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The global physical and biogeochemical environment has been substantially altered in response to increased atmospheric greenhouse gases from human activities. In 2023, the sea surface temperature (SST) and upper 2000 m ocean heat content (OHC) reached record highs. The 0–2000 m OHC in 2023 exceeded that of 2022 by 15 ± 10 ZJ (1 Zetta Joules = 10 21 Joules) (updated IAP/CAS data); 9 ± 5 ZJ (NCEI/NOAA data). The Tropical Atlantic Ocean, the Mediterranean Sea, and southern oceans recorded their highest OHC observed since the 1950s. Associated with the onset of a strong El Niño, the global SST reached its record high in 2023 with an annual mean of ∼0.23°C higher than 2022 and an astounding > 0.3°C above 2022 values for the second half of 2023. The density stratification and spatial temperature inhomogeneity indexes reached their highest values in 2023.

The energy radiated by the Earth towards space does not compensate the incoming radiation from the Sun leading to a small positive energy imbalance at the top of the atmosphere (0.4-1.Wm-2). This imbalance is coined Earth’s Energy Imbalance (EEI). It is mostly caused by anthropogenic greenhouse gases emissions and is driving the current warming of the planet. Precise monitoring of EEI is critical to assess the current status of climate change and the future evolution of climate. But the monitoring of EEI is challenging as EEI is two order of magnitude smaller than the radiation fluxes in and out of the Earth. Over 93% of the excess energy that is gained by the Earth in response to the positive EEI accumulates into the ocean in the form of heat. This accumulation of heat can be tracked with the ocean observing system such that today, the monitoring of Ocean Heat Content (OHC) and its long-term change provide the most efficient approach to estimate EEI. In this community paper we review the current four state-of-the-art methods to estimate global OHC changes and evaluate their relevance to derive EEI estimate on different time scales. These four methods make use of : 1) direct observations of in situ temperature; 2) satellite-based measurements of the ocean surface net heat fluxes; 3) satellite-based estimates of the thermal expansion of the ocean and 4) ocean reanalyses that assimilate observations from both satellite and in situ instruments. For each method we review the potential and the uncertainty of the method to estimate global OHC changes. We also analyze gaps in the current capability of each method and identify ways of progress for the future to fulfill the requirements of EEI monitoring. Achieving the observation of EEI with sufficient accuracy will depend on merging the remote sensing techniques with in situ measurements of key variables as an integral part of the Ocean Observing System.

Heating in the ocean has continued in 2024 in response to increased greenhouse gas concentrations in the atmosphere, despite the transition from an El Niño to neutral conditions. In 2024, both global sea surface temperature (SST) and upper 2000 m ocean heat content (OHC) reached unprecedented highs in the historical record. The 0–2000 m OHC in 2024 exceeded that of 2023 by 16 ± 8 ZJ (1 Zetta Joules = 10 21 Joules, with a 95% confidence interval) (IAP/CAS data), which is confirmed by two other data products: 18 ± 7 ZJ (CIGAR-RT reanalysis data) and 40 ± 31 ZJ (Copernicus Marine data, updated to November 2024). The Indian Ocean, tropical Atlantic, Mediterranean Sea, North Atlantic, North Pacific, and Southern Ocean also experienced record-high OHC values in 2024. The global SST continued its record-high values from 2023 into the first half of 2024, and declined slightly in the second half of 2024, resulting in an annual mean of 0.61°C ± 0.02°C (IAP/CAS data) above the 1981–2010 baseline, slightly higher than the 2023 annual-mean value (by 0.07°C ± 0.02°C for IAP/CAS, 0.05°C ± 0.02°C for NOAA/NCEI, and 0.06°C ± 0.11°C for Copernicus Marine). The record-high values of 2024 SST and OHC continue to indicate unabated trends of global heating.

