Atmospheric CO₂ concentrations would be about 55 ppm (parts per million) higher than their present concentration without the oceanic anthropogenic CO₂ sink. The ocean has hindered the extent of accelerated climate change and will continue to absorb about 33% of fossil-fuel emissions well into the future 1. The ocean CO₂ sink is different to other carbon sinks in that it directly remediates against climate change by sequestering anthropogenic CO₂ on both short and long timescales. The exclusive economic zone (EEZ) is an oceanic zone legally bound to nation states under international law 2. With the global EEZ representing 27% of the oceans area, the question arises as to how significant could the EEZ CO₂ sink be for national carbon accounts. I use Australia as a case study to estimate the EEZ anthropogenic CO₂ sink due largely to the detailed accounting information on CO₂ emissions from fossil fuels and land-use changes 3 along with its very large oceanic territory. In fact excluding the Antarctic territorial claim, Australia's EEZ is one of the largest in the world and covers an area 8.2 × 10⁶ km² 4, which makes it larger than its continental land area (7.7 × 10⁶ km²).

I have calculated the accumulation (storage) of anthropogenic CO₂ within Australia's EEZ (including the Antarctic territory) for the 1990–1999 period to be 2.12 ± 0.7 Pg CO₂ (Pg = 1 × 10¹⁵ g) with an annual increase of about 220 Mt CO₂/yr (Figure 1). The calculations are described in the methods section a the end of the manuscript. This storage of anthropogenic CO₂ within Australia's EEZ does not necessarily imply the flux occurred within the EEZ as the ocean can redistribute CO₂ from its uptake region to other locations. If I assume that Australia's EEZ absorbs anthropogenic CO₂ at a rate equivalent to the global oceanic mean flux (7.3 ± 01.5 Pg CO₂/yr 15), I can obtain an independent flux estimate. As Australia's EEZ accounts for 2.4% of the total surface ocean (3.61 × 10⁸ km²), the mean anthropogenic CO₂ flux for Australia's ocean is about 175 MtCO₂/yr throughout the 1990s. This calculation assumes Australia's EEZ acts in proportion to the global average oceanic anthropogenic CO₂ flux. Even though the ocean is relatively homogenous, most studies suggest the Southern Ocean to be the region of highest uptake 6. Since Australia's EEZ contains both Southern Ocean waters and sub-topical waters, it is likely that large variations occur within the Australian EEZ. Despite this however, my estimated range for anthropogenic CO₂ uptake within the Australian EEZ (175–220 MtCO₂/yr) is in agreement with a recent modelling study 7 that estimates a range between 160 to 340 MtCO₂/yr depending on the areal extent of the Australian EEZ.

Australia's EEZ anthropogenic CO₂ uptake is significant when comparing to Australia's CO₂ emissions via fossil-fuel usage or land-use (Figure 2). Based on my analysis, Australia's oceanic EEZ CO₂ sink over the 1990s (1750–1980 MtCO₂) is about 3 times the magnitude of the CO₂ source due to land-use changes (655 MtCO₂) and about 30–40% of the total magnitude of fossil-fuel emissions (4695 MtCO₂). The implications of including Australia's EEZ or any other nations EEZ within the framework of global carbon trading would be considerable. Furthermore direct human influence of the EEZ CO₂ sink through carbon runoff from land use/irrigation/agricultural practices may also significantly influence national carbon accounts for nations with large EEZs.

Nations with large EEZs (like the USA) aren't necessarily those who will benefit the most from including the EEZ carbon sink in international climate policy, as it depends on the relative amount of EEZ sink in comparison to a nations annual anthropogenic emissions. Despite the USA claiming the worlds largest oceanic territory (~10 × 10⁶ km²), its EEZ anthropogenic CO₂ sink only absorbs less than 3% of its annual fossil-fuel emissions8. On the other hand small island nations (such as in the South Pacific) have the most to gain due to their low very anthropogenic CO₂ emissions relative to their large potential EEZ CO₂ sink. Tonga, Fiji, Samoa, Soloman Islands for example have vast oceanic territories that absorb many times over their annual fossil-fuel CO₂emissions. Although Australia emits near the highest amount of anthropogenic CO₂ per capita in the world, its relatively low population and vast oceanic territory results in the EEZ carbon sink being highly influential to its national carbon accounting. However, this 'natural anthropogenic CO₂ sink' could be used as a disincentive for certain nations to reduce their anthropogenic CO₂ emissions, which would ultimately dampen global efforts to reduce atmospheric CO₂ concentrations. Along with the fact that the oceanic anthropogenic CO₂ sink has little ability to be controlled by human activities, it should be explicitly excluded within current or future climate change policies. The international legality of the EEZ carbon sink and its potential implications requires careful consideration in formulating an equitable future framework for climate policy that aims at reducing atmospheric CO₂ levels.

The global EEZ represents over a quarter of the surface area of the ocean, which undoubtedly acts as important reservoir for sequestering anthropogenic CO₂. Just as nation states have varying degrees of land coverage they also have varying degrees of EEZ extents. To demonstrate the potential implications of the EEZ anthropogenic CO₂ sink, I have roughly estimated the uptake for Australias EEZ which is one of the largest in the world. By comparing the amount of anthropogenic CO₂ sequestered by Australias EEZ to Australias CO₂ emissions via fossil-fuel/land-use, I show that including the EEZ has significant implications for Australias national carbon accounts and any other nation who maintains a large EEZ. As the EEZ carbon sink may introduce legal grounds for nation states to possibly exploit, which would ultimately dampen efforts to reduce atmospheric CO₂ concentrations, current and future international climate change policies should have an explicit 'EEZ' clause excluding its use within national carbon accounts.

Due to the lack of temporal CO₂ measurements within Australia's EEZ, to quantify the EEZ anthropogenic CO₂ sink I use a recently developed method that exploits a purely transient tracer 1. The method uses oceanic measurements of chlorofluorocarbons (CFC) coupled with atmospheric CFC observations 9 to determine water mass ages 10. These water mass ages are then used with knowledge of the CO₂ atmospheric history 11, alkalinity and carbonate chemistry equations 12 to estimate an accumulation of anthropogenic CO₂ from 1990 to 1999. Although the method is indirect, the total uncertainty has been quantified to be between 10–20% by comparing results from direct temporal CO₂ estimates 13 and within a general ocean circulation model 1.