Projecting Future Nitrous Oxide Emissions From Agriculture: Importance of Ecological Feedbacks and the Environmental Benefits of Improved Nitrogen Use Efficiency

Abstract:

Nitrous oxide (N₂O) presents a triple threat to the global environment: it is the third most important anthropogenic greenhouse gas, the largest remaining anthropogenic contributor to stratospheric ozone depletion, and an important component of the nitrogen (N) cascade – where one atom of N can interconvert between a number of forms, each with a unique set of environmental impacts. Here we use a dynamic vegetation model (Princeton-Geophysical Fluid Dynamics Lab (GFDL) LM3 – the interactive land component of the GFDL Earth System Model) to assess how changes in future climate, land-use, and global fertilizer and manure application are projected to affect global N₂O emissions from agriculture by 2050. Agricultural land is defined in this study as the sum of cropland and pasture. In a baseline scenario assuming little improvement in global N use efficiency (NUE) by 2050, the model projects a 24-31% increase in global agricultural N₂O emissions (with the uncertainty range stemming from differences in climate forcing, land-use and fertilizer and manure consumption between RCP2.6 and RCP8.5, the two climate scenarios used in this study) – rising from 2.9 Tg N₂O-N yr^-1 in 1990-2000 to 3.6-3.8 Tg N₂O-N yr^-1 in 2040-2050. This emission increase is considerably less than the projected increases in global fertilizer and manure consumption (42-44%) and previously published projections of global agricultural N₂O emission increases (38-75% - again, the uncertainty range reflecting the differences between the climate scenarios used). This disparity appears to be a result of ecological feedbacks captured by the model, where a considerable portion of the increase in fertilizer and manure use is absorbed by agricultural plant biomass rather than lost to the environment. In addition to this dynamic, the model projects that improvements in global NUE of 20-50% could reduce global N₂O emissions significantly, delivering important climate and stratospheric ozone benefits over the period 2015-2050: avoiding up to 9.9 Gt CO₂ eq. and 567 ODP kt, respectively. This is equivalent to about two years of the CO₂ emissions of all the passenger cars currently on the road and 25-35% of the potential reductions from the destruction of the remaining recoverable stocks of other ozone depleting substances.