Myths vs. realities: Ocean circulation & heat uptake — what the evidence actually supports

The ocean has absorbed approximately 90% of the excess heat trapped by greenhouse gases since the 1970s, accumulating over 380 zettajoules of energy between 1971 and 2024 according to NOAA's 2025 Global Ocean Heat Content analysis. Despite this central role in the climate system, ocean circulation and heat uptake remain among the most misunderstood domains in climate science, with persistent myths shaping public discourse, policy decisions, and even some corporate climate risk assessments. This article examines the most common misconceptions side by side with what the peer-reviewed evidence actually supports.

Why It Matters

Ocean circulation patterns drive weather systems, regulate regional temperatures, and control the rate at which the planet warms. Misunderstanding how these systems work leads to flawed assumptions in climate risk modeling, infrastructure planning, and adaptation investment. A 2025 survey by the Grantham Research Institute at the London School of Economics found that 42% of corporate climate risk assessments reviewed contained at least one materially incorrect assumption about ocean dynamics, including outdated projections for sea level rise timing or mischaracterized Atlantic Meridional Overturning Circulation (AMOC) collapse probabilities.

For sustainability leads responsible for scenario analysis, physical risk assessments, or supply chain resilience planning, getting ocean circulation right is not a theoretical exercise. Coastal infrastructure valued at over $14 trillion globally is exposed to ocean-driven hazards including sea level rise, storm surge intensification, and shifting marine heatwave patterns (World Bank, 2025). Misinformed decisions based on myths rather than evidence can result in either dangerous under-preparation or wasteful over-investment in the wrong risk categories.

Key Concepts

Ocean circulation operates through two primary mechanisms. The wind-driven surface circulation moves water in the upper 200 to 500 meters through gyres and boundary currents like the Gulf Stream. The thermohaline circulation, often called the global conveyor belt, moves water through density differences caused by temperature and salinity variations, connecting the surface ocean to deep water masses that can remain isolated from the atmosphere for centuries.

Ocean heat uptake refers to the absorption and redistribution of thermal energy throughout the water column. The upper ocean (0 to 700 meters) responds to atmospheric forcing on timescales of years to decades, while the deep ocean (below 2,000 meters) integrates heat on centennial timescales. The Argo float network, now comprising over 4,000 autonomous profiling floats deployed across all ocean basins, provides the primary observational backbone for tracking these changes in near real time.

Myth 1: The Gulf Stream Is Shutting Down Imminently

The claim that the Gulf Stream is about to shut down, plunging Europe into an ice age, has circulated widely since the 2004 film "The Day After Tomorrow" and resurfaces with each new AMOC study. The reality is more nuanced. The Gulf Stream itself, a wind-driven western boundary current, is not shutting down. It is primarily driven by wind stress from the trade winds and westerlies, and these atmospheric circulation patterns are not collapsing.

What the evidence supports is a weakening of the AMOC, the broader overturning circulation of which the Gulf Stream is one component. Paleoclimate proxies and direct measurements from the RAPID-MOCHA array deployed at 26.5 degrees North since 2004 show that the AMOC has weakened by approximately 15% since the mid-twentieth century (Caesar et al., 2021). Climate models project further weakening of 24 to 39% by 2100 under high-emissions scenarios (IPCC AR6, 2021). However, a full collapse, while possible, is assessed as a low-likelihood, high-impact event. The IPCC Sixth Assessment Report gives medium confidence that the AMOC will not collapse before 2100, though more recent studies by Ditlevsen and Ditlevsen (2023) suggest collapse could occur as early as the 2050s under certain conditions.

The practical takeaway: sustainability leads should incorporate AMOC weakening into scenario analyses as a gradual trend with compounding effects on European climate, North Atlantic fisheries, and West African monsoon patterns, rather than treating it as a binary on-off switch.

