The role of the terrestrial biosphere in climate change mitigation

The Paris Climate Agreement set an ambition of limiting the global mean temperature to "well below 2°C above pre-industrial levels", with efforts to limit warming to 1.5°C. above pre-industrial levels. These targets are most likely not achievable without the removal of carbon dioxide (CO2) or other greenhouse gases from the atmosphere. The two methods assumed for CO2 removal in IPCC Fifth Assessment Report scenarios are bioenergy with carbon capture and storage (BECCS) and a combination of afforestation/reforestation with avoided deforestation. A third mitigation option is to reduce methane (CH4) emissions, which compared to land-based mitigation is less expensive and does not require a commitment of land. We assess the carbon cycle impact of land use change for two scenarios likely to maintain temperatures below 2°C warming in a land surface model run with an ensemble of climate change patterns from CMIP5. We prescribe atmospheric CO2 and the model includes a simple representation of the ocean carbon cycle, so permissible emissions can be diagnosed. In the model, we calculate the mitigation potential of BECCS by removing a portion of the carbon harvested from bioenergy crops and storing it indefinitely in a geologic reservoir. We find that when bioenergy crops replace forests, carbon emissions from the deforestation are often larger the carbon stored via BECCS, meaning forests are often better mitigation options than large-scale bioenergy plantations. We also compare these options with the potential for mitigation by reducing methane emissions on a regional basis. Methane mitigation has the added benefit of reducing tropospheric ozone (O3), which is also a greenhouse gas and is harmful to human and biological systems. From this analysis, we diagnose a range of permissible emissions to meet a 1.5°C or 2°C global mean temperature change and discuss implications of large-scale land use change for mitigating climate change.