Closing loops to rebalance the global carbon cycle: Biomass flows modelling of global agricultural carbon fluxes

Abstract

Since the beginning of farming, and even before, humans have been actively modifying our environment in order to harvest biomass. With the ‘Great Acceleration’ of the industrial age, the global system of biomass harvest for food production has become a major driver of Earth system processes, and caused multi-dimensional sustainability issues which must be addressed in order to meet continued increases in demand for food and other biomass. In addition, bioenergy generation, with the subsequent storage of some or all of the carbon content of the feedstock (known as bioenergy with carbon storage or BECS), is now seen as an important tool for rebalancing the carbon cycle. This thesis has used a biomass flows modelling approach to examine possible trajectories for the socio-ecological metabolism of humanity, with a focus on fluxes of carbon contained in biomass. This approach connects social and economic drivers of biomass harvest with physical Earth systems processes such as the global carbon cycle. Meeting growing food demand in the years 2000-2050 is likely to be a significant challenge in its own right, necessitating the harvest of over 30% of terrestrial biomass. This can only be done without significant damage to natural ecosystems if large increases in efficiency and intensity of food production are achieved, or diets are altered. The production of livestock products is shown to be a major cause of inefficiency in biomass harvest, and changes to livestock demand or production are particularly powerful in ensuring a less damaging relationship with Earth system processes. If increases in efficiency are achieved, it may be possible to grow dedicated bioenergy crops, which, combined with the biomass available in waste and residue streams can be used to generate significant carbon dioxide removal (CDR) fluxes via BECS. Following this strategy it is possible to have a non-trivial effect on atmospheric CO2 concentration by 2050. Increasing the intensity of biomass harvest, particularly when low intensity pasture is replaced with intense bioenergy cropping, also has significant implications for ecological energy flows, and the potential trade-off between protecting biodiversity and growing bioenergy crops to mitigate climate change is also discussed. This body of work presents several interesting areas of potential conflict in different drivers of biomass harvest, and suggestions are made for ways in which to develop the approach in order to explore them.