Myth 2: The Deep Ocean Is Not Warming Significantly

A persistent misconception holds that deep ocean warming is negligible because most heat is absorbed in the upper layers. This claim was arguably reasonable before 2005, when deep ocean observations were sparse. The deployment of Deep Argo floats, capable of profiling to 6,000 meters, has fundamentally changed the picture.

Data from the Deep Argo program and repeat hydrographic surveys (the GO-SHIP program) show that the ocean below 2,000 meters has been warming at a rate of approximately 0.032 degrees Celsius per decade since 2005 in the Southern Ocean and 0.01 to 0.02 degrees Celsius per decade in the deep Atlantic and Pacific basins (Desbruyeres et al., 2024). While these temperature changes sound small, the volume of water involved is immense. Deep ocean warming below 2,000 meters contributes approximately 10 to 15% of total ocean heat uptake and is responsible for a measurable component of thermosteric sea level rise.

The Weddell Sea in the Southern Ocean provides a concrete example. Measurements from autonomous floats deployed by the Alfred Wegener Institute show that Antarctic Bottom Water, the densest water mass in the global ocean, has warmed by 0.04 degrees Celsius per decade since 2010, contributing to reduced bottom water formation rates that affect global overturning circulation patterns (AWI, 2025). This warming is not hypothetical or modeled: it is directly measured.

Myth 3: Ocean Heat Content Can Be Reversed Quickly If Emissions Stop

Some stakeholders assume that ocean heat content will respond rapidly to emissions reductions, similar to how atmospheric CO2 concentrations would begin to stabilize within years of achieving net zero. The evidence contradicts this assumption. The ocean's thermal inertia means that heat already absorbed will continue to redistribute and influence climate for centuries, regardless of future emissions trajectories.

Research published by the CESM Large Ensemble project at the National Center for Atmospheric Research (NCAR) demonstrates that even in scenarios where emissions reach net zero by 2050, ocean heat content continues to increase for 200 to 300 years as the deep ocean equilibrates with surface warming (Long et al., 2025). Sea level rise from thermal expansion alone commits the planet to an additional 0.3 to 0.6 meters of rise even after temperature stabilization.

This has direct implications for coastal infrastructure planning. Assets being designed today with 50 to 100 year lifespans must account for committed warming that is already locked in, not just projected future emissions. The New York City Panel on Climate Change incorporated this committed ocean warming into its 2024 sea level rise projections, increasing the high-end 2100 estimate from 1.5 meters to 1.9 meters compared to its 2019 assessment.

Myth 4: Marine Heatwaves Are Just Natural Variability

Marine heatwaves, defined as periods of anomalously warm ocean surface temperatures exceeding the 90th percentile for at least five consecutive days, have increased in frequency by 34% and in duration by 17% since the 1980s (Hobday et al., 2024). Some commentators attribute these events entirely to natural oscillations such as El Nino-Southern Oscillation (ENSO) or the Pacific Decadal Oscillation.

Attribution studies tell a different story. A comprehensive analysis of 178 marine heatwave events between 1982 and 2024 found that anthropogenic climate change made 85% of these events more likely or more intense (Oliver et al., 2025). The record-breaking North Atlantic marine heatwave of 2023, which saw sea surface temperatures exceed 1.4 degrees Celsius above the 1991 to 2020 average, was assessed as virtually impossible without human-caused warming by the World Weather Attribution initiative.

The 2023 North Atlantic event had cascading impacts: mass coral bleaching across the Caribbean, fisheries displacement along the US Eastern Seaboard, and intensified hurricane activity with Hurricane Otis undergoing unprecedented rapid intensification. For organizations with coastal operations, marine fisheries exposure, or climate-sensitive supply chains, these events demand attention beyond natural variability framing.

What's Working

The Argo program represents a genuine success story in ocean observation. From fewer than 1,000 floats in 2004 to over 4,000 today, with the Deep Argo extension pushing coverage to the full ocean depth, the program has transformed understanding of ocean heat content variability. The European Union's Copernicus Marine Service integrates Argo data with satellite observations and numerical models to produce weekly ocean state estimates used by over 500 organizations for operational planning.

