In four parks near Irvine, they calculated that emissions were similar to or greater than the amount of carbon dioxide stored through photosynthesis — a finding relevant to policymakers seeking to control the gases that trap heat in the atmosphere and contribute to climate change. “Green spaces may be good to have,” said geochemist AmyTownsend-Small, the lead researcher in the paper published in the journal Geophysical Research Letters. “But they shouldn’t be automatically counted as sequestering carbon.”

The paper is particularly timely, she added, because governments are calculating their carbon footprints, and discussing whether parkland could offset other sources of emissions, such as refineries, power plants and automobiles. Turfgrass covers about 1.9% of the U.S. and is the most commonly irrigated crop. It is increasingly in demand in urban areas.

Townsend-Small and colleague Claudia Czimczik measured the carbon stored in the parks’ soil samples, and compared that with emissions from producing fertilizer, from mowing with gasoline-powered equipment and from pumping water to irrigate the plots. The pumped water was recycled — but if it were fresh water transported from the Colorado River, as is much of Southern California water, emissions would be higher, said Townsend-Small. They also factored in the nitrous oxide released from soil after fertilization. Nitrous oxide is a greenhouse gas 300 times more powerful than carbon dioxide, which is released by fossil fuel combustion.

California has no regulations to control turfgrass, but the study “shows the importance of full life-cycle analysis for greenhouse gases,” said Mary Nichols, head of the California Air Resources Board, which is charged with reducing the state’s carbon footprint. Research is underway, she noted, to develop varieties of grass that need less mowing and use less water.

What about the heat island effect, the vaunted benefit of plants as a way to cool cities? “Irrigating trees in urban Southern California reduces the heat island effect,” said Stephanie Pincetl, author of “Transforming California: A Political History of Land Use and Development.” “But lawns have no such benefits, and also contribute to water pollution because they are heavily fertilized.”
Townsend-Small said that turf emissions vary according to region. Studies would need to be done in wetter northern climates. There, she said, grass might not need irrigation, but it would also store less carbon during cold winter months.

For Southern California residential lawns, she noted, using rakes rather than leaf-blowers and hand mowers rather than gasoline-powered equipment would improve their carbon footprint.

“About 40% of the drinking water we import at great financial and environmental expense is used for outdoor irrigation,” said Paula Daniels, an L.A. Department of Public Works commissioner. “This study hopefully will motivate more of us to make changes in our landscapes.”

–Margot Roosevelt