When you realize that the most advanced climate models ever created by humans may be measuring the wrong things, a certain kind of unease sets in. Not just a little off. structurally and fundamentally lacking in ways that permeate all forecasts, policy choices, and international agreements based on the presumption that we know how much carbon our oceans are truly absorbing.
One recurring culprit is being identified by the most recent research: what happens to light in the deep ocean and how frequently that process has been ignored.
A fairly simple concept underpins the majority of ocean carbon models: sunlight strikes the surface, phytoplankton absorbs it, carbon is fixed near the top of the water column, and some of that sinks. Clear, rational, and dangerously lacking, according to an increasing amount of research. Researcher Rémy Asselot’s study, which was published in the European Earth System Dynamics journal, identified phytoplankton light absorption as a carbon cycle missing link that adds extremely uncertain feedbacks to our more general climate projections. Practically speaking, this means that the models are not only imprecise but also lack a variable that significantly alters the result.
What is below 400 meters is, quite literally, the deeper issue. The dark ocean has long been regarded by scientists as largely passive, a chilly, silent area with little biological activity and sluggish carbon cycling. That assumption was methodically contested in a significant review that was published in the Journal of Marine Systems by Brenda Burd and Richard Thomson, and the results are difficult to ignore. The amount of respiration in the deep ocean is far greater than what could be explained by surface primary production alone. Down there, organic carbon is being produced by something else. Actually, a few things.
It turns out that zooplankton that migrate vertically function as a sort of biological conveyor belt. Approximately one-third of the epipelagic biomass migrates throughout the day, feeding close to the surface at night and moving downward during the day to carry particulate matter and dissolved organic carbon. These migrations have a 3,000-meter depth limit.
Additionally, most models fail to account for the additional 10 to 50 percent of surface primary production that occurs in oxygen minimum zones due to chemoautotrophic production, which is fueled by ammonia from those same migrating animals. Submarine volcanoes, hydrothermal vents, and methane seeps are examples of crustal sources that may contribute between 30 and 50 percent of the oceanic organic carbon flux to the dark ocean. These are not estimates from the fringe. They are present in the literature, but mainstream models ignore them.
It’s difficult to ignore how much of this stems from the challenge of studying something that is actually inaccessible. In comparison to surface oceanography, the deep ocean has historically been underfunded, costly to monitor, and technically difficult to sample. As a result, massive assumptions have been used to fill in the gaps in global carbon budgets, and these assumptions have consistently favored simplicity over complexity.
The image is made even more disturbing by a different line of research from Plymouth Marine Laboratory. The CO2 absorption in the shelf environment decreased by 7% when fish were fully incorporated into marine ecosystem models, not as passive plankton consumers but as active agents feeding back into plankton dynamics. The biomass of mesozooplankton decreased by 17%. Fish biomass decreased by 20%. These are not small changes. Not much carbon was being physically removed by the fish. Predation pressure on plankton populations had an indirect effect that cascaded through the food web. Most models just don’t have that feedback loop.
The gap was referred to as a “major blind spot” in UNESCO’s own assessment, which was published alongside this study. Given the stakes involved, the language used seems almost deliberately restrained. Due to limited observations, the Southern Ocean—one of the planet’s most significant carbon sinks—remains poorly constrained during the winter. Its CO2 sources were significantly underestimated during the colder months, according to a November 2025 study published in Science Advances. The margins of uncertainty continue to grow.
All of this doesn’t lead to a clear picture of scientists making mistakes. It’s a more complex situation where a field is working hard with the tools at hand while the ocean consistently shows that the tools are insufficient. The carbon is traveling through biochemical processes in areas we don’t often visit, through migration corridors that scientists are still mapping, and through darkness. Every projection has a gap that the science itself constantly reminds us of until those dynamics are appropriately incorporated into the models.
