Cities trap more carbon than rain forests
08 Sep 2009
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 journalGlobal 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.
Many cities have already launched ambitious plans for turning gray to green, such as Los Angeles‘ Million Trees LA project, which aims to plant a million trees in the Californian city over several years.
Trees take up CO2 and turn it into carbon in their trunks, branches, and leaves, so planting more trees helps counter some of the excess CO2 in the air.
Likewise trees also cool cities and reduce the need for air-conditioning, according to urban forest expert David Nowak of the U.S. Forest Service in Syracuse, New York.
By planting trees around buildings, he added, “you avoid about four times more CO2 emissions than the trees sequester.”
Study leader Churkina added that “people could [also] try to store more carbon in gardens by smart management of the land. The carbon storage in lawns is quite amazing.”
However, figuring out whether more lawns or trees in cities would actually fight global warming “can be tricky,” said earth scientist Diane Pataki of the University of California, in Irvine.
“Managing urban soils to store more carbon can use energy, and those fossil fuel emissions have to be taken into account,” said Pataki, who was not involved in the research.
For example, the process of making fertilizer typically burns a lot of natural gas. Later, when the fertilizer breaks down in the soils, it releases nitrous oxide—also known as N2O, or laughing gas.
Since N2O is a much more powerful greenhouse gas than CO2, fertilizer can offset some or all of the carbon gain, Pataki added.
Study leader Churkina agreed. “You have to follow the whole life cycle of things, and cannot just think of carbon storage.”
Waste to Energy
Building wood houses instead of using concrete could also help, said Leif Gustavsson, an expert on sustainable technologies at the Mid Sweden University in Õstersund.
However, the main benefit comes from better use of waste from wood industry and construction, not the carbon stored in the structures, said Gustavsson, who was not involved in the new research.
Of the wood harvested for building materials in Sweden, his research found, only about 25 percent winds up in the buildings, while the rest becomes waste.
“We should use all of the byproducts to replace fossil fuels,” he said, burning them instead of coal, oil, or natural gas to generate electricity or heat.
Bricks and concrete also require a lot of energy to create, the new research suggests, whereas harvesting sustainably grown wood uses much less energy—another carbon savings.
(Related: “Hot New High-Tech Energy Source Is … Wood?”)
Overall “it’s a good thing if you can increase the carbon stored in society,” he said.
“Everything makes a small difference.”