In The Field

AGU: Cities as carbon sinks

Cities often take a lot of heat in environmental discussions. All that pavement and pollution damages human lungs and warms the climate. But a few sessions this week suggest that cities may be part of the greenhouse solution.

First a few challenges: Cities are carbon bombs. In the United States, urban areas cover less than 3 percent of the land area but account for 70 to 80 percent of the carbon dioxide emitted by fossil fuel burning. And cities are growing. By 2030, 60 percent of the world population will be urbanites.

Now some of the good news: Cities are carbon vaults. They and other human settlements store a surprising amount of carbon—to the tune of 18 petagrams in the United States. That’s almost 10 percent of the total land carbon storage in the conterminous US, according to Galina Churkina of the Max-Planck Institute for Biogeochemistry in Germany. The carbon is locked in everything from landfills to the books lining library shelves.

Question: What has a higher carbon density—a city or a tropical forest?

Answer after the jump.

According to Churkina, they are about the same. That brings new meaning to the term ‘urban jungle.’

Some cities are strengthening their roles as carbon sinks. Phoenix is doing it inadvertently, as its residents replace desert scrubland with trees and lawns. Los Angeles has plans to plant a million (give or take) trees, in part to mitigate global warming. Once fully grown, the trees will have absorbed enough carbon dioxide to offset only 1 to 2 percent of the annual emissions from LA, but they will also help cool the city with their shade and the process of evapotranspiration, says Diane Pataki of the University of California, Irvine.

As a resident of the Washington DC metropolitan area, I can vouch for the value of urban greenery. When I bicycle home during the summertime, the temperature drops several degrees as I enter Rock Creek Park, a splendid forest that cuts through the city.


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