Marine biological controls on climate via the carbon and sulphur geochemical cycles

Abstract

We review aspects of the influence of the marine biota on climate, focusing particularly on their role in mediating surface temperatures via their influence on atmospheric carbon dioxide (CO ₂ ) and dimethyl sulphide (DMS) concentrations. Variation in natural CO ₂ concentrations occurring over 10 ³ to 10 ⁵ years are set by oceanic processes, and in particular by conditions in the Southern Ocean, so it is to this region that we must look to understand the glacial–interglacial changes in CO ₂ concentrations. It seems likely that marine productivity in the Southern Ocean is limited by a combination of restricted iron supply to the region and insufficient light. Plankton–produced DMS is thought to influence climate by changing the numbers of cloud condensation nuclei available in remote regions; the efficiency of this mechanism is still unknown, but calculations suggest it may be a powerful influence on climate. It has a much shorter time–scale than the CO ₂ effect, and as a consequence may well be a player on the ‘global change’ timescale. The direction of both the CO ₂ and the DMS mechanisms is such that more marine productivity would lead to lower global temperatures, and we speculate that the overall effect of the marine biota today is to cool the planet by ca. 6°C as a result of these two mechanisms, with one–third of this figure being due to CO ₂ effects and two–thirds due to DMS. While the marine biota influence climate, climate also influences the marine biota, chiefly via changing atmospheric circulation. This in turn alters ocean circulation patterns, responsible for mixing up sub–surface nutrients, and also influences the transport of nutrients, such as iron, in atmospheric dust. A more vigorous atmospheric circulation would be expected to increase the productivity of the marine biota on both counts. Thus during glacial time, the colder and drier climate might be expected to stimulate greater marine productivity than occurs today. Since more production leads to greater cooling by reduction in CO ₂ and increase in DMS, the marine biota–climate system appears to have been in positive feedback in the glacial–interglacial transition, with the changes in the climate system being reinforced by changes in the marine biota. In the context of anthropogenic change, we cannot at present say what sign the feedback on climate will have, because we have no clear idea whether circulation will become more or less vigorous in the future.