Three groups of organisms — coccolithophores, foraminifers, and pteropods — build shells of calcium carbonate from dissolved carbon in seawater. When these organisms die, their shells sink, carrying carbon to deeper ocean layers. This is part of the ocean carbon pump — the suite of biological and chemical processes that transfer atmospheric carbon to the deep ocean, where it can remain sequestered for centuries or millennia.
A critical mechanism called shallow dissolution occurs in upper ocean waters. The calcium carbonate shells dissolve before reaching the seafloor, releasing their carbon in mid-water layers rather than sequestering it permanently. The balance between sinking (long-term sequestration) and shallow dissolution (mid-water recycling) determines how much atmospheric carbon the ocean actually absorbs. This balance is largely missing from CMIP6 — the Coupled Model Intercomparison Project Phase 6, the standard suite of Earth system models used for climate projections including IPCC reports.
Patrizia Ziveri and an international team, publishing in Science, identified the omission: climate models often simplify or exclude calcifying plankton because their biology is complex and their contributions are difficult to parameterize. The models run without them. The projections proceed. But the carbon pump operates differently with and without the calcifiers, and the difference affects how much atmospheric CO2 the ocean absorbs over the coming century.
The structural insight is about the relationship between model complexity and model accuracy. Climate models are already among the most complex computational systems ever built. Adding biological detail — three groups of plankton, each with different shell compositions, growth rates, depth distributions, and responses to ocean acidification — increases computational cost and introduces uncertain parameters. The tradeoff between completeness and tractability is real. But the organisms being excluded are not minor actors. They are a primary mechanism of ocean carbon transport. Excluding them simplifies the model at the cost of the carbon cycle itself.
The irony is that ocean acidification — a direct consequence of rising atmospheric CO2 — is dissolving the shells of these organisms faster, shifting the shallow-dissolution balance and potentially weakening the biological pump. The feedback loop (more CO2 → more acidification → less shell production → less carbon export → more CO2) is absent from models that exclude the organisms. The models cannot predict the weakening of a mechanism they do not include.