NITROUS-OXIDE AND GLOBAL WARMING

The climatic impact of nitrous oxide (N2O) emissions is calculated ann ually for the period 1900-2100, using a globally averaged computer mod el. 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 t rends 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, exceed s the forcing derived from climate goals that may be considered enviro nmentally desirable. Limiting equilibrium warming to 0.1-degrees-C per decade would require anthropogenic-induced and warming-induced N2O em issions to be reduced by 80% relative to current trends and to be stab ilised from 2050, so that 10.7 Tg N/year is emitted by 2100. To stabil ise the current concentration or climate forcing of N2O, substantially larger cuts are needed. However, even in an optimistic scenario emiss ions keep increasing up to 14.4 Tg N/year by 2100. A major reason is t he close connection between N2O emissions and human food production. S ynthetic fertiliser use, land-use change and production of manure incr ease almost inevitably as the human population grows. Thus if global w arming is to be limited to 0.1-degrees-C per decade it may be necessar y to set emission reductions for other greenhouse gases relatively hig h to compensate for growth in climatic forcing by N2O.