Deep dive: Ocean circulation & heat uptake — the hidden trade-offs and how to manage them

In 2025, the global ocean absorbed a staggering 23 zettajoules of heat—the largest single-year increase since 2017 and equivalent to 12 Hiroshima bombs exploding in the ocean every second. This marked the ninth consecutive year of record-breaking ocean heat content, with the upper 2000 meters reaching an all-time high according to the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP/CAS). The implications extend far beyond rising sea surface temperatures: ocean circulation patterns that have governed Earth's climate for millennia are showing signs of stress, with potentially irreversible consequences for weather patterns, marine ecosystems, and coastal communities worldwide. For sustainability professionals, understanding these hidden trade-offs is no longer optional—it's fundamental to effective climate strategy.

Why It Matters

The ocean serves as Earth's primary climate regulator, absorbing more than 90% of the excess heat trapped by greenhouse gases and approximately 30% of anthropogenic carbon dioxide emissions. This buffering capacity has effectively shielded terrestrial ecosystems from the full force of climate change, but it comes at a profound cost. The 2024 State of the Climate report confirmed that both global sea surface temperature and ocean heat content reached unprecedented highs, with particularly extreme warming recorded in the Indian Ocean, tropical Atlantic, Mediterranean, and Southern Ocean.

The implications for sustainability strategy are threefold. First, thermal expansion from ocean warming now contributes approximately 1.5 millimeters annually to global sea level rise—a figure that compounds with accelerating ice sheet melt. Second, marine heatwaves have become ubiquitous, with 91% of the ocean surface experiencing at least one heatwave in 2023. These events devastate coral reefs, disrupt fisheries, and alter the base of marine food webs. Third, and most critically for long-term planning, the Atlantic Meridional Overturning Circulation (AMOC)—the system of currents that redistributes heat from the tropics to Europe and regulates weather patterns across the Northern Hemisphere—is showing concerning signs of weakening. Research published in 2025 found evidence of a 0.46 sverdrup per decade slowdown since 1950, with some models suggesting a potential collapse could begin as early as 2063 under high-emission scenarios.

KPI	Current Value	Target Range	Measurement Method
Ocean Heat Content Change (0-2000m)	+23 ZJ/year (2025)	<10 ZJ/year	Argo float network, satellite altimetry
Earth Energy Imbalance	0.66-0.74 W/m²	<0.5 W/m²	Space geodetic observations
Marine Heatwave Days	100 days/year	<30 days/year	SST anomaly monitoring
AMOC Strength Trend	-0.46 Sv/decade	Stable (±0.1 Sv)	RAPID array, hydrographic sections
Sea Level Rise (thermal)	1.5 mm/year	<1 mm/year	Tide gauges, satellite radar

Key Concepts

Ocean Heat Content and the Earth Energy Imbalance

Ocean heat content (OHC) quantifies the total thermal energy stored in seawater, typically measured in the upper 2000 meters where most anthropogenic warming occurs. The Earth Energy Imbalance (EEI)—currently measured at 0.66-0.74 watts per square meter averaged globally—represents the difference between incoming solar radiation and outgoing thermal radiation. This imbalance, primarily driven by greenhouse gas concentrations, manifests almost entirely as ocean warming. The Copernicus Marine Service tracks EEI showing a positive trend of 0.29 W/m² per decade since 1993, indicating accelerating heat accumulation.

Thermohaline Circulation and the AMOC

The global thermohaline circulation operates like a vast conveyor belt, driven by differences in water density created by temperature (thermo) and salinity (haline) gradients. In the North Atlantic, warm surface waters from the tropics release heat to the atmosphere before cooling, becoming denser, and sinking to form North Atlantic Deep Water. This process, central to the AMOC, transports roughly 17 million cubic meters of water per second (17 sverdrups) and delivers substantial heat to Europe. Freshwater influx from melting ice sheets threatens to disrupt this process by reducing surface water density, potentially weakening or destabilizing the circulation.

