Do lawns contribute to climate change? Grass is a plant, and through photosynthesis, it takes up carbon dioxide out of the atmosphere. Carbon dioxide is a greenhouse gas and if we sequester it, we help remove this gas from the atmosphere. However, the maintenance of turfgrass takes a good deal of energy -- think of mowing, watering, and applying fertilizers that all consume fossil fuels. The net balance between sequestration and emission may actually demonstrate that lawns release more greenhouse gases than they sequester.
Using actual data from measurements of trees found on different land use types in Florida (e.g., parks, residential yards, and natural urban remnants), we calculated the source/sink potential of a 4 hectare (9.88 acres) sites in three Florida cities (Gainesville, Orlando, Miami). Only above-ground vegetation values were calculated; soils and below ground organic matter were not included in the calculations.
The take home message is that highly maintained lawns and trees sequester much less carbon dioxide than more natural areas with little maintenance. With more lawn cover than tree canopy cover, the balance can actually shift to emitting carbon dioxide (e.g., older residential areas in Miami-Dade). Of note is that we did not calculate the impact of built surfaces, just vegetative. The calculations were simplified as we did not add the carbon cost of making and maintaining the power equipment or the carbon cost of growing and transporting sod. In particular, we did not calculate the emission of nitrous oxide from fertilization applications. Urban turfgrass typically emits nitrous oxide after fertilization and/or irrigation. Nitrous oxide has a much worse global warming potential (GWP) as its heat-absorbing potential is approximately 300 times more than carbon dioxide. With these unmeasured factors, city parks with high maintenance regimes may have much larger impacts than reported here. Thus, urban areas that have a large amount of mowed, irrigated, fertilized lawns and pruned shrubs and trees can be a source of carbon dioxide rather than a sink. These carbon dioxide emissions are not trivial; for example, a 4-hectare greenspace in Miami-Dade with 85 percent of the land covered in lawn would emit over 11 tons of carbon dioxide per year.
Further, because below-ground soil carbon sequestration was not calculated, full carbon credit could not be assessed and these above-ground numbers reported should be regarded as a first look at the potential carbon value of urban greenspace. At this stage, natural greenspaces in and around urban areas, with little to no maintenance, seem to be the best option for carbon dioxide sequestration. Natural urban greenspaces also have other benefits, such as biodiversity conservation, reduced stormwater runoff, and reduced fertilizer applications. Overall, the conservation of urban open space could play a role in reducing Florida's carbon footprint, but highly maintained urban greenspace could be regarded as a source of greenhouse gases. In relation to cities trying to reduce their carbon footprint, these results indicate that if municipalities and developers are to use greenspaces as carbon dioxide sinks, they will have to justify the creation of such high-maintenance parks and may have to mitigate their effects.