Global ocean physical and chemical trends are reviewed and updated using seven key ocean climate change indicators: (i) Sea Surface Temperature, (ii) Ocean Heat Content, (iii) Ocean pH, (iv) Dissolved Oxygen concentration (v) Arctic Sea Ice extent, thickness, and volume (vi) Sea Level and (vii) the strength of the Atlantic Meridional Overturning Circulation (AMOC). The globally averaged ocean surface temperature shows a mean warming trend of 0.062 ± 0.013°C per decade over the last 120 years (1900–2019). During the last decade (2010–2019) the rate of ocean surface warming has accelerated to 0.280 ± 0.068°C per decade, 4.5 times higher than the long term mean. Ocean Heat Content in the upper 2,000 m shows a linear warming rate of 0.35 ± 0.08 Wm –2 in the period 1955–2019 (65 years). The warming rate during the last decade (2010–2019) is twice (0.70 ± 0.07 Wm –2 ) the warming rate of the long term record. Each of the last six decades have been warmer than the previous one. Global surface ocean pH has declined on average by approximately 0.1 pH units (from 8.2 to 8.1) since the industrial revolution (1770). By the end of this century (2100) ocean pH is projected to decline additionally by 0.1–0.4 pH units depending on the RCP (Representative Concentration Pathway) and SSP (Shared Socioeconomic Pathways) future scenario. The time of emergence of the pH climate change signal varies from 8 to 15 years for open ocean sites, and 16–41 years for coastal sites. Global dissolved oxygen levels have decreased by 4.8 petamoles or 2% in the last 5 decades, with profound impacts on local and basin scale habitats. Regional trends are varying due to multiple processes impacting dissolved oxygen: solubility change, respiration changes, ocean circulation changes and multidecadal variability. Arctic sea ice extent has been declining by −13.1% per decade in summer (September) and by −2.6% per decade in winter (March) during the last 4 decades (1979–2020). The combined trends of sea ice extent and sea ice thickness indicate that the volume of non-seasonal Arctic Sea Ice has decreased by 75% since 1979. Global mean sea level has increased in the period 1993–2019 (the altimetry era) at a mean rate of 3.15 ± 0.3 mm year –1 and is experiencing an acceleration of ∼ 0.084 (0.06–0.10) mm year –2 . During the last century (1900–2015; 115y) global mean sea level (GMSL) has rised 19 cm, and near 40% of that GMSL rise has taken place since 1993 (22y). Independent proxies of the evolution of the Atlantic Meridional Overturning Circulation (AMOC) indicate that AMOC is at its weakest for several hundreds of years and has been slowing down during the last century. A final visual summary of key ocean climate change indicators during the recent decades is provided.

The impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport are investigated by contrasting two ensemble chemistry-climate model simulations in 2005–2099: one with fixed ozone depleting substances (ODSs) and another with decreasing ODSs. In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. Ozone recovery causes a dipole change of the full-depth ocean heat content (OHC) with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, OHC decreases in response to ozone recovery. This ocean heat loss is shown to be driven by weakened poleward ocean heat transport (OHT) across 40°S, which is partly canceled by enhanced heat uptake. The weakening of poleward OHT into the Southern Ocean is caused by the ozone-induced equatorward shift of the meridional overturning circulation.

Changes in ocean heat content (OHC), salinity, and stratification provide critical indicators for changes in Earth’s energy and water cycles. These cycles have been profoundly altered due to the emission of greenhouse gasses and other anthropogenic substances by human activities, driving pervasive changes in Earth’s climate system. In 2022, the world’s oceans, as given by OHC, were again the hottest in the historical record and exceeded the previous 2021 record maximum. According to IAP/CAS data, the 0–2000 m OHC in 2022 exceeded that of 2021 by 10.9 ± 8.3 ZJ (1 Zetta Joules = 10 21 Joules); and according to NCEI/NOAA data, by 9.1 ± 8.7 ZJ. Among seven regions, four basins (the North Pacific, North Atlantic, the Mediterranean Sea, and southern oceans) recorded their highest OHC since the 1950s. The salinity-contrast index, a quantification of the “salty gets saltier—fresh gets fresher” pattern, also reached its highest level on record in 2022, implying continued amplification of the global hydrological cycle. Regional OHC and salinity changes in 2022 were dominated by a strong La Niña event. Global upper-ocean stratification continued its increasing trend and was among the top seven in 2022.

Despite the recently recomputed time series of the Atlantic Meridional Overturning Circulation (AMOC) suggesting greater stability than previously recognized, AMOC retains the potential to influence regional climate fluctuations across multiple timescales through its considerable variability. The sloshing component of AMOC has been identified as a significant mode of short‐term AMOC variability. While it does not cause permanent changes to the AMOC, this sloshing mode can reshape the ocean's thermal state by redistributing warmer water in the upper layers and altering both basin‐wide and regional ocean heat content (OHC). This study examines how the sloshing AMOC component regulates meridional heat transport and OHC across different timescales in the North Atlantic. It offers insights into the mechanism through which the AMOC could affect regional climate variability, even if it maintains a stable strength in the foreseeable future. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) plays a pivotal role in shaping global and regional climates through its substantial northward heat transport. While a slowdown of the AMOC could have significant impacts on the global climate, 20 years of continuous monitoring have revealed that the AMOC might be more stable than previously believed. Despite this stability, the large‐scale variations in AMOC can still impact regional climate fluctuations ranging from seasonal to decadal periods. This study investigates the dominant processes driving AMOC variability across timescales and examines how they influence tropical and subtropical Atlantic climates by modulating heat conditions in the upper ocean. These findings are important for understanding whether and how the AMOC can affect regional climates, even if its overall state remains stable in the future. Key Points Sloshing component is crucial in shaping the impact of Atlantic overturning on upper‐ocean heat content variability A strengthening overturning circulation may reduce upper‐ocean heat content in eastern boundary of the North Atlantic Sloshing processes drive much of the North Atlantic overturning variability up to decadal scale, dominant in the tropics and subtropics

