Ocean heat content (OHC) orocean heat uptake (OHU) is the energy absorbed and stored byoceans.^[2] It is an important indicator ofglobal warming.^[3] Ocean heat content is calculated by measuringocean temperature at many different locations and depths, andintegrating the areal density of a change inenthalpic energy over an ocean basin or entire ocean.^[4]

Between 1971 and 2018, a steady upward trend^[5] in ocean heat content accounted for over 90% of Earth'sexcess energy from global warming.^[6][7] Scientists estimate a 1961–2022 warming trend of 0.43 ± 0.08 W/m², accelerating at about 0.15 ± 0.04 W/m² per decade.^[8] By 2020, about one third of the added energy had propagated to depths below 700 meters.^[9][10] The five highest ocean heat observations to a depth of 2000 meters all occurred in the period 2020–2024.^[5] The main driver of this increase has been human-causedgreenhouse gas emissions.^[11]: 1228

Ocean heat content measurements are critical for models of climate. Since before 1960, research vessels and stations have sampledsea surface temperatures andtemperatures at greater depth all over the world. Since 2000, an expanding network of nearly 4000Argo robotic floats has measured temperature anomalies, or the change in ocean heat content. The upper 2000 meters of the global ocean has experienced warming on average since the 1970s, while the rate of warming varies regionally with the subpolarNorth Atlantic warming more slowly and theSouthern Ocean taking up a disproportionately large amount of heat.^[11]: 1230 Deep-ocean warming below 2000 meters has also been largest in the Southern Ocean compared to other ocean basins.^[11]: 1230

Changes in ocean temperature greatly affectecosystems in oceans and on land. For example, there are multiple impacts oncoastal ecosystems and communities relying on theirecosystem services. Direct effects include variations insea level andsea ice, changes to theintensity of the water cycle, and themigration of marine life.^[12]

The Earth's oceans play a critical role in climate stability. The oceans have highheat capacity so they can store vast amounts of energy little change in temperature. Oceans also cover 70% of the Earth's surface. Measuring ocean heat content and monitoring changes over time is essential for understanding and modeling climate.^[13]

Ocean heat content is a term used inphysical oceanography to describe a type ofthermodynamic potential energy that is stored in the ocean. It is defined in coordination with theequation of state of seawater.TEOS-10 is aninternational standard approved in 2010 by theIntergovernmental Oceanographic Commission.^[15]

Calculation of ocean heat content follows that ofenthalpy referenced to the ocean surface, also calledpotential enthalpy. OHC changes are thus made more readily comparable to seawater heat exchanges with ice, freshwater, and humid air.^[16][17] OHC is always reported as a change or as an "anomaly" relative to a baseline. Positive values then also quantify ocean heat uptake (OHU) and are useful to diagnose where most of planetary energy gains from global heating are going.

To calculate the ocean heat content, measurements ofocean temperature from sampleparcels of seawater gathered at many different locations and depths are required.^[18]Integrating the areal density of ocean heat over an ocean basin, or entire ocean, gives the total ocean heat content. Thus, total ocean heat content is avolume integral of the product of temperature, density, and heat capacity over the three-dimensional region of the ocean for which data is available.^[19] The bulk of measurements have been performed at depths shallower than about 2000 m (1.25 miles).^[20]

The areal density of ocean heat content between two depths is computed as a definite integral:^[4][19]

${\displaystyle H=c_p{\int }_{h2}^{h1}\rho (z)\mathrm{\Theta }(z)dz}$

where${\displaystyle c_p}$ is thespecific heat capacity ofsea water,${\displaystyle h_2}$ is the lower depth,${\displaystyle h_1}$ is the upper depth,${\displaystyle \rho (z)}$ is the in-situseawater density profile, and${\displaystyle \mathrm{\Theta }(z)}$ is theconservative temperature profile.${\displaystyle c_p}$ is defined at a single depth h0 usually chosen as the ocean surface. InSI units,${\displaystyle H}$ has units ofJoules per square metre (J·m⁻²).

In practice, the integral can be approximated bysummation using a smooth and otherwise well-behaved sequence of in-situ data; including temperature (t), pressure (p),salinity (s) and their corresponding density (ρ).Conservative temperature${\displaystyle \mathrm{\Theta }(z)}$ are translated values relative to the reference pressure (p0) at h0. A substitute known aspotential temperature has been used in earlier calculations.^[21]

Measurements of temperature versus ocean depth generally show anupper mixed layer (0–200 m), athermocline (200–1500 m), and adeep ocean layer (>1500 m). These boundary depths are only rough approximations. Sunlight penetrates to a maximum depth of about 200 m; the top 80 m of which is thehabitable zone for photosynthetic marine life covering over 70% of Earth's surface.^[22] Wave action and other surfaceturbulence help to equalize temperatures throughout the upper layer.

