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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.

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.

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 ºC 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 ºC 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.

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.

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.

Understanding the variability of upper ocean thermal conditions is key to regional climate prediction. In recent decades, the sea surface temperature and upper ocean heat content in the South China Sea (SCS SST and SCS OHC) have exhibited contrasting changes. In-situ observations and reanalysis data reveal a linear warming trend in SCS SST during 1975–2010 but a regime shift of SCS OHC during the late 1990s. Mixed layer heat budget analysis shows that the decreasing latent heat flux associated with a weakening surface wind contributes to SCS SST warming trend. The increasing SCS SST reflects a regional footprint of global warming. A simplified upper layer budget diagnosis reveals that more than half of OHC change results from the advection effect, which is caused by an anomalous SCS anticyclonic gyre associated with an anomalous negative wind stress curl. Then, the anomalous anticyclonic circulation deepens thermocline depth at the basin-scale, and result in the regime shift of SCS OHC. Changes in the ocean circulation are found to be related to the enhanced trade wind and a Matsuno-Gill response to cooling in the tropical central Pacific. Further analyses show that the regime shift process is attributed to a phase transition of the Interdecadal Pacific Oscillation (IPO) from positive to negative. Our results indicate that although the IPO is the sea surface low-frequency climate pattern, it could impact on the subsurface thermal variability in the SCS through the oceanic process.

Marine heatwaves pose an increasing threat to the ocean’s wellbeing as global warming progresses. Forecasting marine heatwaves is challenging due to the various factors that affect their occurrence, including large variability in the atmospheric state. In this study we demonstrate a causal link between ocean heat content and the area and intensity of marine heatwaves in the Tasman Sea on interannual to decadal time scales. Ocean heat content variations are more persistent than ‘weather-related’ atmospheric drivers (e.g. blocking high pressure systems) for marine heatwaves and thus provide better predictive skill on timescales longer than weeks. Using data from a forced global ocean sea-ice model, we show that ocean heat content fluctuations in the Tasman Sea are predominantly controlled by oceanic meridional heat transport from the subtropics, which in turn, is mainly characterized by the interplay of the East Australian Current and the Tasman Front. Variability in these currents is impacted by wind stress curl anomalies north of this region, following Sverdrup´s and Godfrey’s ‘Island Rule’ theories. Data from models and observations show that periods with positive upper (2000m) ocean heat content anomalies or rapid increases in ocean heat content are characterized by more frequent, larger, longer and more intense marine heatwaves on interannual to decadal timescales. Thus, the oceanic heat content in the Tasman Sea acts as a preconditioner and has a prolonged predictive skill compared to the atmospheric state (e.g. surface heat fluxes), making ocean heat content a useful indicator and measure of the likelihood of marine heatwaves.

The influence of ocean stratification and heat content on the timing of sea ice formation and its subsequent growth remains an open question. Here we investigate the thermohaline conditions prior to fall sea ice formation as well as the roles of stratification and heat content on sea ice growth rates through the analysis of in situ observations and numerical simulations from a one‐dimensional ocean‐ice‐column model. We find that the simulated time series of sea ice concentration are highly correlated with observations. We identify two clusters of sea ice concentration growth rate, which we name Early Slow and Late‐Fast. We find that cold, shallow mixed layers promote early sea ice freeze‐up. Salinity stratification within the upper pycnocline slows the release of heat into the deepening mixed layer, leading to slower ice growth. However, where salinity stratification above the upper pycnocline is absent, sea ice growth occurs later and, once started, progresses faster.

Research has indicated that the meridional heat transport (MHT) in the North Pacific Ocean (NP) across 24°N increased in the 1980s and 1990s, resulting in different heat distributions: the ocean heat content (OHC) increased in the 1980s, while the net surface heat release was strengthened in the 1990s; however, the reasons for these differences remained unclear. The authors revisited the investigation of the heat budget in the NP mainly using hydrographic observations to understand why the heat distribution was different between these 2 decades and extend the analysis to the 2010s. The OHC in the upper 700 m north of 24°N and east of 137°E exhibited sharp increases around 1990 as well as in the 2010s, while it was nearly stable in between. The northward retreat of the subarctic gyre boundary coincided with the spin-up of the subtropical gyre in the late 1980s, thereby allowing warm anomaly from the subtropics to propagate northeastward. Meanwhile, the concurrent weakening of the wintertime westerlies resulted in the suppression of the surface heat loss in the western NP. In contrast, the southward shift of the subarctic front suppressed the OHC rise, despite the MHT increase in the late 1990s. In the 2010s, unprecedented warming occurred in the eastern NP. The MHT estimation based on hydrographic observations indicates that the net surface release must have been suppressed since the MHT did not increase; however, the latest atmospheric reanalysis datasets failed to reproduce this.