title: A Biogeochemical Modelling Analysis of the Potential For Marine Ecosystems to Regulate Climate By the Production of Dimethylsulphide
Creators: name: Cropp, Roger Allan
Publisher: name: Griffith University
Year: 2003
Subjects: subject: biogeochemical modelling
subject: ecosystem modelling
subject: dimethylsulphide
subject: DMS
subject: climate
subject: marine ecosystems

description: The potential for life to control its environment was first suggested by Lovelock (1972). Charlson et al (1987) proposed a role for marine planktonic ecosystems in global climate regulation via the production and ventilation to the atmosphere of dimethylsulphide (DMS), a by-product of phytoplankton metabolism. Once in the atmosphere DMS contributes to the formation of cloud condensation nuclei, and increases the amount and brightness of cloud. This affects the albedo of the planet, reflecting more incident sunlight back into space, and cooling the earth. In common with many other 'hypotheses' regarding complex adaptive systems, the hypothesis proposed by Charlson et al (1987) is not experimentally testable. The production and ventilation to the atmosphere of DMS is the result of complex interactions between biological, chemical and physical processes. Consequently, increasing use is being made of mathematical models that simulate these processes to advance understanding of it (Archer et al. 2002). This study examines one of the fundamental mechanisms proposed by the Charlson et al (1987) hypothesis, that increasing global temperatures will lead to increased ventilation of DMS from the ocean to the atmosphere. The study develops one-dimensional biogeochemical models of DMS production by upper ocean ecosystems, based on the model proposed by Gabric et al. (1993b). The models are examined to elucidate their fundamental mathematical properties, and are subjected to sensitivity analysis to identify important processes and parameters. These investigations identify a simpler model that can reproduce the predictions of the Gabric et al. (1993b) model. Predictions derived from model simulations forced by climatologies of measured physical data are compared to a global database of measurements of sea surface DMS concentrations, and to observed depth profiles of DMS in the upper ocean. These comparisons confirm that all models are in good qualitative agreement with measured data. The fifteen global climate prediction models currently in use around the globe all predict substantial warming effects from the ventilation of anthropogenic carbon dioxide to the atmosphere. A simplified DMS model is calibrated to climatologies of Antarctic chlorophyll and DMS data and reproduces the data with great precision. The calibrated model is applied in global warming scenarios to 'test' the efficacy of the mechanism proposed by the Charlson et al (1987) hypothesis. This simulation provides evidence that the response predicted by the hypothesis is indeed feasible, and that substantial increases (up to 45%) in the ventilation of DMS to the atmosphere could be possible in some circumstances. The results of the modelling study provide impetus for further examination of field data. If couplings between marine biota and atmosphere are feasible, then they may be operating contemporarily, and may be detectable. Atmospheric DMS is oxidised to form aerosols (Miller et al. 2002) that influence the aerosol optical depth of the atmosphere. Archives of remote sensed ocean chlorophyll a concentration and aerosol optical depth are examined for evidence of the biologically mediated couplings. A clear coupling between aeolian dust and marine phytoplankton is evident from this analysis, suggesting that the deposition of dust from the atmosphere is a major factor controlling phytoplankton growth in many parts of the ocean. A second coupling between marine phytoplankton and atmospheric aerosols is also detected. This coupling is apparently not related to dust and is symmetrical about the equator, despite the substantial differences in the atmospheres and oceans of each hemisphere. It is speculated that this coupling may reflect the influence of the ventilation of DMS produced by marine phytoplankton on the atmosphere. This thesis provides new evidence supporting the important role of marine ecosystems in global climate regulation by the production of DMS. This evidence is principally obtained from a biogeochemical modelling approach, but is supported by analyses of empirical data. The concordance of results obtained from different approaches suggests that the contribution of marine ecosystems to global climate regulation is real, important and currently active.