In:
Biogeosciences, Copernicus GmbH, Vol. 20, No. 7 ( 2023-04-03), p. 1195-1257
Abstract:
Abstract. Ocean alkalinity is critical to the uptake of atmospheric carbon in surface waters and provides buffering capacity towards the associated acidification. However, unlike dissolved inorganic carbon (DIC), alkalinity is not directly
impacted by anthropogenic carbon emissions. Within the context of
projections of future ocean carbon uptake and potential ecosystem impacts,
especially through Coupled Model Intercomparison Projects (CMIPs), the
representation of alkalinity and the main driver of its distribution in the
ocean interior, the calcium carbonate cycle, have often been overlooked.
Here we track the changes from CMIP5 to CMIP6 with respect to the Earth
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. We report an improvement in the representation of alkalinity in CMIP6 ESMs
relative to those in CMIP5, with CMIP6 ESMs simulating lower surface
alkalinity concentrations, an increased meridional surface gradient and an
enhanced global vertical gradient. This improvement can be explained in part
by an increase in calcium carbonate (CaCO3) production for some ESMs,
which redistributes alkalinity at the surface and strengthens its vertical
gradient in the water column. We were able to constrain a particulate inorganic carbon (PIC) export estimate of 44–55 Tmol yr−1 at 100 m for the ESMs to match the observed
vertical gradient of alkalinity. Reviewing the representation of the
CaCO3 cycle across CMIP5/6, we find a substantial range of
parameterizations. While all biogeochemical models currently represent
pelagic calcification, they do so implicitly, and they do not represent
benthic calcification. In addition, most models simulate marine calcite but
not aragonite. In CMIP6, certain model groups have increased the complexity
of simulated CaCO3 production, sinking, dissolution and sedimentation.
However, 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. Although modellers aim to balance the global alkalinity budget in ESMs 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. For 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. As such, the future
impact of ocean acidification on the carbonate pump, and in turn ocean
carbon uptake, is potentially underestimated in current ESMs and is insufficiently constrained.
Type of Medium:
Online Resource
ISSN:
1726-4189
DOI:
10.5194/bg-20-1195-2023
DOI:
10.5194/bg-20-1195-2023-supplement
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2023
detail.hit.zdb_id:
2158181-2
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