Megacities are vast population centers with equally great effects upon local climate. They both concentrate and require significant amounts of resources, posing unique challenges for sustainability and environmentalism generally.

A megacity is a city with a population greater than 10 million. When cities such as Mexico City, Los Angeles, or Hong Kong get this large, it becomes difficult to determine their precise boundaries or true population. The U.S. Census Bureau and the United Nations often disagree in nonsystematic ways about the population of some of the world's largest cities, and the discrepancies between their estimates can represent several million people.

The bureaucracy and infrastructure of megacities can be as complex as those of small nations, and resource allocation within them is particularly difficult. In 1950, New York City was arguably Earth's only megacity. In less than one human lifetime there were at least twenty megacities on the planet.

One of the most challenging aspects of this dramatic increase in the number of megacities is that they are increasingly appearing in some of the poorest nations of the world (Burdett, 2007). In 1950, only three of the world's most populous cities were in the developing world. In 2005, about 75 percent of the world's largest cities were in the developing world. The rise of these megacities represents a profound development in the history of humanity, and the challenges they present with respect to climate change may also be among the greatest opportunities to address that change.

A megacity is a complex structure consisting of a sophisticated built environment that shelters and sustains millions of human agents. These agents, living in close proximity to one another, often represent extremes of human experience, as megacities juxtapose great wealth and poverty, as well as the diversity of human cultures. In their vast urban landscapes, millions of people live and die in sad and tragic conditions of poverty and low life expectancy. At the same time, others live lives of almost unfathomable wealth and freedom to travel about the globe. These urban environments simultaneously represent both a pinnacle of human achievement and a shameful failure to realize human potential.

Cities almost always depend on a hinterland beyond their spatial extent to provide food, water, energy, and raw materials to sustain the lives of their citizens. They also increasingly depend on this hinterland to absorb their sewage, solid waste, and greenhouse gas (GHG) emissions. Historically this hinterland was predominantly nearby. Increasingly, however, hinterlands are farther and farther away from megacities, and, in regard to GHG emissions, the global atmosphere itself may be considered part of the hinterland. The hinterland of Los Angeles is global, as the city receives oil from the Middle East, water from the eastern Sierra, and food from Mexico, Europe, and Asia. Mexico City has built vast tunnels to divert sewage to distant hinterlands. Denver, Colorado, uses a network of tunnels to divert water from the western slope of the Rocky Mountains that would normally flow into Mexico's Sea of Cortez.

Almost all of these processes relate to climate change forcing factors in direct or indirect ways. Nonetheless, a fundamental and primary impact of megacites with respect to climate change is the energy used by these cities to provide electricity (often provided by coal-fired power plants) and the energy used to provide transportation (predominantly generated by fossil-fuel combustion). These urban areas are the most densely populated areas of the world. This density is an opportunity for numerous efficiencies with respect to energy consumption for electricity, transportation, and the myriad other related needs of urban residents that require electricity and transportation. Leveraging the energy efficiency opportunities that these densely populated areas represent will be of paramount importance with respect to humanity's collective response to the challenges of climate change.

Reference

Burdett, Ricky, and Deyan Sudjic. The Endless City: The Urban Age Project by the London School of Economics and Deutsche Bank's Alfred Herrhausen Society. New York: Phaidon, 2007.

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