Carbon footprints of urban landscapes

Climate change is fast becoming one of the most important issues of our time. Not to dismiss the political discussions going on across the U.S., but a recent analysis of public opinion sponsored by the National Science Foundation indicates that 75 percent of the American public believes the earth’s climate is warming and human activities are responsible¹. The Stanford University study also found that about the same percentage of Americans think the U.S. government should be passing regulations limiting emissions of greenhouse gases that are causing the problem and moving towards energy savings and green technologies. No one can predict how fast the political process will respond but because of our dependence on the fossil fuels, oil and coal, it’s fair to say that significant changes are coming.

The earth is in an inter-glacial period when cooling would be expected, yet temperatures are increasing². Scientists estimate that with current trends, temperature increases of at least seven to 12 degrees seem likely over the next 50 to 100 years, which will increase sea levels and change weather patterns in unpredictable ways. The heightened awareness of the American public leads to many asking how they can we help control carbon dioxide (CO₂) emissions and do their part to prevent climate change.

Most strategies being proposed to mitigate global climate change include increasing carbon storage in plant systems³. This is often referred to as “terrestrial carbon sequestration.” If large amounts of CO₂ are removed from the atmosphere by photosynthesis and then held in stable plant material or soil organic matter, it could help offset CO₂ generated by fossil fuel use.

The potential benefit of carbon sequestration provides an opportunity for the landscape industry and its clients to become involved in efforts to control global warming. Much of the CO₂ being released into the atmosphere originates from urban and suburban areas, with household fuel use and automotive travel generating about forty percent of CO₂ emissions in the U.S.⁴ But if substantial amounts of carbon can be stored in trees and in soil beneath turfgrasses, maintained landscapes could help offset some of their CO₂ generation. 

What it Means to You
One of the challenges for university scientists and the landscape industry is to come up with accurate estimates for amounts of carbon that can be sequestered in residential areas and develop ways to optimize the storage. To help with this effort, we have been developing carbon calculators that can be used to estimate residential footprints. The calculators are part of educational programs on managed ecologies being developed by faculty at North Carolina State University (NCSU) and its private sector partner, Bayer Environmental Science.

Using the carbon calculator, we assembled carbon balance summaries for two residential landscapes to provide examples of the factors controlling carbon footprints. The two cases are typical for suburban areas in North Carolina. Trees and turfgrasses are the two types of vegetation contributing carbon to the systems. Negative carbon factors are the release of CO₂ during fertilizer manufacturing and mowing, and the release of nitrous oxide associated with application of nitrogen fertilizer. Nitrous oxide is a greenhouse gas that has a heating potential 310 times greater than CO₂. The summaries also include automobile use to demonstrate the potential of landscape carbon storage to offset one of the primary sources of family CO₂ generation.

The two examples in the case studies clearly show that residential landscapes can have positive carbon footprints, and the landscapes can offset a substantial amount of the CO₂ generated by suburban lifestyles.

Conclusions
Several aspects of the summaries are of particular interest. One is that turfgrasses make a positive contribution to the carbon footprint. Recently, there has been much controversy surrounding the carbon balance of turfgrass systems. We find that turfgrass systems nearly always have neutral to positive footprints in our geographical region.
Still, homeowners must be aware that excessive fertilization and greater numbers of mowings move the carbon equation towards the negative. The extension faculty at NCSU recommends that nitrogen additions stay within the 1- to 4-pound range used in our summaries.

In addition to keeping inputs at a reasonable level, the key to carbon sequestration in residential landscapes is to maintain healthy plants. Sustained carbon accumulation by trees, turfgrasses and other horticultural plants depends on sustained growth. Up to this time, we do not have estimates of the carbon costs for the manufacturing of the chemical inputs which, of course, will have to be taken into account in the carbon balances.

Rufty is a professor of environmental plant physiology at North Carolina State University; Rees is development manager, Bayer Environmental Science; and Hamon is vice president of sustainable development at Bayer Environmental Science.

References:

1. Krosnick J. 2009 The Climate Majority, New York Times op-ed June 8, 2009. Refer to woods.stanford.edu
2. Hansen J. 2009. Storms of My Grandchildren. Bloomsbury publishers, 304 pp.ISBN 978-1-60819-200-7
3. www.fossil.energy.gov
4. Glaeser EL, ME Kahn. The Greenness of Cities: Carbon Dioxide Emissions and Urban Development. http://www.hks.harvard.edu/taubmancenter/pdfs/working_papers/glaeser_08_greencities.pdf . Accessed June 11, 2010.
5. Center for Urban Forest Research Tree Carbon Calculator. http://www.fs.fed.us/ccrc/topics/urbanforests/ctcc . Accessed December 4, 2008.
6. Qian Y, RF Follett. 2002. Assessing soil carbon sequestration in turfgrass systems using long-term soil testing data. Agron. J. 94:930–935.
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8. Portmess R, N Pettinati, C Miller, B Hochstein, T Condzella, FS Rossi. 2008. Can a golf course be carbon neutral? A preliminary assessment. Cornell University Turfgrass Times, Issue 2, Volume 19, Number 2. Cornell University, Ithaca, NY.
9. Schlesinger WH. 1999. Carbon sequestration in soils. Science 284:2095.
10. Bremer DJ. 2006. Nitrous oxide fluxes in turfgrass: Effects of nitrogen fertilization rates and types. J. Environ. Qual. 35:3678-1685.