Natural Emissions Model
The Natural Emissions Model (NEM) is used to simulate the emissions of methane (CH4) and nitrous oxide (N2O) from the terrestrial biosphere to the atmosphere. The natural terrestrial fluxes from soils and wetlands are important contributors to the global budgets for these gases. Because these fluxes are dependent on climate, global models to simulate the relevant biogeochemical processes are incorporated in the IGSM.
The global emission model for N2O, which focuses on soil biogenic N2O emissions, has a 2.5 degree spatial resolution. The model can predict daily emissions for N2O, N2, NH3 and CO2 and daily soil uptake of CH4. It is a process-oriented biogeochemical model including soil C and N dynamic processes for decomposition, nitrification, and denitrification. The model takes into account the spatial and temporal variability of the driving variables, which include vegetation type, total soil organic carbon, soil texture, and climate parameters. Climatic influences, particularly temperature and precipitation, determine dynamic soil temperature and moisture profiles and shifts of aerobic-anaerobic conditions.
The methane emission model is developed specifically for wetlands and has a spatial resolution of 1 degree. For high latitude wetlands, the emission model uses a two-layer hydrological model to predict the water table level and the bog soil temperature, which are then used in an empirical formula to predict methane emissions. For tropical wetlands, a two-factor model (temperature and water availability) is used to model the methane flux by taking into account the temperature and moisture dependence of activity of methanogens. Methane emissions from wet tundra are calculated by assuming a constant small methane flux and an emission season defined by the time period when the surface temperature is above the freezing point. The hydrological model and the two-factor model are driven by surface temperature and precipitation, which links methane.