Global tourism accounts for 8% of total worldwide greenhouse gas emissions, four times more than previously believed, new research says.

Some gases, including carbon dioxide and methane, trap heat in the atmosphere, producing a "greenhouse effect," and so make the planet warmer. The amount of greenhouse gases released by a particular activity is referred to as its "carbon footprint."

The increasing carbon footprint of global tourism between 2009 and 2013 represents a 3% annual growth in emissions, according to University of Sydney researchers.

Their paper was published Monday in the journal Nature Climate Change.

Looking beyond air travel

Worldwide, travel is a trillion-dollar, energy-intensive industry.

Previous research has quantified total emissions from tourism as accounting for 2.5% to 3% of global greenhouse gas emissions. These estimates, however, often did not account for emissions produced by food and beverage, infrastructure and retail services at travel destinations.

Arunima Malik, lead author of the new study and a lecturer in the School of Physics at the University of Sydney, conducted a comprehensive analysis of global tourism based on detailed data sourced from 189 countries around the globe.

Malik and her colleagues tracked tourists from their countries of residence to destinations and back again, and they estimated carbon emissions by including the greenhouse gases produced to create the goods and services purchased by tourists: local transportation, for example, a hotel stay and souvenirs.

"Since we provide a more comprehensive and complete scope, our estimates are higher than those provided by previous studies," said Malik of her study's estimated 8% contribution made by global tourism to total worldwide greenhouse gas emissions.

Half of the 14% total increase of emissions due to global tourism occurred in high-income countries from 2009 through 2013, the study finds. However, middle-income countries -- notably China -- recorded the highest growth rate at 17.4% per year for the period.

Overall, the US tops the carbon footprint ranking, followed by China, Germany and India, Malik and her colleagues estimate. Domestic travel, which includes business travel, makes up a majority of the carbon footprints for each of these countries.

Small-island countries -- such as the Maldives, Mauritius, Cyprus and the Seychelles -- see between 30% and 80% of national emissions from international tourism.

Unraveling the patterns of tourists crisscrossing the globe, Malik and her colleagues describe travel as a "largely a high-income affair," with high-income countries acting as both the origin and destination for travelers.

Because data were unavailable, Malik and her colleagues could not look at net emissions and "alternative scenarios if the tourists had not travelled and stayed at home," said Malik. Travelers, though, live differently on the road than they do at home, she said: "It has been suggested that people consume more processed food and go for carbon-intensive transport options, rather than public transport when they travel."

Innovations are needed

For Daniel Scott, a professor in the Department of Geography and Environmental Management at University of Waterloo in Ontario, the important finding in the study is "the scale of the contribution, at approximately 8%."

"People are often surprised at the size of their travel and tourism emissions," said Scott, who was not involved in the new research. "A frequent flyer can have a larger carbon footprint from their work or leisure travel than their entire carbon footprint at home."

Instead of a buying an electric car or slapping solar panels on your house, sometimes the biggest action you can take to reduce emissions is to travel differently, he said.

Despite some energy and emission efficiency gains within the tourism industry, continued growth in global tourism means emissions will continue to grow, he said.

"All of international tourism depends on air travel, and technological innovations are urgently required for aviation to be part of the decarbonized global economy," Scott said.

Jukka Heinonen, a professor of civil and environmental engineering at the University of Iceland, said the study is "something that should have been done for some time already."

Heinonen, who was not involved in the new research, noted that there have not been sufficient data to conduct this kind of study until recently.

Two years ago, he and his colleagues used a similar "consumption-based approach" for a study of the "Carbon Footprint of Inbound Tourism to Iceland."

The high 8% estimate is expected, Heinonen said.

"More and more people are reaching the level of income where they travel," he said. With costs related to traveling decreasing, "it's one of the sectors in which emissions increase very rapidly, since it's a very high-emission-density activity."

"Many people who are really environmentally concerned and taking actions in their daily lives toward reducing emissions are also global-minded people who have social relationships around the world," he said.

Malik believes tourism will grow at an annual 4% rate, outpacing many other economic sectors. And so it is "crucial" to make it sustainable, she said. "We recommend flying less, where possible. Try to stay Earth-bound to reduce emissions."