Life Cycle Assessment Applied to 95 Representative U.S. Farms

Since World War II, concern for the environmental impacts of human activities has grown. Agriculture plays a significant role in several impact categories including global warming. Governments, including the U.S., have recently begun or are considering the regulation of greenhouse gas (GHG) emission to mitigate the global warming effect. Because agriculture accounts for a large portion of anthropogenic greenhouse gas emissions, it is necessary to establish a baseline measure of the GHG emission of U.S. agriculture at the farm level. The objective of this research is to estimate the GHG emission levels for multicrop farms in the U.S. and identify the major sources of GHG emissions in their supply chains. To accomplish the objective, a partial life cycle assessment (LCA) methodology is used to establish a GHG baseline for the representative farms. LCA as defined by the International Organization for Standardization (ISO) includes four phases: goal and scope definition, inventory, impact assessment, and interpretation. It is a holistic approach that catalogues environmental impacts of all relevant processes at all stages of production, from raw material extraction to disposal. However, this study only catalogues impacts up to the farm gate. Partial LCAs are common in agriculture. Emissions of three GHGs, CO2, CH4, and N2O, are inventoried for 95 U.S. farms. The results are characterized using their 100-year global warming potentials into CO2 equivalents. The CO2 equivalents are then normalized over four functional units: enterprises, acres or head, harvest units, and pounds of production. The variation of GHG intensity between crops and farms is very large. However, it is clear that GHG intensity is affected by three characteristics: location, size, and irrigation practice. Crops grown in their associated regions tend to be more GHG efficient than those grown outside their associated regions. Also, crops grown on large farms tend to be more GHG efficient than the same crop grown on a small farm in the same area. Lastly, with the exceptions of cotton and soybeans, irrigated crops tend to be more GHG intensive than non-irrigated crops. These results combine to suggest that there may be a correlation between production efficiency and carbon efficiency.