Ocean heat content (OHC) is a crucial climate indicator and provides an estimate of Earth’s energy imbalance, but its accuracy and uncertainty have not been fully understood. Here, we constructed a Large-Ensemble to comprehensively quantify the OHC uncertainty from global to regional scales since 1955, and consider eight groups of error sources based on the community’s efforts. We find a six-fold reduction in the global upper 2000 m OHC estimated uncertainty (5–95% quartile error range) since 1955 from ~ 111 ZJ (1955–1965) to ~ 19 ZJ (2014–2023), attributed to the reduction of errors from all sources especially mapping/sampling. This indicates a robust upper 2000 m OHC increase since 1955 of 449 [385, 519] ZJ. Quality-Control (QC) has emerged as a primary error source in the recent decade, because of the difficulties in detecting outliers in large perturbations in eddy-rich regions. The trend uncertainty has different sources from the annual OHC: from 2005–2023, the upper 2000m OHC trend is 10.9 [9.2, 11.4] ZJ yr − 1 or equivalently 0.68 [0.57, 0.71] Wm − 2 , with uncertainty mainly stemming from spatial gaps and mapping errors, QC, and climatology choice. An increase in ocean warming rate from 2005–2023 is small but robust: 0.34 [0.19, 0.46] Wm − 2 dec − 1 . The comprehensive OHC uncertainty estimates provide a basis for model evaluation and a guide for future improvements in the global 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 = 1021 Joules, with a 95

Huge heat anomalies in the atmosphere and ocean in recent years are not yet explained. Strong characteristic patterns in temperatures for upper layers of the ocean occurred from 2000 to 2023 in the presence of global warming from increasing atmospheric greenhouse gases. Here, we show that the deep tropics are warming, although sharply modulated by El Ni & ntilde;o-Southern Oscillation events, with strong heating in the extratropics near 40 degrees N and 40 degrees-45 degrees S but little heating near 20 degrees N and 25 degrees-30 degrees S. The heating is most clearly manifested in zonal-mean ocean heat content and is evident in sea surface temperatures. The strongest heating is in the Southern Hemisphere, where aerosol effects are small. Estimates are made of the contributions to heating of top-of-atmosphere (TOA) radiation, atmospheric energy transports, surface fluxes of energy, and redistribution of energy by surface winds and ocean currents. The patterns of change are not directly related to TOA radiation but are evident in net surface energy fluxes and inferred ocean heat transports, underscoring their coupled origin. Changes in the atmospheric circulation through a poleward shift in ocean jet streams and storm tracks are reflected in surface wind-driven ocean Ekman transports. As well as human-induced climate change, internal natural variability is likely in play. Hence, the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates.

Knowledge of the functioning of the climate system, including the physical, dynamical and biogeochemical feedback processes expected to occur in response to anthropogenic climate forcing, has increased substantially over recent decades. Today, climate science is at a crossroads, with new and urgent demands arising from the needs of society to deal with future climate change, and the need for the climate science community to refine its strategic goals to meet these demands rapidly. All possible—but currently unknown—worlds in 2050, with a larger global population, unprecedented climate conditions with higher temperatures, more frequent extreme weather events, sea level rise, disrupted ecosystems, changes in habitability and increased climate-induced displacement and migration, and the emergence of new geopolitical tensions, will require limiting society’s vulnerability both through mitigation measures to minimize further warming and through the implementation of innovative adaptation initiatives. The development of a skillful climate information system, based on the most advanced Earth system science, will be required to inform decision-makers and the public around the world about the local and remote impacts of climate change, and guide them in optimizing their adaptation and mitigation agendas. This information will also help manage renewable resources in a warmer world and strengthen resilience to the expected interconnected impacts of climate change. In this paper, we summarize the major advances needed to understand the multiscale dynamics of the Earth system. We highlight the need to develop an integrated information system accessible to decision-makers and citizens in all parts of the world, and present some of the key scientific questions that need to be addressed to inform decisions on mitigation and adaptation. Finally, we speculate about the values and ethics of climate science and the nature of climate research in a world that will be increasingly affected by global warming in a geopolitical context very different from that of recent decades.

The ocean is highly stratified. Warm, fresh water sits on top of cold, salty water, influencing vertical oceanic exchange of heat, carbon, oxygen and nutrients. In this Review, we examine observed and projected stratification shifts and their impacts. Changes in ocean temperature and salinity have altered the ocean density field, leading to a 0.8 ± 0.1% dec−1 (90% confidence interval) increase in stratification in the global upper 2,000 m since the 1960s. These increases are most pronounced in the tropics and are primarily temperature driven. Model simulations project ongoing stratification increases in the future, with global 0–2,000 m stratification increasing 0.7 [0.3,1.1; 13–87% confidence interval], 1.4 [0.9,1.8] and 2.9 [2.1,3.8]% dec−1 by 2090–2100 relative to 2010–2020 under Shared Socioeconomic Pathways SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively; regional patterns of projected stratification changes generally follow observed trends. These observed and projected ocean stratification changes have important climate and ecological consequences, including alterations in ocean heat uptake, ocean currents, vertical mixing, tropical cyclone intensity, marine ecosystems and elevation of marine extremes. Further research should better quantify stratification change at critical layers and understand their drivers and impacts. Ocean stratification — density-related layering of seawater — influences oceanographic and climatic processes. This Review outlines observed and projected changes in stratification, noting a 0.8% dec−1 increase in 0–2,000 m stratification from 1960–2024, and a further 1.4% dec−1 increase by 2100 under SSP2-4.5.

