Cities Trap More Carbon Than Rain Forests, Study Says

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Tree Power

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."

Tricky Balance

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.

(Related: "Laughing Gas Biggest Threat to Ozone Layer, Study Says.")

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."

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