Despite efforts to decrease the carbon footprint, crowded cities and advanced technology continue to increase the amount of carbon released. According to the latest study by the Norwegian University of Science and Technology, Seoul, South Korea, with 21 million people, leads the world in its global carbon footprint.

The study, which ranked 13,000 cities globally based on their carbon footprints, focused on the total amount of the heat-trapping greenhouse gas carbon dioxide directly or indirectly produced.

The city of Guangzhou, China, with a population of 44 million, is number two on the list; and New York City, with 13.6 million people, is number three on the list of cities with the largest carbon footprints.

Istanbul with a population of 15 million ranked 26, while Ankara the capital city of Turkey ranked 80.The paper's lead author, Daniel Moran, says he was surprised at just how concentrated carbon footprints are.

"The top 100 highest-footprint cities worldwide drive roughly 20 percent of the global carbon footprint," Moran said. "This means concerted action by a small number of local mayors and governments can significantly reduce national total carbon footprints."The results of the study are published in "Environmental Research Letters."