Huge heat anomalies in the past few years are not explained by climate models1. Strong characteristic patterns in temperatures for upper layers of the ocean occurred from 2000 to 2023 in the presence of global warming from increasing atmospheric greenhouse gases2. Here we show that the deep tropics are warming, although sharply modulated by El Niño-Southern Oscillation events, with strong heating in the extratropics near 40°N and 40 to 45°S, but little heating near 20°N and 25-30°S. The heating is most clearly manifested in zonal mean ocean heat content and is evident in sea surface temperatures. Strongest heating is in the Southern Hemisphere, where aerosol effects are small. Estimates are made of the contributions of top-of-atmosphere (TOA) radiation, atmospheric energy transports, surface fluxes of energy, and redistribution of energy by surface winds and ocean currents. The patterns are not directly related to TOA radiation but are strongly evident in net surface energy fluxes. Changes in the atmospheric circulation, jet streams, and storm tracks are reflected in surface ocean Ekman transports. As well as climate change, natural variability is likely in play. Hence the atmosphere and ocean currents are systematically redistributing heat from global warming, profoundly affecting local climates.

Ocean observational gridded products are vital for climate monitoring, ocean and climate research, model evaluation, and supporting climate mitigation and adaptation measures. This paper describes the 4th version of the Institute of Atmospheric Physics (IAPv4) ocean temperature and ocean heat content (OHC) objective analysis product. It accounts for recent developments in quality control (QC) procedures, climatology, bias correction, vertical and horizontal interpolation, and mapping and is available for the upper 6000 m (119 levels) since 1940 (more reliable after ∼ 1957) for monthly and 1°×1° temporal and spatial resolutions. IAPv4 is compared with the previous version, IAPv3, and with the other data products, sea surface temperatures (SSTs), and satellite observations. It has a slightly stronger long-term upper 2000 m OHC increase than IAPv3 for 1955–2023, mainly because of newly developed bias corrections. The IAPv4 0–2000 m OHC trend is also higher during 2005–2023 than IAPv3, mainly because of the QC process update. The uppermost level of IAPv4 is consistent with independent SST datasets. The month-to-month OHC variability for IAPv4 is desirably less than IAPv3 and the other OHC products investigated in this study, the trend of ocean warming rate (i.e., warming acceleration) is more consistent with the net energy imbalance at the top of the atmosphere than IAPv3, and the sea level budget can be closed within uncertainty. The gridded product is freely accessible at https://doi.org/10.12157/IOCAS.20240117.002 for temperature data (Cheng et al., 2024a) and at https://doi.org/10.12157/IOCAS.20240117.001 for ocean heat content data (Cheng et al., 2024b).

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.

As global surface temperatures have increased with human-induced climate change, notable compound climate extremes in the New Zealand (NZ) region associated with atmospheric heatwaves (AHWs) and marine heatwaves (MHWs) have occurred in the past 6 years. Natural modes of variability that also played a key role regionally include the Interdecadal Pacific Oscillation (IPO), El Ni & ntilde;o/Southern Oscillation (ENSO) and changes in the location and strength of the westerlies as seen in the Southern Annular Mode (SAM). Along with mean warming of 0.8 degrees C since 1900, a negative phase of the IPO, La Ni & ntilde;a phase of ENSO and a strongly positive SAM contributed to five compound warm extremes in the extended austral summer seasons (NDJFM) of 1934/35, 2017/18, 2018/19, 2021/22 and 2022/23. These are the most intense coupled ocean/atmosphere (MHWs/AHWs) heatwaves on record with average temperature anomalies over land and sea +0.8 degrees C to 1.1 degrees C above 1991-2020 averages. The number of days above 25 degrees C and above the 90th percentile of maximum temperature has increased, while the number of nights below 0 degrees C and below the 10th percentile has decreased. Coastal waters around NZ recently experienced their longest MHW in the satellite era (1982-present) of 289 days through 2023. The estimated recurrence interval reduces from 1 in 300-years for the AHW event during the 1930s climate to a 1 in 25-year event for the most recent decade. Consequences include major loss of ice of almost one-third volume from Southern Alps glaciers from 2017 to 2021 with rapid melt of seasonal snow in all four cases. Above-average temperatures in the December/January grape flowering period resulted in advances in veraison (the onset of ripening); and higher-than-average grape yields in 2022 and 2023 vintages. Marine impacts include widespread sea-sponge bleaching around northern and southern NZ.

Current human-induced climate change arises primarily from the heating of the planet mainly from changes in atmospheric composition, and temperature change is one manifestation. The increasing greenhouse gases, notably carbon dioxide from burning fossil fuels, lead to Earth’s Energy Imbalance (EEI), altering the flow of energy through the climate system, and the dissemination of excess energy is partly what determines how climate change is manifested. Some of the extremes being experienced, especially those involving drought, convection, storms, flooding, and the water cycle, are mostly driven by aspects of heating and, while temperature contributes through the water-holding capacity of the atmosphere, it is more a consequence than a cause. Afterall, water is the air conditioner of the planet. The United Nations, and especially the Intergovernmental Panel on Climate Change (IPCC) in their Summary for Policy Makers, focus on global temperature targets rather than broader facets of climate change including EEI, and do not always adequately discriminate between temperature and heating. This also has consequences for future climate if or when heating is brought under control by cutting emissions. Improvements are needed in expressing how the climate is changing by properly accounting for the flow of energy through the climate system.

As a major component of Earth's energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in Earth's energy budget. Here, we investigate the annual cycle of the upper-2000-m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere ra-diation, surface heat flux, ocean heat divergence, and meridional heat transport. Results show that the robust signal of an-nual OHC change is significant down to a 1000-m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0-1500 m) changes from positive anomalies within September-February to negative anomalies within March-August, mainly because of the larger ocean area in the Southern Hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycles in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid-and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport associated with the geostrophic flow is the main driver.

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.