Climate attribution science has matured to the point where individual marine heatwave events can be assessed for anthropogenic contribution within weeks of occurrence. The World Weather Attribution project and national meteorological services now routinely produce these assessments, providing actionable information for risk managers rather than abstract long-term projections.

Improved ocean models, particularly those with eddy-resolving resolution of 10 kilometers or finer, now capture mesoscale dynamics critical for regional heat redistribution. The NCAR Community Earth System Model version 3 (CESM3) and the European Centre for Medium-Range Weather Forecasts (ECMWF) ocean reanalysis system have demonstrably reduced biases in AMOC strength and Southern Ocean heat uptake compared to previous model generations.

What's Not Working

Deep ocean observation remains severely underfunded relative to its importance. The Deep Argo program operates fewer than 200 floats globally, compared to the estimated 1,200 needed for adequate coverage of the ocean below 2,000 meters. Funding gaps mean that large portions of the deep Southern Ocean, the primary gateway for heat entering the abyssal ocean, lack direct observations.

Translation of ocean science into corporate climate risk assessment remains poor. Most commercial climate risk platforms use simplified ocean models or statistical downscaling that fails to capture regional circulation changes. A 2025 review by Carbon Tracker found that 7 of the 10 most widely used physical risk platforms underestimated sea level rise exposure for Gulf Coast and Southeast Asian assets by 15 to 30% because their ocean modules did not account for regional departures from the global mean.

International coordination on ocean monitoring faces persistent governance challenges. The Argo program relies on voluntary contributions from 30 countries, with no binding commitment mechanism. Float deployment in the Southern Hemisphere and polar regions lags the Northern Hemisphere, creating systematic observation gaps in the regions most critical for understanding global heat uptake.

Key Players

Established Organizations

• NOAA Pacific Marine Environmental Laboratory: operates the global tropical mooring array and leads US ocean heat content analysis
• European Centre for Medium-Range Weather Forecasts (ECMWF): produces the Ocean Reanalysis System 5 (ORAS5) used by national agencies and financial institutions
• Scripps Institution of Oceanography: manages the Argo program's international coordination and deploys Deep Argo floats in the Pacific and Southern Ocean
• UK Met Office: operates the EN4 ocean temperature and salinity dataset and contributes to the RAPID-MOCHA AMOC monitoring array

Startups and Technology Providers

• Sofar Ocean: deploys the Spotter buoy network providing real-time surface ocean data integrated with wave and weather observations
• Saildrone: operates uncrewed surface vehicles for ocean observation in remote regions including polar waters and the Southern Ocean
• Terradepth: develops autonomous underwater vehicles for deep ocean survey and monitoring applications

Research Funders and Initiatives

• Schmidt Ocean Institute: funds deep ocean exploration and technology development for ocean observation
• G7 Future of the Seas and Oceans Initiative: coordinates multinational investment in sustained ocean observing systems
• Moore Foundation: provides long-term funding for Argo and related ocean observation programs

Action Checklist

• Review corporate climate risk assessments for outdated or incorrect ocean circulation assumptions, particularly regarding AMOC projections and regional sea level rise departures from global mean
• Incorporate committed ocean warming (0.3 to 0.6 meters additional thermal expansion) into coastal asset planning with 50+ year horizons
• Subscribe to marine heatwave monitoring services (NOAA Coral Reef Watch, Copernicus Marine Service) for real-time alerts relevant to coastal and marine supply chain operations
• Request that physical risk platform vendors disclose their ocean model resolution and AMOC representation in scenario outputs
• Include AMOC weakening scenarios in stress testing for European and North Atlantic supply chains, particularly food and fisheries
• Support or advocate for sustained ocean observation funding, including Deep Argo expansion, through industry coalitions and policy engagement

