The finding places new importance on the waters between Greenland and Newfoundland, where powerful circulation patterns draw oxygen-rich surface water downward and distribute it through the deep ocean. The Labrador Sea is one of the few places on Earth where the atmosphere can become directly connected to the abyss through intense seasonal mixing. During winter, cold air chills the sea surface, increasing the density of the water. As the surface becomes heavier, it sinks, carrying dissolved oxygen from the atmosphere into the depths. This process operates alongside the Atlantic Meridional Overturning Circulation, or AMOC, the planetary-scale system that moves warm, salty water northward near the surface and returns colder, denser water southward at depth.

The new study, published in Nature Geoscience on August 17, shows that the Labrador Sea does not simply participate in this circulation. It exports enough oxygen to meet the biological demand of deep-sea organisms across a huge area of the North Atlantic. That oxygen is consumed by microbes, animals and other organisms as they respire, a process that converts organic matter into energy and returns nutrients to the ocean. By comparing the oxygen leaving the Labrador Sea with estimates of respiration in the deep North Atlantic, the researchers found a striking match. The result suggests that oxygen produced and transported through this region is closely tied to the survival of ecosystems far beyond the sea itself.

“We found that the Labrador Sea exports enough oxygen to meet the biological need across a vast part of the deep North Atlantic Ocean, so it’s very likely crucial to sustain these deep sea ecosystems,” said Una Miller, assistant professor of earth and atmospheric sciences at Cornell and the study’s first author. “Our finding shows that if we’re going to understand the future, especially in the face of these deoxygenation trends, you can’t just look at the strength of AMOC, you also have to understand processes in the Labrador Sea.”

Scientists have long used AMOC strength as a key indicator of change in the North Atlantic. The circulation redistributes heat, carbon, salt and nutrients around the planet and influences weather patterns on both sides of the Atlantic. Observations indicate that AMOC has weakened over approximately the past 75 years, although the magnitude and causes of that decline remain subjects of active research. A major collapse could disrupt regional climates, raise sea levels along parts of the North American coast and alter marine ecosystems. But the Cornell-led research indicates that AMOC strength alone cannot describe how oxygen reaches the deep ocean. Local mixing processes in the Labrador Sea may continue to influence oxygen delivery even when the broader circulation changes.

To measure that delivery, Miller and an international research team analyzed observations from 60 oxygen sensors attached to moorings positioned along the seafloor in the Labrador and western Irminger seas. These instruments continuously recorded changes in dissolved oxygen as water moved through the region. Moorings are particularly valuable in the North Atlantic because they can collect data through storms, darkness and severe winter conditions that make ship-based observations difficult. The sensors allowed the researchers to track oxygen transport at depths where the movement of water is largely hidden from satellites and surface measurements.

The measurements capture a complex physical process. Oxygen enters the ocean at the air-sea boundary, where it dissolves into cold surface water. Strong winds and waves can enhance this exchange, while winter convection carries the oxygen downward. The density structure of the ocean then determines how far that water can travel. In the Labrador Sea, the newly oxygenated water becomes part of deep and intermediate layers that circulate around the basin. A gyre, or large rotating system of currents, helps organize the movement, while the wider AMOC transports the water through the North Atlantic. The oxygen is therefore not created in the deep ocean; it is imported from the atmosphere and delivered downward by the region’s unusual combination of cooling, sinking and circulation.

The researchers estimate that the Labrador Sea exports an amount of oxygen equivalent to sustaining the breathing needs of every person on Earth for at least two months. Although the comparison is designed to illustrate the scale of the transport rather than describe a direct human resource, it highlights how much oxygen is involved. Deep-sea animals consume oxygen as they use energy, while bacteria break down falling organic material known as marine snow. This continuous rain of particles links the sunlit surface to the ocean floor. If oxygen delivery weakens substantially, respiration becomes more difficult, decomposition changes and low-oxygen conditions can spread through habitats that support fish, invertebrates and microbial communities.

The discovery is especially significant as the global ocean loses oxygen. Warmer water holds less dissolved oxygen than colder water, and rising temperatures can also strengthen stratification, separating the surface from the deep ocean and making vertical mixing more difficult. At the same time, changes in winds, freshwater input from melting ice and the formation of dense water can affect the structure of North Atlantic circulation. Scientists have observed expanding areas of deoxygenation in many parts of the ocean, driven by both warming and biological activity. The Labrador Sea may temporarily buffer some of these pressures by continuing to ventilate deep waters, but the researchers warn that its future role cannot be assumed.