Upper‐ocean heat content (UOHC) modulates air–sea exchange in tropical regions; however, its variability at (sub)mesoscale remains insufficiently resolved. Here we combine Surface Water and Ocean Topography (SWOT) observations with high‐resolution glider sections in the northeastern tropical Pacific, where mesoscale eddies and sharp frontal structures associated with currents up to 1 m s−1${\\mathrm{s}}^{-1}$were sampled. The gliders reveal strong spatial variability of UOHC along the transects, with localized anomalies reaching 20 kJ cm−2${\\text{cm}}^{-2}$ , superimposed on a mesoscale background state. Mesoscale eddies and fronts define the background thermal structure, within which submesoscale frontal dynamics drive localized and intermittent vertical heat redistribution. In these structures, a complex interplay of ageostrophic circulations, Ekman‐related processes, and enhanced mixing generates vertical heat fluxes reaching O(102${10}^{2}$ –103${10}^{3}$ ) W m−2${\\mathrm{m}}^{-2}$ , comparable to air–sea heat fluxes. These results provide observation‐based constraints on submesoscale vertical heat redistribution and its quantification, which remain poorly represented in ocean models.

The AMOC FINGERPRINT and GYRE INDEX are two widely used metrics by the oceanographic community to assess whether northward ocean heat transport, and consequently temperature variability in the subpolar North Atlantic, is primarily governed by the Atlantic overturning circulation or the horizontal gyre circulation. Although these metrics are presented as distinct measures of subpolar ocean circulation, we contend that they are not unique indicators of the dynamics they are intended to represent. Instead, our analysis demonstrates that both metrics are essentially capturing variability in upper ocean heat content, which can result from a number of mechanisms. This conclusion, corroborated with direct overturning measurements from the OSNAP and RAPID programs, raises concerns about the use of these metrics to infer the dynamics responsible for past and projected changes in subpolar ocean temperatures. Plain Language Summary Two indices have been widely used by the oceanographic community to determine whether the sinking of waters to greater depths or shifts in ocean currents regulate the amount of heat moving northward from the subtropical to the subpolar North Atlantic. While these indices were designed as two separate measures of ocean circulation in the North Atlantic, we caution that they are not unique indicators of what they purport to measure. Instead, both metrics reflect changes in upper ocean temperatures, which can be impacted by sinking waters, current shifts and several other mechanisms. Thus, neither metric should be used to infer the mechanism causing temperature variability in the North Atlantic. Key Points The AMOC FINGERPRINT and GYRE INDEX do not uniquely describe changes in the subpolar overturning and horizontal gyre circulation The AMOC FINGERPRINT and GYRE INDEX both reflect ocean heat content variability in the subpolar North Atlantic Over the AMOC observational period, neither metric explains the variability of the dynamics they were designed to capture

During deglaciations, Earth takes up vast amounts of energy, about half of which heats the global ocean. Thus, ocean heat content (OHC) is a key metric to assess Earth's energy budget. Recent modeling studies suggest that OHC changes not only in response to orbitally driven climate change but is also modulated on millennial timescales by the Atlantic Meridional Overturning Circulation (AMOC). Here, we present the first OHC record for the last four deglaciations using noble‐gas ratios in the EPICA Dome C ice core. The record reveals millennial‐scale OHC variability in all studied deglaciations, most prominently as OHC maxima at the end of Terminations II, III, and IV. These millennial‐scale OHC changes are anti‐correlated with AMOC strength, suggesting that the AMOC modulates OHC across different climate states. Furthermore, given the magnitude of the end‐of‐termination OHC maxima, AMOC‐induced OHC changes may be an important control of early interglacial atmospheric CO2, sea level, and climate. Plain Language Summary Earth's energy balance determines whether the planet experiences a net gain or loss of energy. Due to its high heat capacity, the ocean is one of Earth's dominant energy reservoirs and ocean heat content (OHC) therefore a key metric to assess the global energy balance. Recently, OHC has been suggested to not only change on glacial–interglacial timescales, but to also be affected by ocean circulation on millennial timescales. Our OHC record reveals that millennial‐scale OHC changes occur concomitantly with changes in ocean circulation across different climate states during the last four glacial–interglacial transitions. This suggests that ocean circulation plays a crucial role controlling millennial‐scale ocean heat uptake, which has consequences for atmospheric CO2, sea level, and climate. Key Points First ocean heat content (OHC) record covering the last four deglaciations All studied deglaciations show millennial OHC variability anti‐correlated with Atlantic meridional overturning circulation (AMOC) strength Prominent OHC maxima at the ends of Termination II–IV point to AMOC‐induced OHC changes as important control of early interglacial climate