Global tourism accounts for eight percent of greenhouse gas emissions, four times more than previously estimated, according to a study published in the Journal Nature Climate Change on 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. The US tops the rankings followed by China, Germany and India.

Tourism is a huge and booming global industry worth over $7 trillion, and employing one in ten people around the world. 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).

“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.

The research also indicated that new technologies such as renewable energy can help reduce the impact of tourism on global emissions but that so far they haven’t been able to keep pace with the rapid growth in global travel.

The findings were based on an analysis of 189 countries and more than 1 billion supply chains and their effects on the atmosphere that have been incorporated into vast, previously established databases.

The United Nations World Tourism Organization has suggested two ways to reduce the carbon footprint of global tourism: Travelers could choose destinations closer to home and use more public transportation, and governments could offer tourism providers incentives to boost their energy efficiency. To date, neither approach has been wildly successful, the researchers note.