Terrestrial carbon-cycle feedback to climate warming: Synthesis of modeling results and experimental evidence

All the coupled carbon-climate models reported in the literature so far predict a positive feedback between terrestrial carbon cycle and climate warming.  The positive feedback results from decreases in simulated net primary production (NPP) by most of the models and increases in simulated respiratory carbon release by all the models under climate warming.  Results from a limited number of experimental warming studies, however, show that ecosystem photosynthesis and NPP mostly increased under warming and initial stimulation of respiratory carbon release by warming declined over time due to acclimation and other ecosystem-scale adjustment.  Consequently, experimental warming caused either decreases, increases, or no change in net ecosystem carbon exchange.  Net ecosystem carbon storage by measuring net changes of carbon in plant and soil pools in a few experimental studies shows either decreases or no change under warming.  Mechanisms underlying the observed changes in net ecosystem exchange or net carbon storage in the warming experiments include extended growing seasons, enhanced nutrient availability, shifted species composition, and altered ecosystem water dynamics.  Those indirect mechanisms often override direct responses of biochemical processes involved in photosynthesis and respiration to increasing temperature.  The latter biochemical responses of photosynthesis and respiration to temperature are primary mechanisms that have been incorporated into the global models.  To improve model predictions of the carbon-climate feedback, the modeling community needs to incorporate some of the key indirect mechanisms into the global models to regulate responses of carbon cycles to climate warming by temperature-induced changes in phenology, nutrient availability, water dynamics, and species composition.  The experimental research community has the potential to make contributions to understanding of the carbon-climate feedback if whole-ecosystem warming experiments can be conducted in wide ranges of biomes to improve empirical knowledge basis on major ecosystem processes.