Climate implications of biomass appropriation: Integrating bioenergy and animal feeding systems

A review of biomass use by humans and the greenhouse gas (GHG) implications of using biomass to supply energy services is presented. Human appropriation of global terrestrial net primary productivity is a ratio of appropriation and production; it currently stands at 15-30% of NPP. Over half of used biomass is fed to animals, while equal amounts are eaten directly by humans and burned for energy services. Published estimates of the potential for biomass to supply global energy services vary widely, but a range of 200-500 EJ/ yr has been proposed that considers limitations due to water scarcity, food demand and biodiversity protection. With respect to climate change, uncertainty regarding soil carbon storage capacity, nitrous oxide (N2O) emissions, and emissions from land use change (LUC) influence the attractiveness of biomass energy (bioE) for GHG abatement. While humans have altered terrestrial biomass patterns for millennia, recent rates of carbon emissions from LUC have been relatively large. In the US, expanded biomass production for bioE could exacerbate the already unsustainable use of water for irrigation, although substantial opportunities exist for increased irrigation efficiency. I calculate that plausible life cycle GHG balances for annual and perennial bioE feedstocks in the Unites States (US) are 14 and 7 g CO2eq/ MJ, respectively, not including any land use or C stock changes. The influence of biofuel policies on land use change and the degree to which C storage in terrestrial ecosystems can reduce cost of climate change abatement is reviewed. I conclude Chapter 1 with an estimate that appropriating all US biomass exports and all roughage fed to beef cattle would supply about 8 EJ of bioE feedstock. 1.1 Primary production of biomass Autotrophs such as trees and grasses are living organisms which use light (electromagnetic radiation) as an energy source in order to build complex organic materials (e.g. carbohydrates). The organic compounds produced by autotrophs are referred to as “primary production”. Gross primary production is a measure of the amount of organic matter produced, and includes the 1 Some organisms harness energy stored in the chemical bonds of inorganic compounds (e.g. H2S and NH3) to function and build new biomass, and use CO2 as the carbon source. These chemoautotrophes are often found around under-ocean thermal vents and other extreme environments. They are primary producers as long as their inorganic energy sources are produced within the Earth’s interior (not biogenic).