SYDNEY: Domestic and international tourism account for eight percent of greenhouse gas emissions, four times more than previously estimated, according to a study published Monday. The multi-trillion dollar industry’s carbon footprint is expanding rapidly, driven in large part by demand for energy-intensive air travel, researchers reported in the journal Nature Climate Change.

“Tourism is set to grow faster than many other economic sectors,” with revenue projected to swell by four percent annually through 2025, noted lead-author Arunima Malik, a researcher at The University of Sydney’s business school.

Holding the sector’s carbon pollution in check will likely require carbon taxes or CO2 trading schemes for aviation, the researchers concluded.

As in past decades, the United States is the single largest emitter of tourism-related carbon emissions, with other wealthy nations — Germany, Canada and Britain — also in the top ten.

But burgeoning middle classes have moved emerging economies up the ranking, with China in second place and India, Mexico and Brazil 4th, 5th and 6th, respectively.

International travel involving long-haul flights is among the fastest growing sectors, and could threaten efforts to reign in planet-warming carbon pollution.

The total number of air passengers is expected to almost double by 2036 to 7.8 billion per year, according to the International Air Transport Association (IATA).

The aviation industry accounts for two percent of all human-generated C02 emissions, and would rank 12th if it were a country.

“We see very fast tourism demand growth from China and India over the past few years, and also expect this trend will continue in the next decade or so,” Ya-Sen Sun, a professor at The University of Queensland Business School in Australia, and co-author of the study, told AFP.

“Besides the sheer population number, what’s worrying is that people with a rising income tend to travel further, more frequently, and with a higher reliance on aviation.”