Tipping Points and Committed Warming

Ocean systems exhibit tipping point dynamics—thresholds beyond which changes become self-reinforcing and potentially irreversible. The AMOC represents a well-documented tipping element; once weakened beyond a critical threshold, positive feedbacks could accelerate collapse regardless of emission reductions. Equally important is the concept of committed warming: the heat currently stored in deep ocean layers will continue releasing for decades, locking in additional surface warming regardless of near-term emission cuts. This oceanic thermal inertia means that even aggressive decarbonization cannot immediately halt warming trends.

What's Working and What Isn't

What's Working

Global Ocean Observing Infrastructure: The international Argo program now maintains over 4,000 autonomous profiling floats worldwide, generating more than 2 million temperature profiles extending to 2000 meters depth. This unprecedented coverage enables near-real-time monitoring of ocean heat content and has revolutionized our understanding of subsurface warming patterns. The network has been instrumental in identifying that warming is penetrating ever-deeper layers, with significant heat accumulation now detected below 2000 meters.

Satellite-Based Earth Observation: Advances in space geodesy and satellite altimetry have transformed ocean monitoring capabilities. The Copernicus Marine Service now provides high-resolution (1 kilometer) products for European coastal waters using Sentinel-1 synthetic aperture radar data. These observations enable precise tracking of sea level changes, ocean currents, and thermal expansion contributions—critical inputs for climate modeling and adaptation planning.

Coordinated International Research: Multi-institutional collaborations have accelerated understanding of ocean-climate dynamics. The January 2025 ocean heat content study, published in Advances in Atmospheric Sciences, involved over 50 scientists from 31 institutions globally. Such coordination ensures methodological consistency and enables detection of trends that would be invisible to isolated research programs.

Operational Forecasting for Maritime Industry: Companies like Sofar Ocean have successfully commercialized ocean intelligence, deploying networks of Spotter buoys to collect real-time wave, temperature, and wind data. Their Wayfinder platform enables shipping operators to optimize routes based on high-accuracy ocean forecasts, reducing fuel consumption and emissions while demonstrating that ocean monitoring can deliver immediate commercial value.

What Isn't Working

Detection of AMOC Trends: Despite decades of research, fundamental uncertainties persist regarding AMOC behavior. A January 2025 Woods Hole Oceanographic Institution study found no decline over 60 years based on air-sea heat flux data, directly contradicting June 2025 findings from UC Riverside showing century-long weakening. This methodological divergence—surface temperature versus heat flux versus salinity approaches—leaves policymakers without clear signals for intervention timing.

Deep Ocean Monitoring Gaps: While the Argo network excels in the upper 2000 meters, observations of the deep ocean remain sparse. Given evidence of heat penetration to greater depths and the potential for deep circulation changes, this monitoring gap represents a significant blind spot. Deep Argo deployments are expanding but remain insufficient for comprehensive coverage.

Integration into Climate Policy: Despite robust scientific evidence, ocean dynamics remain underweighted in national climate strategies and corporate net-zero plans. The Ocean Climate Action Plan—a whole-of-government initiative led by NSF and federal partners—highlights that ocean-based solutions could reduce greenhouse gas emissions by 21% by 2050, yet implementation lags far behind potential.

Economic Valuation of Ocean Services: Quantifying the economic value of ocean climate regulation remains challenging, limiting its integration into cost-benefit analyses for decarbonization investments. While the ocean's absorption of 90% of excess heat represents an immense service, this buffering capacity is neither priced nor protected in mainstream economic frameworks.

Key Players

Established Leaders

National Oceanic and Atmospheric Administration (NOAA): The U.S. federal agency maintains the world's largest climate data archive at its National Centers for Environmental Information (NCEI), holding over 60 petabytes of oceanic and atmospheric data. NOAA received $3.3 billion under the Inflation Reduction Act for climate programs, including ocean research infrastructure and supercomputing capabilities.

Copernicus Marine Service: The European Union's operational ocean monitoring program provides free, real-time data products covering global and regional ocean conditions. Its Ocean State Report 9, published in 2024, documented that no ocean region remains untouched by the "triple planetary crisis" of climate change, biodiversity loss, and pollution.

