Lawns Not so Green After All

Friday, 22 January 2010

Emily Sohn

Discovery News

It turns out the things we do to tend to our lawns produce four times as much carbon dioxide as a lawn can soak in.

A new study has found that the humble lawn is doing more harm than good for the environment.

Despite the fact grass pulls heat-trapping carbon dioxide out of the atmosphere, mowing, leaf-blowing, watering and other lawn-tending activities produce far more greenhouse gasses than lawns and fields can suck up.

The findings, which appeared in the journal Geophysical Research Letters, don't necessarily mean that people should give up on grass altogether.

Instead, the results might encourage people to think about less destructive ways of tending to their yards, like switching from gas mowers to push mowers or planting vegetation that requires less carbon-spewing maintenance.

Not an easy solution

The study also suggests that yards aren't going to be an easy solution to global warming, as previous research had suggested.

"People assume that if something is green, it's a biological sink for carbon dioxide," says Dr Claudia Czimczik, a biogeochemist at the University of California, Irvine.

"There has been a lot of discussion about using green space in developed areas to help sequester some greenhouse gases. Our study points out that we can't grow lawns to store carbon."

Building parks and planting lawns have been appealing strategies for alleviating impacts from global warming because grasses have lots of roots that grow quickly and then die. That makes them good at pulling carbon out of the air and into the soil.

When Czimczik and colleague Dr Amy Townsend-Small looked at ornamental lawns around Southern California, they confirmed what other studies have shown: In the soil beneath grassy lawns, carbon levels increase year after year. The lawns they looked at ranged in age from new to 35 years old - the oldest ones held the most carbon.

The researchers took their study a step further, looking not just at what lawns take in, but also what they emit.

NO2 emitter

By collecting gasses coming out of the ground, the scientists found that grass-bound soil steadily emits nitrous oxide, which is 300 times better at trapping heat compared to carbon dioxide. Fertiliser, which on the one hand allows more carbon-absorbing grass to grow, also accelerates the release of nitrous oxide.

What's more, the study, found that typical lawn maintenance activities emit four times more carbon dioxide than grass can absorb. Devices that burn fuel, including mowers and leaf-blowers, contribute a bulk of emissions. Fertiliser production and irrigation add to the totals.

Soccer and baseball fields are even worse offenders, the study found. Playing fields spit out both carbon dioxide and nitrous oxide, but they fail to pull any carbon out of the atmosphere because they get trampled to death every year.

"In the lawns we studied there would be more benefit to the environment if the lawn wasn't there," says Townsend-Smith.

The scientists can't yet say whether their findings apply beyond Southern California, as environmental conditions and maintenance needs vary from place to place.

Better methods

Still, the study should prod people to imagine better ways to customise their front and backyards, says Paul Robbins, a geographer at the University of Arizona, Tucson, and author of the book Lawn People.

To reduce maintenance needs, for example, homeowners could plant prairie grasses in the Midwest, clover in the Northeast, and drought-tolerant cactuses in the Southwest.

The research also points out how important it is to look at both sides of the equation when considering whether something is good or bad for the environment.

"It reminds us that there are two sides to the ledger on everything we do," says Robbins. "We have to look at both sides of the equal sign."

Source: Discovery News