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

Interannual variation in terrestrial net primary production (NPP) was modeled using the global production efficiency model (GLO-PEM), a semimechanistic plant photosynthesis and respiration model driven entirely with satellite advanced very high resolution radiometer (AVHRR) observations. The model also estimated a wide range of biophysical variables at 10-day intervals for the period 1982-1989, including air temperature, vapor pressure deficit, soil moisture, biomass, autotrophic respiration, canopy-absorbed photosynthetically active radiation, gross primary production, and light use efficiency. 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 variability (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 coefficient of variation in NPP of sparsely vegetated areas (mostly semiarid) on an interannual basis was as much as 80%, whereas densely vegetated areas (broadleaf evergreen and seasonally deciduous forests) vaned comparatively little (0-10%). Mean annual NPP of the latter decreased 36 gC m-2 yr over the time series examined. There was extreme seasonal and moderate interannual variation (10-60%) in NPP of middle- to high-latitude regions (temperate and boreal forests) with evidence for a slight trend toward increased values through time (+3 to 12 gC m(-2) yr(-1)). The results indicate significant interannual 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 temperature. 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.