AN EFFICIENT AND ACCURATE REPRESENTATION OF COMPLEX OCEANIC AND BIOSPHERIC MODELS OF ANTHROPOGENIC CARBON UPTAKE

Establishing the link between atmospheric CO2 concentration and anthro pogenic carbon emissions requires the development of complex carbon cy cle models of the primary sinks, the ocean and terrestrial biosphere. Once such models have been developed, the potential exists to use puls e response functions to characterize their behaviour. However, the app lication of response functions based on a pulse increase in atmospheri c CO2 to characterize oceanic uptake, the conventional technique, does not yield a very accurate result due to nonlinearities in the aquatic carbon chemistry. Here, we propose the use of an ocean mixed-layer pu lse response function that characterizes the surface to deep ocean mix ing in combination with a separate equation describing air-sea exchang e. The use of a mixed-layer pulse response function avoids the problem arising from the nonlinearities of the carbon chemistry and gives the refore more accurate results. The response function is also valid for tracers other than carbon. We found that tracer uptake of the HILDA an d Box-Diffusion model can be represented exactly by the new method. Fo r the Princeton 3-D model, we find that the agreement between the comp lete model and its pulse substitute is better than 4% for the cumulati ve uptake of anthropogenic carbon for the period 1765 to 2300 applying the IPCC stabilisation scenarios S450 and S750 and better than 2% for the simulated inventory and surface concentration of bomb-produced ra diocarbon. By contrast, the use of atmospheric response functions give s deviations up to 73% for the cumulative CO2 uptake as calculated wit h the Princeton 3-D model. We introduce the use of a decay response fu nction for calculating the potential carbon storage on land as a subst itute for terrestrial biosphere models that describe the overturning o f assimilated carbon. This, in combination with an equation describing the net primary productivity permits us to exactly characterize simpl e biosphere models. As the time scales of biospheric overturning are o ne key aspect to determine the amount of anthropogenic carbon which mi ght be sequestered by the biosphere, we suggest that decay response fu nctions should be used as a simple and standardized measure to compare different models and to improve understanding of their behaviour. We provide analytical formulations for mixed-layer and terrestrial biosph ere decay pulse response functions which permit us to easily build a s ubstitute for the ''Bern'' carbon cycle model (HILDA). Furthermore, mi xed-layer response functions for the Box-Diffusion, a 2-D model, and t he Princeton 3-D model are given.