Global Soil Nitrous Oxide Emissions in a Future Climate

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) with a global warming potential of approximately 300 times more than CO2. In addition, it is becoming the most important species for protecting stratospheric ozone due to its high ODS-weighted emissions. To understand and quantify soil nitrous oxide emissions, we expanded the Community Land Model with prognostic Carbon and Nitrogen (CLM-CN) by inserting a module to estimate annually- and seasonally-varying nitrous oxide emissions between 1978 and 2000. We evaluate our soil N2O emission estimates against existing emissions inventories, other process-based model estimates, and observations from two forest sites in the Amazon and one in the United States. The model reproduces soil temperature and soil moisture relatively well, and it reconfirms the important relationship between N2O emissions and these parameters. The model also reproduces observations of N2O emissions well in the Amazonian forests but not during the winter in the USA. Applying this model to estimate the past 23 years of global soil N2O emissions, we find that there is a significant decrease in soil N2O emissions associated with drought and El Nino years. We further carry out five future simulations using different forcing datasets representing two scenarios - a scenario with policy to stabilize CO2 emissions to meet 550 CO2e, and the other scenario without policy. No significant increase is found in the future under the policy scenario, whereas an increase of more than 3.5 TgN year-1 between 2010 and 2100 is found under no policy. The results are robust under five different forcing forecasts, and the increase is due to a hotter and a wetter climate without effective policy to constrain GHGs. This natural soil emissions increase is equivalent to the historical surge in N2O emissions due to agriculture and manure, which led to a more than 30ppb rise in atmospheric N2O. This therefore could potentially increase the atmospheric GHG concentrations substantially and destroy stratospheric ozone in the future.