The largest city in Saudi Arabia is also the most polluted, mainly due to its industrial activity. Looking at 13,000 cities, researchers found that highly populated centers contributed a major part of countries' total emissions. Large urban areas use up more than 70 percent of the world's total energy — meaning metropolises hold the power to change the global climate situation.

Only about 2 percent of new cars sold in Japan's capital are environmentally friendly. The Tokyo-Yokohama urban area, with the world's largest urban population, emits a massive amount of CO2 every year — 62 million tons for Tokyo alone. But the recent adoption of the Tokyo Declaration gives some hope: 22 global megacities have committed to tackling air pollution and promote zero-emission vehicles.

It's the third-most-populous city in the US, and it also has the third-largest carbon footprint. Pollution in the Chicago metropolitan area increased significantly between 2014 and 2016, according to a study by the American Lung Association. Chicago has also been ranked as the third-dirtiest US city. Any guess as to the other two?

The many industries in this city-state are behind most of its CO2 emissions; the manufacturing sector will account for 60 percent of its emissions by 2020. But the government has realized it's time to act, and declared 2018 the year of climate action. It has also announced a carbon tax on highly polluting facilities. Singapore is also a main importer of sand — a scarce natural resource.

It isn't surprising to find Shanghai in this ranking, as it's among the world's most populated cities. Congestion there has led to serious environmental problems, including air and water pollution. As in many other Chinese cities, power plants and nonstop traffic are the main causes for its carbon emissions. That's why it isn't uncommon to see people wearing masks as they move through the city.

Not everything in LA is like in the movies. The city's air quality has been ranked as the worst in the US. But California has set ambitious targets to reduce greenhouse gas emissions by 40 percent by 2030, using clean energy and supporting electric or hybrid cars. California Governor Jerry Brown has taken a leading role in the fight against climate change, opposing the Trump administration.

The autonomous territory in southeastern China is densely populated; thousands of vehicles clog its roads, and coal-fired power plants and polluting industries spew smog into the air. In addition, its cargo shipping sector is responsible for up to 50 percent of the city's carbon emissions, according to its Environmental Protection Department.

The most populous city in the US takes the bronze medal for carbon footprint. In January, the iconic metropolis sued the world's five largest oil companies — BP, Chevron, ConocoPhillips, ExxonMobil and Royal Dutch Shell — for their contribution to climate change and its effects on the city. The city has been intensively working to reduce its emissions, but still has a lot of work to do.

In China's third-most populous city, factories and vehicles are continuously emitting harmful emissions. Smog is an everyday occurrence. But Guangzhou has committed to replacing its entire fleet of fossil-fuel powered buses and taxis with pure electric vehicles by 2020, after major campaigns by environmental groups such as Greenpeace.

Seoul is the world's top city when it comes to carbon footprint. Air pollution is its biggest environmental and health concern: More than 30,000 tons of harmful pollutants are emitted from just 10 old coal-fired power plants, accounting for around 20 percent of South Korea's total pollution. In recent years, the city has suspended plant operations in an effort to tackle the problem.