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  • Review Article
  • Published:

Past and future ocean warming

Abstract

Changes in ocean heat content (OHC) provide a measure of ocean warming, with impacts on the Earth system. This Review synthesizes estimates of past and future OHC changes using observations and models. The top 2,000 m of the global ocean has significantly warmed since the 1950s, gaining 351 ± 59.8 ZJ (1 ZJ = 1021 J) from 1958 to 2019. The rate of warming increased from <5 to ~10 ZJ yr−1 from the 1960s to the 2010s. Observed area-averaged warming is largest in the Atlantic Ocean and southern oceans at 1.42 ± 0.09 and 1.40 ± 0.09 × 109 J m2, respectively, for the upper 2,000 m over 1958–2019. These observed patterns of heat gains are dominated by heat redistribution. Observationally constrained projections suggest that historic ocean warming is irreversible this century, with net warming dependent on the emission scenario. By 2100, projected warming in the top 2,000 m is 2–6 times that observed so far, ranging from 1,030 [839–1,228] ZJ for a low-emission scenario to 1,874 [1,637–2,109] ZJ for a high-emission scenario. The Pacific is projected to be the largest heat reservoir owing to its size, but area-averaged warming remains strongest in the Atlantic and southern oceans. Ocean warming has extensive impacts that pose risks to marine ecosystems and society. The projected changes necessitate a continuation and improvement of observations and models, along with better uncertainty estimation.

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Fig. 1: The role of ocean warming in the climate system.
Fig. 2: Observed and projected ocean heat content changes.
Fig. 3: Observed and projected regional OHC changes.
Fig. 4: Relationship between transient climate response and the vertical structure of temperature and OHC.
Fig. 5: Simulated past and projected future ocean heat content change.

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Data availability

The observation and model data used in this review are available at http://www.ocean.iap.ac.cn/. CMIP6 model data is available at https://esgf-node.llnl.gov/search/cmip6/.

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Acknowledgements

L.C. acknowledges financial support from the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB42040402), National Natural Science Foundation of China (42122046, 42076202), Youth Innovation Promotion Association, CAS (2020-077). The National Center for Atmospheric Research is sponsored by the US National Science Foundation (NSF). L.Z. is supported by NSF OCE award 2048576. J.F. is supported by NASA awards 80NSSC17K0565 and 80NSSC22K0046, and by the Regional and Global Model Analysis (RGMA) component of the Earth and Environmental System Modeling Program of the US Department of Energy’s Office of Biological & Environmental Research (BER) via NSF IA 1947282. M.E. and J.Z. are supported by the Australian Research Council (SR200100008, LP200100406, DP190100494). Y.Y. is supported by National Natural Science Foundation of China (91958201, 42130608). We thank S. Simoncelli for discussion on the Mediterranean Sea, V. Gouretski and F. Reseghetti for discussion on observations. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output through Earth System Grid Federation. We also acknowledge the International Argo Program and the national programmes that contribute to it. The Argo Program is part of the Global Ocean Observing System.

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All authors contributed to writing and editing the article. L.C., K.v.S., J.P., K.T. and M.M led the overall conceptual design and the activity. L.C. led and coordinated the writing. K.v.S. and L.C. led the Introduction section. J.P.A. and K.v.S. led the observations and OHC estimates section. L.C., K.T., J.Z. and M.E. led the past OHC changes section. L.Z., J. F., M.E., L.C. and Y.-Q.Y. led the future projections section. K.T., L.C. and K.v.S led the impacts and consequences section. L.C., M.M. and K.T. led the final section. Y.-Y.P., J.Z., E.N., B.B., L.P. and L.C. contributed to data processing.

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Correspondence to Lijing Cheng.

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Glossary

Earth energy imbalance

EEI. The net downwelling radiation at the top of the atmosphere, represented as the balance of absorbed solar radiation (allowing for reflection and scattering) and outgoing longwave radiation.

Ocean heat content

OHC, or ocean heat storage (OHS). A change or anomaly of the thermal energy of the ocean assumed to have a fixed volume (Vx,y,z, in units of J), and vertical integration (Vz, in units of J m−2). Calculated as OHC (x,y,z) = \({C}_{p}{\iiint }_{V(x,y,z)}\rho TdV(x,y,z)\) following TEOS-10 standards, where cp is a constant of ~3,991.9 J (kg K)1, ρ is potential density in kg m3 and T is conservative temperature measured in degrees Celsius.

OHC trend

Or alternatively, OHC rate, or tendency. The time derivative of OHC (dOHC/dt), given in units of J yr1 or W m2.

Representative concentration pathway

RCP. The RCPs are scenarios of concentrations, and thereby emissions, of the full suite of greenhouse gases, aerosols and chemically active gases, as well as land use/land cover. In RCP2.6: radiative forcing peaks at ~3 W m2 and then declines, to be limited at 2.6 W m2 in 2100. In RCP4.5 and RCP8.5: the radiative forcing reaches ~4.5 W m2 and >8.5 W m2 in 2100, respectively.

Ocean heat uptake

OHU. The accumulated contribution of heat added into the ocean (heat gain) or removed from the ocean (heat loss) through heat fluxes at the air–sea, ice–sea and land–sea interfaces (in units of W m2). Globally, it is synonymous with ‘OHC change, trend, rate, tendency’. Regionally, OHU and redistribution contribute to local OHC change.

Ocean heat redistribution

The transport of heat within the ocean without involving any net global ocean warming or cooling through advection, convection, eddy mixing and small-scale diffusion.

Mode water

Formed when winter mixed layers are convectively overturned owing to gravitational instability, and characterized by low potential vorticity and nearly vertically homogeneous temperature, salinity and density.

Shared socioeconomic pathway

The SSPs are a set of plausible trajectories of societal development and radiative forcing. SSP1-2.6 is a relatively low-emission scenario, representing the pathways to limit the global surface warming below 2 °C, which requires immediate, rapid and large-scale reductions in greenhouse gas emissions. SSP5-8.5 is a higher emissions scenario with projected warming >3 °C by 2100.

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Cheng, L., von Schuckmann, K., Abraham, J.P. et al. Past and future ocean warming. Nat Rev Earth Environ 3, 776–794 (2022). https://doi.org/10.1038/s43017-022-00345-1

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