Unlikesurface temperatures which decrease with latitude, deep-ocean temperatures are relatively cold and uniform in most regions of the world.^[23] About 50% of all ocean volume is at depths below 3000 m (1.85 miles), with thePacific Ocean being the largest and deepest of five oceanic divisions. The thermocline is the transition between upper and deep layers in terms of temperature, nutrient flows, abundance of life, and other properties. It is semi-permanent in the tropics, variable intemperate regions (often deepest during the summer), and shallow to nonexistent in polar regions.^[24]

Ocean heat content is derived from ocean temperature measurements. Ocean temperature measurements come with difficulties, especially before the deployment of theArgo profiling floats.^[20] Due to poor spatial coverage and poor quality of data, it has not always been easy to distinguish between long termglobal warming trends andclimate variability. Examples of these complicating factors are the variations caused byEl Niño–Southern Oscillation or changes in ocean heat content caused by majorvolcanic eruptions.^[3]

Argo is an international program of roboticprofiling floats deployed globally since the start of the 21st century.^[26] The program's initial 3000 units had expanded to nearly 4000 units by year 2020. At the start of each 10-day measurement cycle, a float descends to a depth of 1000 meters and drifts with the current there for nine days. It then descends to 2000 meters and measures temperature, salinity (conductivity), and depth (pressure) over a final day of ascent to the surface. At the surface the float transmits the depth profile and horizontal position data throughsatellite relays before repeating the cycle.^[27]

Starting 1992, theTOPEX/Poseidon and subsequentJason satellite seriesaltimeters have observed vertically integrated OHC, which is a major component of sea level rise. Since 2002,GRACE and GRACE-FO have remotely monitored ocean changes usinggravimetry.^[28] The partnership between Argo and satellite measurements has yielded ongoing improvements to estimates of OHC and other global ocean properties.^[25]

Human-caused increases in greenhouse gases have reduced the outgoing infrared radiation from Earth's atmosphere creating an increase in planet-wide heat.^[29]Over 90% of this planetary heat uptake is manifest in ocean heat content. This high percentage is a result of the oceans highheat capacity.^[30] Most extra energy that enters the planet via the atmosphere is taken up and retained by the ocean.^[31][32][33]

Planetary heat uptake or heat content accounts for the entire energy added to or removed from the climate system.^[34] It can be computed as an accumulation over time of the observed differences (orimbalances) between total incoming and outgoing radiation.Changes to the imbalance have been estimated from Earth orbit byCERES and otherremote instruments, and compared againstin-situ surveys of heat inventory changes in oceans, land, ice and the atmosphere.^[6][35][36] Achieving complete and accurate results from either accounting method is challenging, but in different ways that are viewed by researchers as being mostly independent of each other.^[35] Increases in planetary heat content for the well-observed 2005–2019 period are thought to exceed measurement uncertainties.^[29]

From the ocean perspective, the more abundant equatorialsolar irradiance isdirectly absorbed by Earth's tropical surface waters and drives the overall poleward propagation of heat. The surface also exchanges energy that has been absorbed by the lowertroposphere through wind and wave action. Over time, a sustained imbalance inEarth's energy budget enables a net flow of heat either into or out of greater ocean depth viathermal conduction,downwelling, andupwelling.^[37][38] Releases of OHC to the atmosphere occur primarily viaevaporation and enable the planetarywater cycle.^[39] Highsea surface temperatures help drivetropical cyclones,atmospheric rivers,atmospheric heat waves and otherextreme weather events that can penetrate far inland.^[40] Altogether these processes enable the ocean to be Earth's largestthermal reservoir which functions to regulate the planet's climate; acting as both asink and a source of energy.^[31]

Surface air temperatures over land masses have been increasing faster than thesea surface temperature.

Thegreenhouse effect traps heat in the lower atmosphere and oceans, so that the upper atmosphere, receiving less reflected energy, cools.^[41]

From the perspective of land and ice covered regions, their portion of heat uptake is reduced anddelayed by the dominant thermal inertia of the ocean. Although the average rise in land surface temperature has exceeded the ocean surface due to the lower inertia (smaller heat-transfer coefficient) of solid land and ice, temperatures would rise more rapidly and by a greater amount without the full ocean.^[31] Measurements of how rapidly the heat mixes into the deep ocean have also been underway to better close the ocean and planetary energy budgets.^[42]

Global upper-2000 m ocean heat content reached a new record in 2024, exceeding the 2023 value by 16 ± 8 ZJ, continuing a run of annual records over the past eight years.^[43] Numerous independent studies in recent years have found a multi-decadal rise in OHC of upper ocean regions that has begun to penetrate to deeper regions.^[6][20]Numerous independent studies in recent years have found a multi-decadal rise in OHC of upper ocean regions that has begun to penetrate to deeper regions.^[6][20] The upper ocean (0–700 m) has warmed since 1971, while it is very likely that warming has occurred at intermediate depths (700–2000 m) and likely that deep ocean (below 2000 m) temperatures have increased.^[11]: 1228 The heat uptake results from a persistent warming imbalance inEarth's energy budget that is most fundamentally caused by the anthropogenic increase in atmosphericgreenhouse gases.^[44]: 41 There is very high confidence that increased ocean heat content in response to anthropogenic carbon dioxide emissions is essentially irreversible on human time scales.^[11]: 1233

