Comparing global models of terrestrial net primary productivity (NPP): analysis of differences in light absorption and light-use ef®ciency A. RUIMY 1 , L. KERGOAT 2,3 , A. BONDEAU 4³ and T H E P A R T I C I P A N T S O F T H E POTSDAM NPP MODEL INTERCOMPARISON* 1 Laboratoire d'Ecophysiologie Ve Âge Âtale, Ba Ãtiment 362, Universite  Paris-Sud, F-91405 Orsay Cedex, France, 2 Centre d'Etudes Spatiales de la Biosphe Áre, bpi 2805, 18, Av Edouard Belin, F-31055 Toulouse Cedex, France, 3 Present address: Laboratoire d'Ecologie Terrestre, 13, Av Col Roche, BP 4403, F-31405 Toulouse Cedex 4, France, 4 Potsdam Institut fu Èr Klimafolgenforschung e.V. (PIK), Postfach 60 12 03, Telegrafenberg, D-14412 Potsdam, Germany *The following participated in the Potsdam NPP Model Intercomparison: A. Bondeau (previous name: A. Fischer), G. Churkina, W. Cramer, G. Colinet, J. Collatz, G. Dedieu, W. Emanuel, G. Esser, C. Field, L. Franc Ëois, A. Friend, A. Haxeltine, M. Heimann, J. Hoffstadt, J. Kaduk, L. Kergoat, D. W. Kicklighter, W. Knorr, G. Kohlmaier, B. Lurin, P. Maisongrande, P. Martin, R. McKeown, B. Meeson, B. Moore III, R. Nemani, B. Nemry, R. Olson, R. Otto, W. Parton, M. Plo È chl, S. Prince, J. Randerson, I. Rasool, B. Rizzo, A. Ruimy, S. Running, D. Sahagian, B. Saugier, A. L. Schloss, J. Scurlock, W. Steffen, P. Warnant, and U. Wittenberg. ³Previous name: A. Fischer. Abstract Twelve global net primary productivity (NPP) models were compared: BIOME3, CASA, CARAIB, FBM, GLO-PEM, HYBRID, KGBM, PLAI, SDBM, SIB2, SILVAN and TURC. These models all use solar radiation as an input, and compute either absorbed solar radiation directly, or the amount of leaves used to absorb solar ra- diation, represented by the leaf area index (LAI). For all models, we obtained or estimated photosynthetically active radiation absorbed by the canopy (APAR). We then computed the light use ef®ciency for NPP (LUE) on an annual basis as the ratio of NPP to APAR. We analysed the relative importance for NPP of APAR and LUE. The analyses consider the global values of these factors, their spatial patterns represented by latitudinal variations, and the overall grid cell by grid cell variability. Spatial variability in NPP within a model proved to be determined by APAR, and differences among models by LUE. There was a compensation between APAR and LUE, so that global NPP values fell within the range of `generally accepted values'. Overall, APAR was lower for satellite driven models than for the other models. Most computed values of LUE were within the range of published values, except for one model. Keywords: NPP, model, global, intercomparison, light use ef®ciency Introduction Terrestrial net primary productivity (NPP) is the differ- ence between photosynthesis, or gross primary produc- tivity (GPP), and autotrophic respiration (R A ), integrated over a year. NPP is an important variable in studies of the global carbon cycle, as it determines the rate of absorption of atmospheric carbon by land vegetation. Recently, several models of terrestrial NPP have been developed, re¯ecting increasing concern about anthro- pogenic perturbations to the global carbon cycle and global climate. In contrast to oceanic NPP, global terrestrial NPP estimates have not varied much histori- cally, being usually in the 40±80 Pg C year ±1 range (Cramer et al. 1999). However, models that agree on the value of certain outputs (e.g. annual NPP) may disagree on the underlying processes (e.g. differences in rates of Correspondence: Dr L. Kergoat, fax: + 33±5-61558544, E-mail: kergoat.let@cesbio.cnes.fr Global Change Biology (1999), 5 (Suppl. 1), 56±64 56 # 1999 Blackwell Science Ltd.