Washington, DC — Viewing the world's biggest cities as analogous to living, breathing organisms is fostering a deep new understanding of how poor air quality in megacities can cause harm.
Charles Kolb of the Center for Atmospheric and Environmental Chemistry and the Center for Aerosol and Cloud Chemistry of Aerodyne Research says large cities can be seen as living entities that consume energy, food, water, and other raw materials, and release wastes.
"Carbon dioxide and other pollutants in megacities make them immense drivers of climate change," he said. "They impact climate on both a regional and global level because these long-lived greenhouse gases are dispersed around the world."
More than half the world's population today lives in cities, and the world's largest urban areas are growing rapidly. The number of megacities — metropolitan areas with populations exceeding 10 million — has grown from just three in 1975 to about 20 today.
Kolb said that the most highly polluted megacities are in developing countries. They include Dhaka, Bangladesh; Cairo, Egypt; and Karachi, Pakistan. Even the cleanest megacities like Tokyo/Osaka in Japan and New York City and Los Angeles in the United States still have serious problems, Kolb said.
The hot weather and frequent atmospheric inversions in southern California, for instance, foster Los Angeles' legendary smog problem.
Controlling urban growth in the developing world is key to improving the world's air quality, Kolb said. Urban pollutant levels in poor countries will remain high, with increased emissions expected as the city populations and economic activities increase. Until megacities are rich enough to devote significant funds to reduce their emissions, two factors will invariably increase the stresses on their environment — increasing vehicular traffic and industrial growth.
Kolb says that megacities in Asia and Africa urgently need to modify their urban metabolism in similar ways. A few fundamental changes could pay off quickly.
"We need to start with low-hanging fruit," he said. "In some cities in Asia and Africa, they still have lead in their gasoline. In the developed world, we can institute emissions controls on diesel vehicles, which create hazardous fine particles, and we can also reduce pollution by using more rail-based mass transport or setting up specialized bus routes."
The urban metabolism model can reveal how developed-world megacities, such as Tokyo, New York and Los Angeles, have improved their air quality despite a rise in population. The study also assesses how developing-world megacities are seriously grappling with the problem.