Unaccountable counting: the folly of incorporating open ocean carbon sinks in Nationally Determined Contributions

The national climate action plans to cut greenhouse gas emissions, referred to as Nationally Determined Contributions (NDCs), currently include anthropogenic land-based carbon fluxes but typically exclude open ocean carbon sinks within Exclusive Economic Zones (EEZs). Here, we utilise a high-resolution global ocean biogeochemical model alongside available observations to illustrate why including EEZ carbon sinks in NDCs would be both conceptually and practically highly problematic. To demonstrate this, we focus on the case of the French EEZ, where we simulate an anthropogenic carbon sink of 0.21 GtCO2·yr–1 over the past decade, representing nearly 50% of France’s territorial greenhouse gas emissions over the same period. However, it should be noted that this anthropogenic carbon uptake results from the physical and biogeochemical properties of water masses, rather than from human management practices. Additionally, the EEZ oceanic carbon sink is already considered in estimates of remaining carbon budgets to meet the goals of the Paris Agreement. Therefore, integrating these fluxes into NDCs would lead to inconsistency between national objectives and global climate goals. We argue that to identify the additionality of management measures aimed at enhancing the anthropogenic carbon sink in EEZ waters, robust assessments of the baseline carbon sink and how it would have evolved without intervention will be needed. As we show, this is complicated by the spatially and temporally dynamic nature of ocean carbon fluxes. More fundamentally, there is no colocalization between air–sea fluxes and the subsequent sequestration of carbon in the ocean due to circulation and mixing. Consequently, the measurement, reporting, and verification requirements for national stocktakes would be highly challenging. Moreover, as fluxes predominantly occur in the high latitude oceans and sequestration in the subtropics, the national allocation of credits would involve contentious issues of equity and justice.

Les plans d’action nationaux pour le climat, connus sous le nom de contributions déterminées au niveau national (CDN), intègrent les flux de carbone anthropiques continentaux, mais excluent généralement le puit de carbone océanique des zones économiques exclusives (ZEE). Dans cette étude, nous utilisons un modèle biogéochimique océanique global à haute résolution ainsi que les observations disponibles pour illustrer les raisons pour lesquelles l’intégration du puits de carbone des ZEE dans les CDN serait conceptuellement et concrètement problématique. Pour le démontrer, nous nous penchons sur le cas de la ZEE française, pour laquelle nous simulons un puits de carbone anthropique de 0,21 GtCO2·yr–1 au cours des dix dernières années. Nous argumentons que pour identifier l’additionnalité de mesures d’atténuation, des évaluations solides du puits de carbone de référence sont nécessaires. Cette tâche est rendu complexe, comme nous le d’emontrons, en raison de la nature dynamique des flux de carbone océaniques.