Reducing Greenhouse Gas By Ocean Nourishment
Authors: Jones Ian S.F. , Young Helen E. 
Published: October 2008

The burning of fossil fuels has produced vast amounts of carbon dioxide as a waste product. The United Nations Framework Convention on Climate Change requires industrialised countries to mitigate the rate of increase of atmospheric carbon dioxide. The most promising approach for mitigating climate change is to enhance biological sinks, the potentially largest of which is the ocean. Ocean nourishment is a technique for enhancing ocean sequestration by providing nutrients to regions of the ocean where they are in short supply. This process has the potential to generate tradeable carbon credits. It can be a very effective use for stranded offshore gas. Ocean nourishment can turn a waste product, carbon dioxide, into marine protein suitable for human consumption.

Introduction

The Greenhouse gas problem offers ocean engineering new opportunities in demonstrating environmental responsibility. The ocean provides the largest potential sink for atmospheric carbon dioxide but its uptake is restricted by a shortage of nitrogen and other nutrients. Phytoplankton forms the base of the photosynthetic carbon cycle in which atmospheric carbon dioxide is absorbed into the ocean. It plays a similar role to grass in the terrestrial food chain. By providing nutrients to the upper ocean photosynthesis can be enhanced, a process called ocean nourishment.

The objective of the UN Framework Convention on Climate Change is to stabilise greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. All Parties to the Convention are required to "promote and cooperate in the conservation and enhancement, as appropriate, of sinks and reservoirs of all greenhouse gases not controlled by the Montreal Protocol, including biomass, forests and oceans as well as other terrestrial, coastal and marine ecosystems". [4.1.d]

One method proposed to make mitigation of carbon dioxide efficient is to have tradeable carbon credits. By producing a sink of carbon dioxide this commodity can be sold to offset the emissions of carbon into the atmosphere. If the sink of carbon dioxide can be produced more cheaply than the alternatives available to a carbon dioxide emitter (such as increased effiency or fuel switching) purchase of a carbon credit is attractive.

The ocean nourishment process can produce carbon credits at a low cost when the feedstock is offshore natural gas. Jones and Cappelen-Smith (1999) talk about exploiting these fields. When one considers stranded natural gas exploited with a relocatable ocean nourishment plant, the economics are even more attractive.

Primary production in the ocean

Photosynthesis occurs in the upper sunlit zone of the ocean and combines carbon, nitrogen and phosphate (together with other micro-nutrients) in approximately the Redfield ratio. This ratio of C:N:P of 6.6:1:0.06 is a convenient generalisation. Phytoplankton die or are grazed upon by higher species in the food chain and the organic carbon and nitrogen are either recycled in the photic zone or fall through the water column to the deeper ocean.