@article{fdi:010087598,
  title = {{T}he representation of alkalinity and the carbonate pump from {CMIP}5 to {CMIP}6 {E}arth system models and implications for the carbon cycle},
  author = {{P}lanchat, {A}. and {K}wiatkowski, {L}. and {B}opp, {L}. and {T}orres, {O}. and {C}hristian, {J}. {R}. and {B}utenschon, {M}. and {L}ovato, {T}. and {S}eferian, {R}. and {C}hamberlain, {M}. {A}. and {A}umont, {O}livier and {W}atanabe, {M}. and {Y}amamoto, {A}. and {Y}ool, {A}. and {I}lyina, {T}. and {T}sujino, {H}. and {K}rumhardt, {K}. {M}. and {S}chwinger, {J}. and {T}jiputra, {J}. and {D}unne, {J}. {P}. and {S}tock, {C}.},
  editor = {},
  language = {{ENG}},
  abstract = {{O}cean alkalinity is critical to the uptake of atmospheric carbon in surface waters and provides buffering capacity towards the associated acidification. {H}owever, unlike dissolved inorganic carbon ({DIC}), alkalinity is not directly impacted by anthropogenic carbon emissions. {W}ithin the context of projections of future ocean carbon uptake and potential ecosystem impacts, especially through {C}oupled {M}odel {I}ntercomparison {P}rojects ({CMIP}s), the representation of alkalinity and the main driver of its distribution in the ocean interior, the calcium carbonate cycle, have often been overlooked. {H}ere we track the changes from {CMIP}5 to {CMIP}6 with respect to the {E}arth system model ({ESM}) representation of alkalinity and the carbonate pump which depletes the surface ocean in alkalinity through biological production of calcium carbonate and releases it at depth through export and dissolution. {W}e report an improvement in the representation of alkalinity in {CMIP}6 {ESM}s relative to those in {CMIP}5, with {CMIP}6 {ESM}s simulating lower surface alkalinity concentrations, an increased meridional surface gradient and an enhanced global vertical gradient. {T}his improvement can be explained in part by an increase in calcium carbonate ({C}a{CO}3) production for some {ESM}s, which redistributes alkalinity at the surface and strengthens its vertical gradient in the water column. {W}e were able to constrain a particulate inorganic carbon ({PIC}) export estimate of 44-55 {T}mol yr(-1) at 100m for the {ESM}s to match the observed vertical gradient of alkalinity. {R}eviewing the representation of the {C}a{CO}3 cycle across {CMIP}5/6, we find a substantial range of parameterizations. {W}hile all biogeochemical models currently represent pelagic calcification, they do so implicitly, and they do not represent benthic calcification. {I}n addition, most models simulate marine calcite but not aragonite. {I}n {CMIP}6, certain model groups have increased the complexity of simulated {C}a{CO}3 production, sinking, dissolution and sedimentation. {H}owever, this is insufficient to explain the overall improvement in the alkalinity representation, which is therefore likely a result of marine biogeochemistry model tuning or ad hoc parameterizations. {A}lthough modellers aim to balance the global alkalinity budget in {ESM}s in order to limit drift in ocean carbon uptake under pre-industrial conditions, varying assumptions related to the closure of the budget and/or the alkalinity initialization procedure have the potential to influence projections of future carbon uptake. {F}or instance, in many models, carbonate production, dissolution and burial are independent of the seawater saturation state, and when considered, the range of sensitivities is substantial. {A}s such, the future impact of ocean acidification on the carbonate pump, and in turn ocean carbon uptake, is potentially underestimated in current {ESM}s and is insufficiently constrained.},
  keywords = {},
  booktitle = {},
  journal = {{B}iogeosciences},
  volume = {20},
  numero = {7},
  pages = {1195--1257},
  ISSN = {1726-4170},
  year = {2023},
  DOI = {10.5194/bg-20-1195-2023},
  URL = {https://www.documentation.ird.fr/hor/fdi:010087598},
}