A central unanswered question is how oxygenation will respond if AMOC weakens further, if the Labrador Sea’s winter mixing changes, or if both processes occur together. A weaker circulation could reduce the movement of oxygen-rich water through the North Atlantic, yet shifts in local convection might produce different outcomes in the Labrador Sea. Freshwater entering the North Atlantic can make surface water less dense, potentially inhibiting the sinking that ventilates the deep ocean. Conversely, changing atmospheric conditions could alter wind-driven mixing and the timing or intensity of winter convection. Understanding these interactions will require sustained observations, improved ocean models and measurements from other critical ventilation zones.

Miller is extending this work to the Southern Ocean around Antarctica, another place where surface waters connect with the deep ocean and help regulate the distribution of oxygen and carbon. Together, the Labrador Sea and Southern Ocean offer scientists natural laboratories for studying how the atmosphere communicates with the ocean interior. The Cornell study suggests that the health of deep-sea ecosystems may depend on the behavior of specific, geographically limited regions rather than on global ocean circulation alone. As climate change reshapes temperature, ice, winds and freshwater flows, tracking these gateways could become one of the fastest ways to detect whether the deep ocean is still receiving the oxygen it needs.

Subject of Research: The role of the Labrador Sea in transporting oxygen to deep-sea ecosystems in the North Atlantic and its relationship with the Atlantic Meridional Overturning Circulation.

Web References: https://www.nature.com/articles/s41561-026-02057-3 ; https://news.cornell.edu/stories/2026/08/life-deep-atlantic-depends-labrador-sea

References: Nature Geoscience; National Science Foundation; National Oceanic and Atmospheric Administration; Canada Excellence Chair in Ocean Science and Technology; Canada First Research Excellence Fund.

Keywords

Labrador Sea, ocean circulation, Atlantic Meridional Overturning Circulation, AMOC, deep-sea oxygen, ocean deoxygenation, ocean temperature, oxygen transport, North Atlantic, deep-sea ecosystems, ocean physics, marine science

The oceans cover more than 70% of our planet’s surface and have historically absorbed approximately 25 to 30% of human-made CO₂ emissions annually. This natural absorption mitigates the pace of atmospheric warming, acting as a vital stabilizer against the intensifying effects of climate change. Yet, the new data highlight an alarming vulnerability: the ability of the ocean to continue soaking up carbon is not infinite, nor is it guaranteed under extreme environmental stressors. The record-high sea surface temperatures (SSTs) observed globally in 2023 have pushed the ocean carbon sink to a precipice, resulting in a marked reduction in carbon uptake.

At the core of this shift is the interplay between physical and biological processes that govern oceanic carbon sequestration. Warmer sea surface temperatures affect the solubility of CO₂ in seawater: as water warms, its capacity to dissolve gases diminishes. This thermodynamic principle means that the ocean’s surface layers are less capable of absorbing CO₂ from the atmosphere when SSTs increase dramatically. Moreover, elevated temperatures can alter ocean stratification, reducing the vertical mixing that usually transports carbon-rich surface waters to the ocean interior. Such stratification inhibits the deeper, more permanent sequestration of carbon, leading to a build-up of CO₂ in near-surface waters and ultimately decreasing net carbon uptake.

Beyond these physical limitations, biological feedbacks offer additional complexity. Phytoplankton, the microscopic photosynthetic organisms responsible for approximately half of global primary production and a critical component of the biological carbon pump, are sensitive to temperature changes. The study points to a significant reduction in phytoplankton biomass during 2023, particularly in key regions known for their high productivity and carbon export potential. Warmer waters tend to favor smaller phytoplankton species, which are less efficient at exporting carbon to the deep ocean. This shift diminishes the biological sequestration pathway that moves carbon from surface waters to abyssal depths on timescales of decades to centuries.

Compounding these effects, the ocean carbon sink decline aligns with an array of unprecedented oceanographic phenomena recorded in 2023. Heatwaves affected vast oceanic expanses, with surface temperatures soaring to levels unseen in the historical record. These heat extremes not only influence chemical and biological processes but also stress marine ecosystems, inducing harmful algal blooms and altering food web dynamics. Such stressors could further suppress phytoplankton productivity or change the community structure in ways unfavorable to carbon export mechanisms.

