Who conducts greenhouse gas inventories and why?

Combine harvesting rice.

A combine harvesting rice. Photo courtesy of Gene Miyao.

In order to better understand the effects of greenhouse gas emissions on climate change and to prioritize mitigation strategies, national, state, and local agencies conduct periodic inventories of greenhouse gas emissions from various sectors. In California, the energy and industrial sectors are now required to report their greenhouse emissions as a part of a proposed cap and trade system set to begin in 2012. While agriculture is exempt from this mandate, incentives exist to promote the voluntary mitigation of agricultural emissions, such as the sale of credits for sequestering carbon or reducing emissions. California counties like Yolo conduct greenhouse gas inventories as part of their general plan updates or by developing separate climate action plans so that they are in compliance with the state's Global Warming Solutions Act (AB 32). In March 2011, the Yolo County Board of Supervisors passed a climate action plan that included an inventory of emissions from all major economic sectors including agriculture.

How do GHG inventories affect agriculture?

Although agricultural greenhouse gas emissions are relatively small compared to other sources such as transportation or energy, studying agricultural greenhouse gas emissions can benefit agriculture in several ways:

  • By providing a contrast between agricultural/rural emissions and urban emissions, and presenting agricultural land preservation as an urban planning strategy to limit greenhouse gas emissions.
  • By identifying improvements in farming strategies that may reduce agricultural greenhouse gas emissions

What do agricultural GHG emissions look like in California and Yolo County?

Statistics from the California Air Resources Board's (CARB) statewide inventory in 2008 indicate that transportation, electricity production and industry are responsible for the majority of the California's GHG emissions, and that agriculture accounts for only 6%. At the county level, agriculture accounts for approximately 14% of Yolo County's GHG emissions (Figure 1). Certain GHG emissions indirectly related to agriculture, including transportation and processing costs, may not be classified under the agriculture sector but rather under other sectors such as transportation.

Greenhouse gases by sector

Figure 1. Greenhouse gas emissions from Yolo County in 1990 by sector.

What can we learn from Yolo's Agricultural GHG inventory?

In addition to the GHG inventory done as a part of Yolo County's Climate Action Plan, Haden et al. (in prep) conducted a more detailed study of Yolo's agricultural emissions for 1990 and 2008. The methods used to carry out the inventory were adapted from both IPCC and CARB inventory methodologies. Below we highlight several findings from this recent study. First, urban land uses emit 70 times more emissions per unit area than irrigated cropland. In more urbanized counties, this number is likely to be even greater. This indicates that preserving agricultural land is likely to be an important strategy to help stabilize or reduce future emissions.

Land area and average emission rates

Table 1. Land area and average emissions rates (MT CO2e acre-1 yr-1) for rangeland and irrigated cropland and urbanized land in Yolo County during 1990 and 2008 [2]_.

Second, the study found that Yolo County agricultural greenhouse emissions have decreased by 10% between 1990 and 2008. The reduction in emissions was largely caused by 5.7% decrease in crop acreage. Additionally, the change in emissions was caused by a decrease in the amount of applied nitrogen (driven by a switch to less nitrogen-demanding crops and by a reduction in N rate for some crops). The study also indicated that methane emissions livestock and rice cultivation had increased by about 20% due to higher cattle populations and more cropland allocated to rice. For more examples of GHG reduction studies in CA, please visit http://www.plantsciences.ucdavis.edu/Agroecology/outreach.html .

Are there opportunities to reduce GHG emissions from agriculture?

While agricultural greenhouse gas emissions in Yolo have declined over the years, opportunities still exist for mitigation and adaptation. Table 2 lists a range of agricultural strategies to reduce emissions as well as some of the other benefits and tradeoffs. Drip irrigation and fertigation have been shown to reduce N2O emissions in tomato cropping systems; for a synopsis of the results please visit http://www.plantsciences.ucdavis.edu/Agroecology/Outreach/tomato.html .

Tradeoffs of agricultural GHG mitigation strategies

Table 2. Trade-offs and co-benefits of various agricultural GHG mitigation strategies in Yolo County.

References

[1]Yolo County Planning Department (YCPD). 2010. Yolo County Climate Action Plan: A Strategy for Smart Growth Implementation, Greenhouse Gas Reduction, and Adaptation to Global Climate Change. Public Review Draft.
[2]Haden, V.R., et al. In prep. Involving local agriculture in California's bottom-up climate change policies: Lessons from an inventory of agricultural greenhouse gas emissions in Yolo County.
[3]California Air Resources Board (CARB). 2008. Climate Change Draft Scoping Plan: A Framework for Change. June 2008 Discussion Draft Pursuant to AB 32. Sacramento, California.
[4]Jackson, L.E., F. Santos-Martin, A.D. Hollander, W.R. Horwath, R.E. Howitt, J.B. Kramer, A.T. O’Geen, B.S. Orlove, J.W. Six, S.K. Sokolow, D.A. Sumner, T.P. Tomich, and S.M. Wheeler. 2009. Potential for adaptation to climate change in an agricultural landscape in the Central Valley of California. Report from the California Climate Change Center. CEC-500-2009-044-F. 165 pp.