Effect of model structure on the response of terrestrial biosphere models to CO2 and temperature increases

The sensitivity of a number of different globally aggregated models of the terrestrial biosphere to changes of atmospheric CO2 and temperature is investigated. Net primary production (NPP) or net photosynthesis (NP) is modeled as a logistic function, with enhancement due to increased CO2 using the β factor widely used in global carbon cycle models. NPP also increases with temperature using a Q10 of 1.4, while respiration and coefficients for translocation and for detritus to soil, and soil to soil, carbon transfers increase with a Q10 of 2.0. The pathway of carbon flow to the slowly overturning soil reservoir has a significant effect on equilibrium sensitivity of total carbon mass to temperature increases if the transfer coefficient from the rapidly to slowly overturning soil reservoir is fixed; maximum sensitivity occurs if all the carbon entering the slowly overturning reservoir first passes through the rapidly overturning reservoir. If the transfer coefficient increases in parallel with the increase of soil respiration coefficient, the carbon pathway has no effect on equilibrium sensitivity, although the transient response depends strongly on the subdivision of the soil reservoir. Allowing the detritus to soil transfer coefficient to increase in parallel with the coefficient for detrital respiration reduces the equilibrium sensitivity of total carbon mass to temperature increases by about half. The variation in model response to CO2 and temperature increases using different model structures is generally comparable to the variation resulting from uncertainty in feedback parameters.