the Indian province of Bihar, or to the White Creek Wind Project in Roosevelt, Wash.

There are many other offset providers to choose from, some for-profit, others nonprofit, but all angling to redirect dollars. Critics of the system claim that, at worst, all this offsetting can be a distraction from the challenge: using less fossil fuel in the first place.

"Is it like the Titanic, just rearranging the deck chairs?" said Joe Romm, a senior fellow at the Center for American Progress who keeps the blog climateprogress.org.

People won't cut back on flying as long as they can assuage a guilty conscience, that logic goes. And then there will be little hope of meeting targets such as cutting carbon emissions by 80 percent over the next 40 years.

Even some supporters of offsets contend the unregulated voluntary market leaves customers with too little certainty about whether their offset purchase will do any good.

A key part of the debate revolves around "additionality": whether an environmental project would have happened without the new money created by the offset purchase. If the project isn't new, the argument goes, nothing has been added to counter the impact of the flight to D.C. or other carbon-producing activity.

Some offset dollars, for example, go to a methane-burning project run by a big corporation. Would that project not happen without the offset money?

For consumers, one useful guidepost is still a 2006 report published by Clean Air Cool Planet, a nonprofit environmental group. The report, which ranks individual providers, is still available online at cleanair-coolplanet.org; officials plan to update it later this year. The Tufts Climate Initiative (tufts.edu/tci/carbonoffsets) also maintains a helpful travel guide for consumers.

Many big travel companies, including Virgin Atlantic and Continental Airlines, take pains to tell individual buyers in their offset programs which projects the money is financing. Virgin even sells offsets in flight.

And they point to connections to specific providers as proof of transparency. Continental and Virgin partnered with Sustainable Travel International, which uses industry-generated standards, including the so-called Gold Standard system, and Green-e certification from the Center for Resource Solutions. Virgin describes the Gold Standard thusly:

"The daddy of all the offsetting standards, with backing from everyone and their dog, including lots of environmental groups, Gold Standard methodology was created to facilitate the move from fossil fuels to renewable energies."

Mark Trexler, a consultant who conducted the original Clean Air Cool Planet review, said there remains plenty to clarify.

"It still sort of remains to be seen how good the different standards are in accomplishing what we want them to," Trexler said.

Indeed, someone at a computer terminal can rightly wonder: If only 85 percent of the money, for example, goes to the offset project, that means the other 15 percent goes toward running an offset operation that presumably emits its own carbon into the atmosphere. And the projects themselves, many involving international relationships or the production of new technologies, produce new carbon. What, after all, is the net gain from buying the offset?

The bottom line, advocates say, is to get efforts moving in the right direction.

On a broad level, if the voluntary market can flourish, it could encourage policymakers to set tough regulatory standards for corporations to offset their carbon emissions. Individually, if consumers begin to measure the carbon impact of one flight, perhaps they will soon begin calculating, and offsetting, how they drive or heat their homes.

From that awareness can come lifestyle changes - trying to travel, or heat, more efficiently, for example - that reduce the carbon emitted in the first place.

"You don't have to move out to the middle of nowhere, live in a yurt, and just eat roots and grass," said Billy Connelly, marketing director for NativeEnergy.

"It's all about the incremental change."

No Impact Man agrees. Colin Beavan, as he is also known, undertook an experiment last year to live a completely carbon neutral life.

Beavan describes himself as a typical guilty guy who wanted to try to undo the environmental damage he has done. But the New York resident now talks about unexpected upsides to scaling back.

Getting rid of the television has meant more family time. Walking to local markets has meant more exercise.

That, he says, is where the battle can be won. He points to the stress of a getaway weekend, and how even if people begin to take one longer vacation rather than two weekend escapes, their number of flights, and travel stress, would be cut in half.

"I'm not saying let's be killjoys and never fly," Beavan says. "But if we can find a way to make ourselves happier, and fly less, let's do that first."

That kind of quality-of-life swing could drive habits toward the world Hansen, the Norwegian farmer, sees, in which people fly less. Before the engines thrust and the jet speeds down the runway, travelers with a better understanding of the offset system, and the environmental and personal cost of the journey ahead, can ask the obvious question: Do we need to go there?

Tom Haines can be reached at thaines@globe.com.