Ocean fertilization: a potential means of geoengineering?

The oceans sequester carbon from the atmosphere partly as a result of biological productivity. Over much of the ocean surface, this productivity is limited by essential nutrients and we discuss whether it is likely that sequestration can be enhanced by supplying limiting nutrients. Various methods of supply have been suggested and we discuss the efficacy of each and the potential side effects that may develop as a result. Our conclusion is that these methods have the potential to enhance sequestration but that the current level of knowledge from the observations and modelling carried out to date does not provide a sound foundation on which to make clear predictions or recommendations. For ocean fertilization to become a viable option to sequester CO2, we need more extensive and targeted fieldwork and better mathematical models of ocean biogeochemical processes. Models are needed both to interpret field observations and to make reliable predictions about the side effects of large-scale fertilization. They would also be an essential tool with which to verify that sequestration has effectively taken place. There is considerable urgency to address climate change mitigation and this demands that new fieldwork plans are developed rapidly. In contrast to previous experiments, these must focus on the specific objective which is to assess the possibilities of CO2 sequestration through fertilization.

Physical and biochemical aspects of the flow across the Mascarene Plateau in the Indian Ocean.

This paper presents results from a detailed hydrographic survey of the Mascarene Plateau and surrounding area undertaken by the RRS Charles Darwin in June-July 2002. We examine how the westward-flowing South Equatorial Current (SEC) crosses the plateau, and how the structure of the flow determines the supply of nutrients to the surface waters. We find that the flow of the SEC across the plateau is highly dependent on the complex structure of the banks which make up the plateau, and that a large part of the flow is channelled between the Saya de Malha and Nazareth Banks. Furthermore, the SEC forms a sharp boundary between subtropical water masses from further south, which are low in nutrients, and waters from further north, which are relatively nutrient rich. Overall, the SEC delivers relatively high levels of nutrients to the near-surface waters of the central and northern regions of the plateau, compared with the southern regions of the plateau. This is partly due to uplifting of density surfaces through Ekman suction on the northern side of the SEC, and partly due to the higher levels of nutrients on those density surfaces on the northern side of the SEC. This may drive increased production of phytoplankton in these areas, which would in turn be expected to fuel increased abundances of zooplankton and higher levels of the food chain.

Halocarbon and dimethyl sulphide studies around the Mascarene Plateau.

Air-sea exchange is thought to be one of the major routes by which halocarbons and dimethyl sulphide reach the troposphere and stratosphere. Once there, in different ways, they participate in chemical reactions that have implications for ozone depletion and climate change. The gases are released by phytoplankton and other algae, but our present understanding of the sources and sinks is insufficient to establish a balanced global budget. Published data suggest that there are regions of coastal and ocean waters that constitute a major source, but, for halocarbons, in other regions the ocean is a net sink. For example, in many open oceanic areas, the rate of degradation of methyl bromide outweighs production. Here we present data from the Central Indian Ocean, a region considered to be low in terms of biological productivity. Little is known about trace-gas release from the Central Indian Ocean and without such data it is impossible to even hazard a guess at the global ocean source to the atmosphere.