Methods for greenhouse gas offset accounting: A case study of ocean iron fertilization

Reducing atmospheric carbon concentration by removing past emissions can extend our rapidly diminishing emission budgets corresponding to the target of limiting the temperature increase to 2 °C above preindustrial levels. Forestation measures to offset carbon emissions have already entered the Kyoto Protocol. Other carbon offset options like ocean iron fertilization or chemically enhanced weathering are currently being analyzed. The analysis and comparison of such options requires determination of the amount of carbon credits generated that can be used for compliance. In our analysis we assess the impact of various accounting methods applied to large-scale sink enhancement projects, taking into account the partly temporary storage characteristics arising from such projects. We apply the various accounting methods to hypothetical large-scale Southern Ocean iron fertilization projects for different durations. From an economic perspective, issuing temporary carbon credits would provide the largest number of carbon credits at an early stage. This is equivalent to the existing tCER regulation under the Kyoto Protocol. Issuing temporary carbon credits for short-term ocean iron fertilization would also benefit the environment, as all credits would have to be replaced in the next commitment period. As some carbon will be stored permanently, this reduces atmospheric carbon concentration.