The Massachusetts Institute of Technology class study involved the carbon emissions of people in a wide variety of lifestyles -- from the homeless to multimillionaires -- and compared them with people in other nations.

The students found that in the United States, even people with the lowest energy usage account for, on average, more than double the global per-capita carbon emission, researchers said. And those emissions rise steeply from that minimum as people's income increases.

"Regardless of income, there is a certain floor below which the individual carbon footprint of a person in the U.S. will not drop," said Professor Timothy Gutowski.

The results of the study will be presented next month in San Francisco during the IEEE International Symposium on Electronics and the Environment.