International tourism accounts for 8 percent of all carbon emissions worldwide—about three times more than previously thought, according to a study released yesterday.

The study, published in Nature Climate Change, estimates that global tourism—including transportation, accommodations, activities, food consumption, and all the energy and infrastructure required to accommodate visitors—produced about 4.5 billion tons of carbon dioxide equivalent in 2013. Air travel accounts for one-fifth of these emissions.

And the United States has the biggest footprint of all. It’s responsible for about a billion metric tons of CO2 per year, both from visitors coming into the country and Americans traveling outside of it.

By including a comprehensive assessment of all the factors contributing to tourism-related emissions, from transportation to souvenirs, the report presents “a complete life-cycle assessment of global tourism, ensuring we don’t miss any impacts,” senior study author Arunima Malik, of the University of Sydney in Australia, said in a statement.

The research used information from multiple databases cataloguing tourism-related expenditures, including data collected by dozens of individual nations as well as the United Nations’ World Tourism Organization. They analyzed the data in two ways: by assigning all tourism-related emissions to the travelers’ countries of origin, and by assigning all emissions to the travelers’ destination countries.

For the world’s highest emitters, including the United States, China and India, these values were similar no matter which accounting method was applied. But for other nations, particularly small island nations popular among travelers, including the footprint of international visitors makes a big difference. In some of these places, international tourism may account for as much as 80 percent of their national emissions.

This presents a difficult situation for such nations, the researchers wrote, noting that “these islands benefit substantially from the incomes from tourists, so their governments face a challenge of how to impose national mitigation strategies without reducing tourism income.”

In general, affluence has a major influence on global tourism emissions—as the researchers note, “wealthier people travel more.” The association is so strong, in fact, that for groups making any more than $40,000 per capita, a 10 percent increase in wealth results in a carbon footprint increase of 13 percent.

A transportation expert said the study comports with current estimates of aviation’s contribution to emissions. “Globally, aviation we think is responsible for between 2.5 and a little under 5 percent of anthropogenic climate change effects,” said Dan Rutherford, aviation and marine program director for the International Council on Clean Transportation.

In recent years, climate activists have pushed the International Civil Aviation Organization to introduce stricter emissions standards for aircraft. In the meantime, some airlines have also announced plans to begin or expand their use of biofuels on certain flights, or have begun offering carbon offset programs.

Still, projected increases in air travel underscore the need to reduce emissions further, Rutherford said. Some research suggests that the number of air passengers may double over the next 20 years.

Climate-minded travelers may make the choice to take trips closer to home, he suggested. But he also advocates for airlines or booking agencies providing more transparent information to consumers about the carbon intensity of different flights.

“There’s this question of, if you need to fly, how can you choose a less polluting flight?” he said. “We’d really like to see the industry step up.”