NITROUS-OXIDE AND GLOBAL WARMING

The climatic impact of nitrous oxide (N2O) emissions is calculated annually for the period 1900-2100, using a globally averaged computer model. Emissions of N2O have been increasing up to an estimated 12.7 Tg N/year in 1990 by human activities and global warming. If the current trends continue, emissions are estimated to be 25.7 Tg N/year by 2100, with fossil fuel use and human food production as major contributors. The resulting equilibrium temperature increase, 0.37-degrees-C, exceeds the forcing derived from climate goals that may be considered environmentally desirable. Limiting equilibrium warming to 0.1-degrees-C per decade would require anthropogenic-induced and warming-induced N2O emissions to be reduced by 80% relative to current trends and to be stabilised from 2050, so that 10.7 Tg N/year is emitted by 2100. To stabilise the current concentration or climate forcing of N2O, substantially larger cuts are needed. However, even in an optimistic scenario emissions keep increasing up to 14.4 Tg N/year by 2100. A major reason is the close connection between N2O emissions and human food production. Synthetic fertiliser use, land-use change and production of manure increase almost inevitably as the human population grows. Thus if global warming is to be limited to 0.1-degrees-C per decade it may be necessary to set emission reductions for other greenhouse gases relatively high to compensate for growth in climatic forcing by N2O.