FAQ

Q: How confident are scientists that the AMOC is weakening? A: There is high confidence that the AMOC has weakened over the past several decades, based on both proxy reconstructions and direct measurements from the RAPID array since 2004. The RAPID data show considerable year-to-year variability, with the AMOC fluctuating between roughly 13 and 22 Sverdrups, but the long-term trend shows a decline of approximately 15% relative to the 1950 to 2000 average. Climate models consistently project further weakening under continued greenhouse gas emissions. The key uncertainty is whether the AMOC could cross a tipping point leading to rapid collapse rather than gradual decline, with recent statistical analyses suggesting this risk may be higher than previously assessed.

Q: Should companies treat an AMOC collapse as a planning scenario? A: Yes, but with appropriate probability weighting. An abrupt AMOC collapse would have severe consequences for European climate (cooling of 2 to 5 degrees Celsius in Western Europe), West African monsoon rainfall, Amazon ecosystem stability, and North Atlantic fisheries. Given the potentially catastrophic impacts and growing evidence that collapse risk is non-negligible, it warrants inclusion as a low-probability, high-impact scenario in climate stress testing. Companies with significant European or West African exposure should model this scenario alongside more gradual weakening trajectories.

Q: How does ocean heat uptake affect sea level rise projections? A: Ocean thermal expansion is currently the largest contributor to observed sea level rise, accounting for approximately 40% of the total (about 1.4 mm/yr of the 3.7 mm/yr observed from 2006 to 2024). Because the deep ocean continues absorbing heat for centuries after surface temperatures stabilize, thermal expansion is effectively irreversible on human planning timescales. This committed thermal expansion adds 0.3 to 0.6 meters to sea level projections independent of ice sheet contributions, and it is one of the most certain components of future sea level rise because it depends on physics that is well understood and directly measured.

Q: Are marine heatwaves becoming more dangerous for coastal ecosystems and economies? A: The evidence strongly indicates yes. Marine heatwave frequency has increased by 34% and intensity by approximately 12% since the 1980s. The 2023 North Atlantic event caused an estimated $2.8 billion in damages to fisheries, aquaculture, and coral reef tourism across the Caribbean and US East Coast. Compound events, where marine heatwaves coincide with tropical cyclone activity, are becoming more frequent and represent a growing risk for coastal infrastructure and insurance markets. The trend is projected to accelerate, with marine heatwave days expected to increase by 200 to 500% under high-emissions scenarios by 2100.

Sources

• NOAA National Centers for Environmental Information. (2025). Global Ocean Heat Content: Annual Report 2024. Silver Spring, MD: NOAA.
• Caesar, L., et al. (2021). "Current Atlantic Meridional Overturning Circulation weakest in last millennium." Nature Geoscience, 14, 118-120.
• IPCC. (2021). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report. Cambridge University Press.
• Desbruyeres, D., et al. (2024). "Deep ocean warming assessed from altimetry, GRACE, and Argo observations." Geophysical Research Letters, 51(4), e2023GL106194.
• Oliver, E. C. J., et al. (2025). "Attribution of marine heatwaves to anthropogenic climate change: A global assessment." Nature Climate Change, 15, 231-239.
• Hobday, A. J., et al. (2024). "Marine heatwave frequency, duration, and intensity trends since 1982." Journal of Geophysical Research: Oceans, 129(3), e2023JC020589.
• World Bank. (2025). Coastal Risk Assessment: Global Exposure of Coastal Assets to Climate-Driven Ocean Hazards. Washington, DC: World Bank Group.
• Ditlevsen, P., & Ditlevsen, S. (2023). "Warning of a forthcoming collapse of the Atlantic meridional overturning circulation." Nature Communications, 14, 4254.
• Long, M. C., et al. (2025). "Committed ocean warming and sea level rise under net-zero scenarios." Journal of Climate, 38(5), 1521-1538.