When asked about the cost of a gallon of gas or the calories in a gallon of milk, survey respondents were generally close. “Even if you’re off, you’re not extremely off,” said Grinstein, associate professor of marketing in the D’Amore-McKim School of Business. But when asked to estimate the amount of CO₂ emitted from a gallon of gas, most people weren’t even close. The correct answer is 9 kilograms, or about 20 pounds of carbon. Some study participants responded with an answer in tons.

“That’s like going into Starbucks and thinking a cup of coffee would cost $30,000,” said Ory Zik, a co-author of the study and chief executive officer of the renewable energy company Qnergy.

These inflated estimates were outliers. The researchers found that most people tend to estimate the CO₂ emitted from a gallon of gas to be much lower than it really is. This is a common psychological phenomenon, Grinstein said. When people engage in activities that are thought to have “low moral perception,” such as burning fossil fuels, they tend to underestimate their personal impact. It’s like a psychological defense mechanism that allows people to minimize their negative behavior, Grinstein said.

The study included a survey of 100 Boston-area college students, a demographic one might expect to be more environmentally-conscious than the average Joe. But even this group’s guesses were consistently off target.

If we can’t estimate the CO₂ associated with something as ubiquitous as a gallon of gas, we probably don’t know how much carbon is emitted from the food we eat, the energy our home consumes, or any other daily activity. In other words, the term “carbon footprint” is commonly used, but not at all understood.

How can we change this? Grinstein suggested taking a lesson from nutrition labels, which list the calories in food and the percentage those calories claim of a daily limit. Our society has grown accustomed to these labels as a tool for making informed choices.

“If I tell you, ‘there are 300 calories in this hamburger,’ that makes sense to some degree,” Grinstein said. “It’s an abstract concept, but people have been trained to understand calories and use them to make responsible decisions, or at least understand the implications.”

Just as food comes with a nutrition label, perhaps there should be a carbon label on goods, at gas stations and on flight-booking websites, for example. Grinstein said people can use a carbon calculator to measure their personal carbon footprint and determine how it may be reduced. However, even the estimations these calculators make are imperfect.

“Having a better understanding of how CO₂ emissions play a role in everyday life allows people to decide if they’re willing to step up and change,” Grinstein said.