Woods Hole Oceanographic Institution: The world's largest independent oceanographic research institution leads breakthrough studies on ocean-atmosphere interactions and circulation dynamics. Their RAPID array, maintained jointly with the UK National Oceanography Centre, provides the most direct observations of AMOC strength.

Institute of Atmospheric Physics, Chinese Academy of Sciences: IAP/CAS leads the international consortium publishing annual ocean heat content assessments, providing authoritative tracking of this critical climate indicator.

Emerging Startups

Saildrone: The Alameda-based company has deployed autonomous wind and solar-powered uncrewed surface vehicles that have logged over 1 million nautical miles and 60,000 days at sea. With $325 million in total funding and partnerships with NOAA, NASA, and defense agencies, Saildrone demonstrates the scalability of autonomous ocean observation.

Sofar Ocean: Having raised $70 million through its 2024 Series B-III round, Sofar deploys free-drifting Spotter buoys and offers the Wayfinder voyage optimization platform. The company also developed Bristlemouth, an open marine hardware standard created in partnership with DARPA to accelerate ocean technology development.

Terradepth: Focused on autonomous underwater data collection and AI-powered ocean analytics, Terradepth operates a fleet of unmanned underwater vehicles for subsea survey and monitoring applications.

Key Investors & Funders

National Science Foundation (NSF): NSF's Division of Ocean Sciences funds fundamental oceanographic research, infrastructure, and the U.S. National Ocean Decade Committee. The agency allocated $42 million over five years for Ocean and Human Health Research Centers in 2024.

Ocean 14 Capital: A specialized fund investing in sustainable ocean economy solutions, including companies developing ocean monitoring and marine conservation technologies.

Breakthrough Energy Ventures: Bill Gates-backed climate investment fund with $3.5 billion deployed across climate technologies, though ocean-specific investments remain a smaller portion of the portfolio.

In-Q-Tel: The U.S. intelligence community's strategic investor has backed Sofar Ocean, recognizing the strategic importance of ocean data for national security and climate intelligence.

Examples

NOAA's Arctic Report Card 2024: NOAA's annual assessment documented that all 18 lowest September minimum ice extents have occurred in the last 18 years, with the most shallow Arctic seas running 2-4°C warmer than 1991-2020 averages in August 2024. This comprehensive monitoring enables detection of cascading impacts from ice loss to ocean warming to ecosystem disruption, demonstrating the value of sustained observation programs.
Sofar Ocean and Berge Bulk Partnership: Sofar's deployment of Spotter buoys and Wayfinder optimization for shipping operator Berge Bulk achieved measurable fuel consumption reductions through improved weather routing. This commercial application demonstrates that ocean monitoring can deliver immediate decarbonization benefits while generating data that enhances climate science—a model for economically sustainable ocean observation.
Copernicus Marine Service FOCCUS Project: Launched in October 2024 under Horizon Europe, the FOCCUS project is building coastal extensions for the Copernicus Marine Service to address gaps in resolution and accuracy for complex coastal areas. By creating seamless integration from global to coastal monitoring, this initiative addresses a critical need for climate adaptation planning in vulnerable shoreline communities.

Action Checklist

Integrate ocean heat content and AMOC monitoring data into organizational climate risk assessments, updating scenario analyses annually with latest observations from IAP/CAS and NOAA
Evaluate supply chain exposure to marine heatwave impacts on fisheries, aquaculture, and coastal infrastructure using Copernicus Marine Service products
Assess capital investments for sea level rise scenarios incorporating both thermal expansion and ice melt contributions, using IPCC AR6 regional projections
Engage with industry groups advocating for ocean monitoring in policy frameworks, supporting Ocean Climate Action Plan implementation
Consider ocean-based carbon dioxide removal approaches cautiously, ensuring robust measurement, reporting, and verification (MRV) given circulation uncertainties
Monitor emerging research on AMOC tipping points, adjusting long-term planning assumptions if consensus develops on weakening trajectories

FAQ

Q: How does ocean heat uptake affect sea level rise projections? A: Ocean warming drives sea level rise through two mechanisms: thermal expansion of seawater (contributing approximately 1.5 mm/year currently) and acceleration of ice sheet melt. The 23 zettajoules absorbed in 2025 translates directly into expansion of the water column. Critically, this thermal contribution represents "committed" sea level rise—heat already stored will continue driving expansion for decades regardless of emission reductions. Organizations should plan for sea level rise scenarios at the higher end of IPCC projections, recognizing that thermal contributions are highly certain while ice sheet dynamics introduce additional uncertainty.

