This is an interesting presentation, given by Eric Davidson of Woods Hole Research Center, at the recent Planet under Pressure conference in March 2012. The presentation is a summary of a paper he has forthcoming in Environmental Research Letters.
It makes the point that reducing N2O emissions isn’t a question of focusing on dietary change OR on technological improvements – both are needed if stabilisation of atmospheric N2O is to be achieved.
Atmospheric concentrations of nitrous oxide (N2O) have been increasing since the Industrial Revolution, as livestock herds increased globally and as the use of synthetic-N fertilizers increased after WWII. Significantly reducing those emissions while also improving the diets of the growing global human population will be very challenging. Increases in atmospheric N2O since 1860 are consistent with emissions factors of 2.5% of annual fertilizer-N usage and 2.0% of annual manure-N production being converted to N2O. These factors include both direct and indirect emissions attributable to these sources. Here I present projections of N2O emissions for the following scenarios: (1) FAO population/diet scenarios with no changes in emission factors; (2) per-capita protein consumption in the developed world declines to 1980 levels by 2030 and only half of that is obtained from animal products; (3) improvements in N-use efficiency and manure management reduce the emission factors by 50% by 2050; (4) same as 3 but industrial and transportation emissions are similarly reduced by 50% by 2050; and (5) all mitigations together. These projections are compared to the four representative concentration pathways (RCPs) developed for the IPCC-AR5. With no further mitigation, the projections are consistent with RCP8.5, with atmospheric N2O at 368 ppb in 2050. Major reductions in per-capita meat consumption in the developed world reduce projected 2050 N2O to 356 ppb, which is in line with RCP6.0. Cutting emission factors in half but without diet change would also lower projected 2050 N2O to 352ppb. Adding 50% improvements in other sectors reduces the 2050 N2O to 350ppm, which is in line with RCP4.5. Only by combining these improved efficiencies with reduced meat consumption can the most optimistic scenario of RCP3PD be achieved, with stabilization of atmospheric N2O at about 341 ppb in 2050.