Potsdam Net Primary Production Model Intercomparison Project (NPP)


Wolfgang Cramer (wolfgang.cramer@pik-potsdam.de)

(the activity has been concluded)

Seventeen global models of terrestrial biogeochemistry have been compared with respect to annual and seasonal fluxes of net primary productivity (NPP) for the land biosphere. The comparison, sponsored by IGBP-GAIM/DIS/GCTE, used standardised input variables wherever possible and was carried out through two international workshops and over the internet. The models differed widely in complexity and original purpose, but could be grouped in three major categories: Satellite-based models that use data from the NOAA/AVHRR sensor as their major input stream (CASA, GLO-PEM, SDBM, SIB2 and TURC), models that simulate carbon fluxes using a prescribed vegetation structure (BIOME-BGC, CARAIB 2.1, CENTURY 4.0, FBM 2.2, HRBM 3.0, KGBM, PLAI 0.2, SILVAN 2.2 and TEM 4.0), and models that simulate both vegetation structure and carbon fluxes (BIOME3, DOLY and HYBRID 3.0). The simulations resulted in a range of total NPP values (44.4 - 66.3 Pg C yr-1), after two outliers (which produced extreme results as artifacts due to the comparison) had been removed. The broad global pattern of NPP and the relationship of annual NPP to the major climatic variables coincided in most areas. Differences could not be attributed to the fundamental modeling strategies, with the exception that nutrient constraints generally produced lower NPP. The regional and global sensitivity of NPP against the simulation method for the water balance was clearly illustrated. Seasonal variation among models was high, both globally and locally, providing several indications for specific deficiencies in some models. A negative correlation between total absorbed photosynthetically active radiation (APAR) and light use efficiency (LUE) across the majority of models was found after decomposition of annual NPP for those models that do not use remote sensing data into APAR and LUE, and subsequent comparison of these values against those used by the remote sensing models. This may indicate that the models are (consciously or unconsciously) calibrated to achieve 'commonly accepted values' of total NPP, despite widely differing spatial and seasonal patterns. An analysis of the resulting net ecosystem productivities (NEP), using a three-dimensional atmospheric transport model and observed seasonal CO2 observations from the flask sampling network indicates that the uncertainties are larger in water limited systems of the tropics than elsewhere.

Reference:
Cramer, W., Kicklighter, D.W., Bondeau, A., Moore, B., III, Churkina, G., Nemry, B., Ruimy, A., Schloss, A.L. & Participants of "Potsdam'95" 1999. Comparing global models of terrestrial net primary productivity (NPP): Overview and key results. Global Change Biology 5 (Suppl.1):1-15.




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