for National Geographic News
There may be something more to the phrase "urban jungle."
Compared with tropical rain forests—the densest natural ecosystems—cities store more carbon, acre for acre, in their trees, buildings, and dirt, a "surprising" new study says.
With Earth's temperature rising due to increased emissions of greenhouse gases, such as carbon dioxide, scientists are taking a closer look at all the places that naturally store carbon—and how to lock up more.
"Everyone thinks about the tropical forests, but I don't think people consider cities as a way to store carbon," said study leader Galina Churkina of the Leibniz Centre for Agricultural Landscape Research in Germany.
Although a lot of studies have focused on carbon in forests, grasslands, and other natural ecosystems, looking at cities—which now house half of the world's population—is relatively new, Churkina said.
Churkina and colleagues pulled together previous evidence looking at various stores of organic carbon—carbon that comes from living things, as well as from such as plants and animals, wood, dirt, and even garbage.
Cities—including both dense metropolises and sprawling suburbs—store about a tenth of all the carbon in U.S. ecosystems, the study estimated.
In total, U.S. cities contain about 20 billion tons of organic carbon, mostly in dirt, according to the new study to be published in an upcoming issue of the journal Global Change Biology.
Some of this carbon-rich topsoil is in parks and under lawns, but it's also sealed underneath buildings and roads—a remnant of grasslands or forests that were there before development.
Of all this urban carbon, about three billion tons are locked up in human-made materials—two-thirds of it in garbage dumps, and the rest in building materials such as wood.
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