Studies based on Argo measurements indicate that ocean surfacewinds, especially thesubtropical trade winds in thePacific Ocean, change ocean heat vertical distribution.^[46] This results in changes amongocean currents, and an increase of thesubtropical overturning, which is also related to theEl Niño andLa Niña phenomenon. Depending on stochastic natural variability fluctuations, during La Niña years around 30% more heat from the upper ocean layer is transported into the deeper ocean. Furthermore, studies have shown that approximately one-third of the observed warming in the ocean is taking place in the 700–2000 meter ocean layer.^[47]

Model studies indicate thatocean currents transport more heat into deeper layers during La Niña years, following changes in wind circulation.^[48][49] Years with increased ocean heat uptake have been associated with negative phases of theinterdecadal Pacific oscillation (IPO).^[50] This is of particular interest to climate scientists who use the data to estimate theocean heat uptake.

The upper ocean heat content in mostNorth Atlantic regions is dominated by heat transport convergence (a location where ocean currents meet), without large changes to temperature and salinity relation.^[51] Additionally, a study from 2022 on anthropogenic warming in the ocean indicates that 62% of the warming from the years between 1850 and 2018 in the North Atlantic along 25°N is kept in the water below 700 m, where a major percentage of the ocean's surplus heat is stored.^[52]

A study in 2015 concluded that ocean heat content increases by the Pacific Ocean were compensated by an abrupt distribution of OHC into the Indian Ocean.^[53]

A large-ensemble reanalysis of ocean warming published in 2024 estimated a 1961–2022 warming trend of 0.43 ± 0.08 W/m², along with a statistically significant acceleration rate of 0.15 ± 0.04 W/m² per decade^[8] which is consistent with similar independent analysis.^[54][55] The net rate of change in the top 2000 meters from 2003 to 2018 was+0.58±0.08 W/m² (or annual mean energy gain of 9.3 zettajoules).^[3]

Warming oceans are one reason forcoral bleaching^[62] and contribute to themigration of marine species.^[63]Marine heat waves are regions of life-threatening and persistently elevated water temperatures.^[64] Redistribution of the planet's internal energy byatmospheric circulation andocean currents produces internalclimate variability, often in the form of irregularoscillations,^[65] and helps to sustain the globalthermohaline circulation.^[66][67]

The increase in OHC accounts for 30–40% of globalsea-level rise from 1900 to 2020 because ofthermal expansion.^[68][69]It is also an accelerator ofsea ice,iceberg, andtidewater glacier melting. The ice loss reduces polaralbedo,amplifying both the regional and global energy imbalances.^[70] The resulting ice retreat has been rapid and widespread forArctic sea ice,^[71] and within northernfjords such as those ofGreenland andCanada.^[72]Impacts toAntarctic sea ice and the vastAntarctic ice shelves which terminate into theSouthern Ocean have varied by region and are also increasing due to warming waters.^[73][74] Breakup of theThwaites Ice Shelf and itsWest Antarctica neighbors contributed about 10% of sea-level rise in 2020.^[75][76]

The ocean also functions as a sink and source of carbon, with a role comparable to that of land regions in Earth'scarbon cycle.^[77][78] In accordance with the temperature dependence ofHenry's law, warming surface waters are less able to absorb atmospheric gases including oxygen and the growing emissions of carbon dioxide and other greenhouse gases from human activity.^[79][80] Nevertheless the rate in which the ocean absorbs anthropogenic carbon dioxide has approximately tripled from the early 1960s to the late 2010s; a scaling proportional to the increase in atmospheric carbon dioxide.^[81] The increase in CO₂ levels causes ocean acidification, which is where the pH of the ocean decreases due to the uptake of CO₂. This impacts the various species including reducing growth and calcification rates for calcifiers, lowering the capacity of acid base regulation in bivalves, and being harmful to the metabolic pathways of organisms which can lower the amount of energy these organisms are able to produce.^[82]

Warming of the deep ocean has the further potential to melt and release some of the vast store of frozenmethane hydrate deposits that have naturally accumulated there.^[83]

• Ocean acidification – Decrease of pH levels in the ocean
• Ocean deoxygenation – Reduction of the oxygen content of the oceans
• Ocean stratification – Layering of ocean water due to density differences
• Special Report on the Ocean and Cryosphere in a Changing Climate – 2019 IPCC report
• Tropical cyclones and climate change – Impact of climate change on tropical cyclones

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