Q: What would an AMOC collapse mean for European climate and business operations? A: Research suggests an AMOC collapse could reduce European temperatures by up to 30°C over a century—a transformation that would fundamentally alter agriculture, energy demand, and infrastructure requirements. More proximate impacts include altered rainfall patterns, shifted jet stream behavior, and approximately 1 meter of additional sea level rise along Atlantic coasts. While complete collapse remains uncertain—a February 2025 Nature study found the AMOC resilient across 34 climate models—the potential consequences justify inclusion in long-term scenario planning. Organizations with significant European operations should develop contingency frameworks for circulation weakening scenarios.

Q: How reliable is current ocean monitoring for detecting climate tipping points? A: The Argo float network and satellite observations provide unprecedented coverage of upper ocean conditions, enabling high-confidence tracking of heat content trends. However, significant gaps remain: deep ocean monitoring below 2000 meters is sparse, AMOC detection methods yield conflicting results, and coastal resolution remains limited. For tipping point detection, current systems would likely provide years to decades of warning for gradual changes but might miss rapid threshold transitions. Organizations should treat monitoring data as best-available rather than definitive, incorporating uncertainty ranges in planning scenarios.

Q: What ocean-based climate solutions show the most promise? A: The Ocean Climate Action Plan identifies five priority sectors: marine conservation, ocean renewable energy (particularly offshore wind targeting 380 GW by 2030), shipping decarbonization, sustainable aquatic food systems, and coastal ecosystem restoration. Offshore wind offers near-term scalability with proven technology. Nature-based solutions like seagrass and mangrove restoration provide co-benefits for carbon sequestration and coastal protection. More speculative approaches—ocean alkalinity enhancement, macroalgae cultivation for carbon removal—show potential but require substantial MRV development before deployment at scale.

Q: How should sustainability leads prioritize ocean monitoring investment? A: Start with freely available resources: Copernicus Marine Service and NOAA Climate provide comprehensive data at no cost. For organizations with maritime operations, commercial partnerships with providers like Sofar Ocean can deliver operational benefits while supporting broader monitoring networks. Philanthropic investment through organizations like Ocean Conservancy or support for expanded Argo deployments offers leverage for systemic improvement. Most critically, advocate for policy frameworks that internalize the value of ocean climate services—the ocean's absorption of 90% of excess heat represents an unpriced subsidy to all emission-intensive activities.

Sources

Cheng, L., et al. (2025). "Ocean Heat Content Sets Another Record in 2025." Advances in Atmospheric Sciences. Institute of Atmospheric Physics, Chinese Academy of Sciences.
Copernicus Marine Service. (2024). "Ocean State Report 9." European Commission. https://marine.copernicus.eu
NOAA National Centers for Environmental Information. (2024). "Assessing the Global Climate in 2024." https://www.ncei.noaa.gov/news/global-climate-202413
van Westen, R.M., et al. (2025). "Physics-Based Indicators for the Onset of an AMOC Collapse Under Climate Change." Journal of Geophysical Research: Oceans. Wiley.
World Meteorological Organization. (2026). "WMO Confirms 2025 Was One of Warmest Years on Record." https://wmo.int
Li, Y. & Liu, Z. (2025). "The Atlantic's Chilling Secret: A Century of Data Reveals Ocean Current Collapse." Communications Earth & Environment. UC Riverside.
Jackson, L., et al. (2025). "Continued Atlantic Overturning Circulation Even Under Climate Extremes." Nature. https://www.nature.com/articles/s41586-024-08544-0