Interannual variability of global terrestrial primary production: Results of a model driven with satellite observations

Interannual variation in terrestrial net primary production (NPP) was model ed using the global production efficiency model (GLO-PEM), a semimechanisti c plant photosynthesis and respiration model driven entirely with satellite advanced very high resolution radiometer (AVHRR) observations. The model a lso estimated a wide range of biophysical variables at 10-day intervals for the period 1982-1989, including air temperature, vapor pressure deficit, s oil moisture, biomass, autotrophic respiration, canopy-absorbed photosynthe tically active radiation, gross primary production, and light use efficienc y. The accuracy of the simulated variables has previously been shown to be within 10-30% of field measurements, depending on the specific variable. We analyze here interannual changes in NPP, which showed large spatial variab ility (0-1500 gC m(-2) yr(-1)) and trends that differed regionally over the 8-year period. Annually integrated global NPP was found to vary as much as 12% between years and was very sensitive to air temperature. The coefficie nt of variation in NPP of sparsely vegetated areas (mostly semiarid) on an interannual basis was as much as 80%, whereas densely vegetated areas (broa dleaf evergreen and seasonally deciduous forests) vaned comparatively littl e (0-10%). Mean annual NPP of the latter decreased 36 gC m-2 yr over the ti me series examined. There was extreme seasonal and moderate interannual var iation (10-60%) in NPP of middle- to high-latitude regions (temperate and b oreal forests) with evidence for a slight trend toward increased values thr ough time (+3 to 12 gC m(-2) yr(-1)). The results indicate significant inte rannual and regional differences in responses to climate variability, with boreal regions increasing 39 gC m(-2) yr(-1) compared to a decrease of 116 gC m(-2) yr(-1) in tropical regions for each 1 degrees C rise in air temper ature. We explore a few of the possible reasons for these observations and discuss some of the issues and limitations to the use of the current global AVHRR observational record.