If you live in the United States, you are responsible for five times the carbon emissions of the average person on the globe, according to a recent MIT study presented in May at an Institute of Electrical and Electronics Engineers symposium.

What’s more, mechanical-engineering professor Tim Gutowski and the students in his spring 2007 course Environmentally Benign Design and Manufacturing have found that while wealthy people in the United States make the largest mark, even children and the homeless are responsible for high rates of carbon emission.

It’s easy to see how people with money could have significant carbon footprints. But in creating spending profiles for 18 different lifestyles, the students learned that once you factor in money spent by sources such as the government and insurance providers, even someone who eats in soup kitchens and sleeps in a homeless shelter can account for 8.5 metric tons of carbon dioxide emissions annually. Although that’s far below the national average of 20 metric tons, it’s more than twice the worldwide average of four metric tons.

So how can we reduce the country’s carbon emissions? Although altering individual spending habits to cut energy consumption is a step in the right direction, the study shows that it’s only a baby step. Lifestyle adjustments such as driving fewer miles per year, using a fuel-efficient car, and turning the thermostat down a few degrees during the winter could bring down the average ­middle-income American’s energy consumption by about 30 percent at most–still leaving it well above the world average. That limit is in part the result of what Gutowski calls the “rebound effect”: when you save energy, you save money, but the way you spend those savings affects the environment, too. For example, you might ditch your Hummer for a hybrid, but if you put the money you’ve saved on gas toward a plane ticket to Europe, your footprint grows again.

Cutting carbon emissions by more than 30 percent would require more drastic changes, such as moving closer to work, going vege­tarian, and dropping the thermostat to 60 degrees. But Gutowski says most people won’t

adopt those measures on their own. “­People are aware of the problem, but they don’t want to be martyrs,” he says. Until communities and elected leaders make going green a ­priority, people are unlikely to make big lifestyle changes–and even then, stabilizing carbon emissions won’t happen overnight. “It will take a generation,” Gutowski says.

“I don’t want to discourage people from behaving the way they should to have a lower carbon footprint,” he says, “but it’s a systemic problem that requires more than individual action. It requires adjustments at the policy level, in society as a whole. It’s not going to go away because someone decides to bicycle to work.”