A United Nations report has found that the number of megacities in the world has increased almost three-fold in the past 24 years. A megacity is defined as an urban area with over 10 million inhabitants—New York City formed the world’s first in the 1950s—and as of 1990, ten such cities had grown up around the globe. But as of this year, there are now a staggering 28 megacities. And while dense urban environments may provide certain environmental benefits, they also create significant hazards.

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The UN’s World Urbanization Prospects report notes that in 1990, the 10 megacities were home to 153 million people; in 2014, the 28 megacities are home to some 453 million people — 12 percent of the world’s urban dwellers. These cities form an environmental double-edged sword. Taken as whole, cities are responsible for 70 percent of the world’s carbon emissions, but major urban centers are doing more than other regions to curb those emissions. The Climate Action in Megacities 2.0 report, released in early 2014, showed that megacities have doubled the number of steps they are taking to reduce carbon emissions over the previous two years.

So where are these megacities? The report explains “Tokyo remains the world’s largest city with 38 million inhabitants, followed by Delhi with 25 million, Shanghai with 23 million, and Mexico City, Mumbai and São Paulo, each with around 21 million inhabitants. Osaka has just over 20 million, followed by Beijing with slightly less than 20 million. The New York-Newark area and Cairo complete the top ten most populous urban areas with around 18.5 million inhabitants each.”

Related: World Population Expected to Reach Almost 11 Billion People by 2100

On a case by case basis, the burgeoning megacities pose some alarming problems. The Smithsonian notes that megacities with a dependence on dirty energy create health risks for inhabitants. For instance, Shanghai has the highest cancer mortality rate in China, a statistic that is linked to air pollution. In cities such as Los Angeles, commuters of the outlying areas diminish the positive environmental impact of the concentrated central population.

The UN expects that the number of megacities will continue to grow: by 2030 they predict that there will be 41 megacities, and while Tokyo may shrink a little, it will remain the largest of them all. Large urban growth is expected in India, China and Nigeria, with urban populations to add an additional 404, 292 and 212 million individuals respectively by 2050.

Which brings about the question of how to best manage this growth in both population and concentration. The report suggests that a well managed city must “offer important opportunities for economic development and for expanding access to basic services, including health care and education, for large numbers of people,” in addition to “[p]roviding public transportation, as well as housing, electricity, water and sanitation for a densely settled urban population, [which] is typically cheaper and less environmentally damaging than providing a similar level of services to a dispersed rural population.”

Via Smithsonian Magazine

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