The NASA-CASA (Carnegie-Ames-Stanford Approach) model simulates net primary production (NPP) and soil heterotrophic respiration (Rh) at regional to global scales.

Calculation of monthly terrestrial NPP is based on the concept of light-use efficiency, modified by temperature and moisture stress scalars. Soil carbon cycling and Rh flux components of the model are based on a compartmental pool structure, with first-order equations to simulate loss of CO₂ from decomposing plant residue and surface soil organic matter (SOM) pools. Model outputs include the response of net CO₂ exchange and other major trace gases in terrestrial ecosystems to interannual climate variability (1983 to 1988) in a transient simulation mode.

Support for computing resources provided by the High Performance Computing and Communications (HPPC) Program by granting access to their testbed computers at the Numerical Aerodynamic Simulation (NAS) facility at NASA-Ames.

Monthly gridded 1 degree climate anomalies for air surface temperature and precipitation are used together with long-term (30-year) mean values, and surface solar irradiance measurements for the period 1983-1998. The fraction of absorbed photosynthetically active radiation (FPAR) is derived using the Normalized Difference Vegetation Index (NDVI) from the NOAA Advanced Very High Resolution Radiometer (AVHRR) satellite.

Major findings

• The model predicts annual global fluxes in terrestrial net primary production (NPP) at 47-60 Pg C, depending on the yearly conditions, with over 70% of terrestrial net production typically taking place between 30 N and 30 S latitude.
• Zonal estimates of net ecosystem production correlate significantly with seasonal variations in atmospheric CO₂ concentrations measured previously at flask sampling stations.
• From 1985 to 1988, the northern middle-latitude zone (between 30 and 60 N) was the principal region driving progressive increases in global NPP (i.e., the terrestrial biosphere sink for carbon).
• The average annual increase in NPP over this predominantly northern forest zone was on the order of +0.4 Pg C per year, resulting in part from notable expansion of the growing season for plant carbon fixation toward the zonal latitude extremes.
• A net biosphere source flux in 1983-1984 associated with the El Nino event was followed by a major recovery of global terrestrial production in 1985 which lasted through 1987 as a net sink.
• Tropical dry forests and savannas are important source areas for N trace gas emissions. The model predicts the annual N₂O:NO soil flux ratio at a mean value of 1.2 in wet tropical forests, decreasing to around 0.6 in the seasonally dry savannas.
• The model estimates global net consumption of methane in soils at 17-23 Tg per year, with over 40% of the predicted soil sink for methane occurs in warm and relatively dry ecosystems, such as semi-arid steppe, tropical savanna, and tropical seasonal forest.

Interannual results from the NASA-CASA model