En route to greener life? You'll need a map

Carbon footprints not easily gauged

In a warming world, just strolling down a grocery store aisle brings people face to face with dozens of carbon conundrums. Is filling your basket with only locally produced food the best way to shrink your dinner's carbon footprint? (Not always.) Is microwaving dinner better for the climate than baking? (Usually.)

A growing body of research is putting our everyday lifestyle under the microscope to discern the heat-trapping greenhouse gases that our choices create. Studies of products, from lettuce to laptops, reveal some surprising truths - for example, don't be so quick to ditch your old PC for a new, energy-efficient model. Slowing global warming, it turns out, isn't always as straightforward as driving a Prius or screwing in energy-efficient lightbulbs.

Scientists are developing methods for calculating carbon footprints of everyday items and working with companies to trace the complicated environmental impact of products, accounting for everything from the fertilizer on the farm, to the fuel used to power equipment, to the packaging, to the transport. Some foresee a day when consumers count carbon in the same way they now count calories.

"How green is your lunch?" said Edgar Blanco, a scientist at the MIT Center for Transportation and Logistics. "I'm a believer that we should have a notion of a budget of carbon we should use."

What the scientists are finding often defies intuition.

Take what Blanco calls the "distance paradox," best embodied in bottled water. Tap water is most earth-friendly, of course, but if you're going to drink bottled water, those from Fiji and France sound like particular environmental villains. But the distance something is shipped often matters less than the inputs at the beginning of the process, which are virtually invisible to a consumer.

Blanco didn't single out brands or examine the practices that individual companies use to make their products greener. Instead, he looked at the predominant energy source for bottling plants. In France, such plants use electricity generated mostly by nuclear energy, and in the Pacific Islands, geothermal is a common energy source; both are clean from a carbon dioxide standpoint. In the United States, by contrast, bottling plants are powered mostly by fossil fuels such as coal and natural gas.

His conclusions were simple: The mix of power in the United States means that local bottled water has a much bigger carbon footprint than the stuff shipped thousands of miles.

That doesn't mean you should run out and buy imported French water. Nobody suggests that greenhouse gas emissions alone should govern consumer decisions; after all, there are many other environmental considerations - such as the radioactive waste from nuclear plants - not to mention factors such as price and taste. But as carbon consciousness spreads, Blanco says such thinking will drive broader changes among consumers and companies.

Ulf Sonesson, research coordinator for environmental research at the Swedish Institute for Food and Biotechnology, has looked at the most energy-efficient way of cooking. While scenarios can vary, a rough rule of thumb is: The microwave uses the least energy, followed by frying and boiling and, finally, the conventional oven. (Think how wasteful it is to heat up an entire oven just to cook a baked potato or two.)

And since it's more energy-efficient to cook lots of food at once, Sonesson adds: "Eat with a friend. That saves a lot of energy, actually."

Research tracking greenhouse gas emissions over the life of a product is often aimed at companies looking for ways to make their products more environmentally friendly. But consumers probably don't need to know that a banana racks up precisely 118 grams of carbon dioxide from the farm to the shelves of the local grocery store; it's enough to know what choices are most effective.

A key example of understanding broad-brush ecofriendly rules comes from a study published last year by Christopher Weber, a Carnegie Mellon University professor who studies international trade and the environment and also happens to be a foodie.

"When the Oxford Dictionary named 'locavore' the word of the year, I couldn't really believe it," Weber said. It spurred him to do a study of the carbon impact of eating an all-local diet.

He found that people could commit to eating all-local to shrink their carbon footprint, but they could accomplish the same thing more simply by replacing less than a day's worth of calories from red meat and dairy in their diet with chicken, fish, eggs, or fruits and vegetables each week. Cows, because of everything from the amount of feed they require to the methane gas they release during digestion, have large carbon footprints.

The research isn't limited to the dinner table.

In a study of desktop computers, Eric Williams of Arizona State University found that it's better for the atmosphere to use an older machine for a few more years than to buy a slick, more energy-efficient model, because of the large amount of energy used in manufacturing.

"So much of this is new in the public consciousness," Nathan Pelletier, a graduate student studying ecological economics at Dalhousie University in Halifax, Nova Scotia, said at a recent conference, where he also suggested that people could trim their carbon footprints if they stopped overeating.

Carolyn Y. Johnson can be reached at cjohnson@globe.com.