If you're not sure what "living green" means, it's not surprising.

Official definitions are vague. No one is quite sure who coined the term "green," and it's not in the Environmental Protection Agency's glossary of environmental terms.

"Green is a buzzword," said Elise Amel, director of environmental studies at the University of St. Thomas in St. Paul. "It's a shorthand way of talking about sustainability."

Sustainability is meeting the needs of the current generation without compromising the needs of future generations, said Sean Gosiewski with the Alliance for Sustainability, a Minneapolis nonprofit group. The definition has evolved in the past 30 years, and has become more complex, but it basically means living with a smaller footprint.

What footprint?

Yours. The effect of your life on Earth. Your footprint is determined by the amount of this planet's resources you need to support how you live, eat and move about.

A carbon footprint is the amount of energy consumed: the electricity, natural gas and gasoline. An ecological footprint means the amount of food and water you use. By shrinking your carbon and ecological footprints, you are living more sustainably; you are, in effect, living greener.

Sounds simple enough. But knowing isn't doing.

There are barriers that keep people from living green, said Amel, who works in the field of environmental psychology.

It's confusing, for one thing, she said. Our footprints can be adjusted with simple conservation, but without feedback, we don't know where to correct our habits. An electric bill, for example, isn't specific. It doesn't say that the refrigerator ate this much, or that particular outlet uses that much.

"If we knew that the plasma TV uses seven times as much power as anything else in the house, we wouldn't leave it on," Amel said.

Feedback helps Prius drivers rack up good mileage. The Toyota has instruments that indicate miles per gallon at any given moment, so drivers learn quickly what reduces mpg and can adjust their driving habits, Amel said.

But sometimes it's unclear what the effect is of any one decision. Take the average American hamburger. It's not exactly green. Meat from feedlots requires a lot of corn and that means pesticides, fertilizer and huge amounts of water. Animal waste finds its way into our water. Yet, not all meat is the same, Amel said. Grass-fed meat can come from a local farmer who has a very small carbon footprint.

Nor is local food always the better choice, Gosiewski said. Food produced farther away could be greener than locally produced food, despite the carbon emissions of shipping, if it's raised without a lot of fuel, toxins and water.

"The green label is more of a green tartan than a single color, with lots of threads, lots of shades," said state energy specialist Phil Smith. Compact fluorescent bulbs (CFLs), for example, contain mercury, but are considered green because they conserve so much energy that less mercury is released by coal-burning power plants than if conventional light bulbs are used.

Then there's the green message. Saving the planet often has been about doing without and making sacrifices for something far removed.

"When it comes down to paying the rent today or saving the whales, we are genetically programmed to pay the rent," Amel said.

Change the message and people will respond. Frame it to show benefits rather than sacrifice, and it'll encourage green behaviors, Amel said. Saving money by being green is a message that gets the public's attention.

Another factor in the slow process of getting others to go green is this: We are copycats. We model others' behavior.

"We can swear that we don't care what other people think of us, but we do," Amel said. "We are primed by nature to see what other people are doing, and take our cues from there."

It's all about social norms. If others don't bring bags to the store, we won't either. Eventually, habits and norms change.

The proof is in recycling. It's second nature in Minnesota, which is one of the top recycling states in the nation. But elsewhere, such as Florida, recycling lags behind.

To spread green behavior takes some early adapters. They are all around us: the ones biking to work, those using rain barrels for their gardens, shoppers bringing their own bags and buying local food. Eventually, living green becomes the norm.