Abstract
It is now widely recognized that in order to reach the target of limiting global warming to well below 2°C above pre-industrial levels (as the objective of the Paris agreement), cutting the carbon emissions even at an unprecedented pace will not be sufficient, but there is the need for development and implementation of active Carbon Dioxide Removal (CDR) strategies. Among the CDR strategies that currently exist, relatively few studies have assessed the mitigation capacity of ocean-based Negative Emission Technologies (NET) and the feasibility of their implementation on a larger scale to support efficient implementation strategies of CDR. This study investigates the case of ocean alkalinization, which has the additional potential of contrasting the ongoing acidification resulting from increased uptake of atmospheric CO2 by the seas. More specifically, we present an analysis of marine alkalinization applied to the Mediterranean Sea taking into consideration the regional characteristics of the basin. Rather than using idealized spatially homogenous scenarios of alkalinization as done in previous studies, which are practically hard to implement, we use a set of numerical simulations of alkalinization based on current shipping routes to quantitatively assess the alkalinization efficiency via a coupled physical-biogeochemical model (NEMO-BFM) for the Mediterranean Sea at 1/16° horizontal resolution (~6 km) under an RCP4.5 scenario over the next decades. Simulations suggest the potential of nearly doubling the carbon-dioxide uptake rate of the Mediterranean Sea after 30 years of alkalinization, and of neutralizing the mean surface acidification trend of the baseline scenario without alkalinization over the same time span. These levels are achieved via two different alkalinization strategies that are technically feasible using the current network of cargo and tanker ships: a first approach applying annual discharge of 200 Mt Ca(OH)2 constant over the alkalinization period and a second approach with gradually increasing discharge proportional to the surface pH trend of the baseline scenario, reaching similar amounts of annual discharge by the end of the alkalinization period. We demonstrate that the latter approach allows to stabilize the mean surface pH at present day values and substantially increase the potential to counteract acidification relative to the alkalinity added, while the carbon uptake efficiency (mole of CO2 absorbed by the ocean per mole of alkalinity added) is only marginally reduced. Nevertheless, significant local alterations of the surface pH persist, calling for an investigation of the physiological and ecological implications of the extent of these alterations to the carbonate system in the short to medium term in order to support a safe, sustainable application of this CDR implementation.
Highlights
The Paris agreement of 2015 has set a cornerstone in international climate policy by defining the goal of limiting global warming to well below 2◦C, a goal that is driven by the necessity to avoid irreversible change and to restrain intolerable risks to humanity, and supported by the feasibility to achieve this goal and by its simplicity as a tangible target bridging the communication gap between science and policy (Schellnhuber et al, 2016)
Its application to the Mediterranean Sea relies on a spatial grid with a horizontal resolution of 1/16◦ degree and a vertical z-coordinate discretization in 72 levels, with spacing ranging from 3 m in surface layer to 350 m at the bottom
The carbonate system dynamics described by the prognostic evolution of Dissolved Inorganic Carbon (DIC) and Total Alkalinity (TA) and parameterizations for chemical dissociation rates and gas exchanges were set in agreement with the protocol described in Orr et al (2017)
Summary
The Paris agreement of 2015 has set a cornerstone in international climate policy by defining the goal of limiting global warming to well below 2◦C, a goal that is driven by the necessity to avoid irreversible change and to restrain intolerable risks to humanity, and supported by the feasibility to achieve this goal and by its simplicity as a tangible target bridging the communication gap between science and policy (Schellnhuber et al, 2016). González and Ilyina (2016) constructed an alkalinization scenario based on a RCP8.5 with gradually increasing alkalinity additions calibrated to reach the atmospheric CO2 levels of a corresponding RCP4.5 projection, concluding that 114 Pmol would be required to achieve the goal over the period from 2018 to 2100, at the price of unprecedented ocean biogeochemistry perturbations with unknown ecological consequences None of these works considered the practicalities of an actual large-scale implementation of ocean liming or investigated the relative efficiencies and sensitivities of different strategies with respect to the oceanic uptake of CO2 and the mitigation of ocean acidification. This strategy allows us to assess the evolution of the system for each individual approach and compare their respective potentials in mitigating the atmospheric CO2 increases and the ocean acidification
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