The researchers employed an integrative approach, harnessing satellite observations, in situ measurements, and sophisticated Earth system models to unravel the complex drivers behind the weakening carbon sink. This multidisciplinary methodology allowed for robust attribution of the decline to temperature anomalies while quantifying the consequent decrease in oceanic carbon uptake. Model simulations further suggest that if SSTs persist or continue to climb along current trajectories, the ocean carbon sink may experience additional reductions, destabilizing a critical planetary carbon buffer.

Intriguingly, the study underscores regional disparities in the response of the ocean carbon sink to warming. While some areas exhibited pronounced declines in carbon uptake, others showed resilience or even localized increases. These spatial heterogeneities relate to differences in ocean circulation, nutrient availability, and ecosystem composition among ocean provinces. The patchwork nature of these responses complicates global predictions and highlights the pressing need for enhanced monitoring networks tailored to capture fine-scale variability.

The implications of this unexpected decline extend far beyond oceanography, reverberating through climate policy and mitigation strategies. The ocean’s role as a carbon sink has often been considered a stable, albeit slow-reacting, component of the Earth system. The identification of rapid declines linked to temperature extremes challenges this assumption and emphasizes the urgency of curbing greenhouse gas emissions. If the ocean’s mitigation capacity falters, atmospheric CO₂ concentrations could rise more swiftly, thereby accelerating global warming and intensifying extreme weather, sea level rise, and ecological disruptions.

Moreover, the findings raise critical questions regarding the long-term feedback loops in the climate system. Reduced ocean carbon uptake could induce a positive feedback mechanism, wherein warming diminishes oceanic absorption, which in turn exacerbates atmospheric CO₂ accumulation and further warming. This cycle threatens to spiral, potentially complicating efforts to stabilize global temperatures under international goals such as those outlined in the Paris Agreement.

The study also pinpoints opportunities for future research aimed at refining climate projections and adaptation measures. Improved understanding of the thresholds and tipping points for ocean carbon sink decline is essential to predict the timeline and magnitude of potential feedbacks. Additionally, investigating how anthropogenic factors such as pollution, overfishing, and habitat degradation interact with warming to affect marine carbon cycling will be critical for comprehensive ecosystem management.

In practical terms, these insights necessitate an expansion of ocean observing capabilities globally. Continuous and detailed monitoring of SSTs, biogeochemical parameters, and biological productivity must be prioritized to identify emerging trends and anomalies in real-time. Coupled with enhanced model fidelity, this will empower the scientific community and policymakers to formulate adaptive strategies that mitigate risks associated with declining ocean carbon sequestration.

The unexpected decline in ocean carbon storage amid record-breaking temperatures serves as a stark reminder of the fragile balance underpinning Earth’s climate system. It emphasizes how interconnected and delicate the marine carbon cycle is, and how susceptible it is to disturbances induced by human influence. The ocean, often perceived as an inexhaustible absorber of CO₂, now appears vulnerable to rapid shifts that could undermine decades of climate stabilization efforts.

As the study’s authors eloquently summarize, these revelations call for urgent international collaboration to reduce emissions and to protect ocean health comprehensively. Mitigation strategies must integrate not only terrestrial but also marine ecosystem conservation and restoration to preserve the ocean’s capacity to buffer climate change. Recognizing and responding to this early-warning signal is paramount if humanity is to avoid cascading environmental consequences.

Ultimately, the 2023 ocean carbon sink decline harbingers a new era in climate dynamics, where the resilience of natural systems may be dwarfed by unprecedented anthropogenic pressures. This watershed moment challenges scientists, policymakers, and society at large to heed the ocean’s distress signals and bolster global efforts toward a sustainable climate future.

Subject of Research: Ocean carbon sink variability and its response to record-high sea surface temperatures

Article Title: Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023

Article References:
Müller, J.D., Gruber, N., Schneuwly, A. et al. Unexpected decline in the ocean carbon sink under record-high sea surface temperatures in 2023. Nat. Clim. Chang. (2025). https://doi.org/10.1038/s41558-025-02380-4

Image Credits: AI Generated