The oceans have a huge thermal mass compared to the atmosphere and land surface. They act as the planet’s heat storage and transportation system, as the ocean currents redistribute the heat. This is important because if we look at the global surface temperature as an indication of warming, we’re only getting some of the picture. The oceans act as a huge storage heater, and will continue to warm up the lower atmosphere (no matter what changes we make to the atmosphere in the future).

The average surface temperature of the world's oceans has been increasing since about 1910.

The total heat content of the Earth has been rising at a relatively costant pace since about 1965. Most of the heat (about 80%) has been absorbed by the oceans.

Direct observations of ocean chemistry have shown that the chemical balance of seawater has shifted to a more acidic state (lower pH) [Figure 7]. Some marine organisms (such as corals and some shellfish) have shells composed of calcium carbonate which dissolves more readily in acid. As the acidity of sea water increases, it becomes more difficult for them to form or maintain their shells.

While ocean heat content varies significantly from place to place and from year-to-year (as a result of changing ocean currents and natural variability), there is a strong trend during the period of reliable measurements. Increasing heat content in the ocean is also consistent with sea level rise, which is occurring mostly as a result of thermal expansion of the ocean water as it warms.

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Energy absorbed by the Earth 1970-2010 - Most of the heat is going into the oceans
As a result of the increase in atmospheric CO2, the Earth has been absorbing an excess of about 8 zeta joules of energy/year (the equivalent of a 50 megaton nuclear bomb being exploded every 15 minutes), with almost all of the energy going in to warming the oceans.
Global Ocean Heat Content 1979-2020 (relative to 1981-2010)

1979: near -100 Zettajoules; 2020: near +250 Zettajoules (+350 ZJ)
Global upper 2000 m ocean heat content from 1958 through 2020

Fig. 1. (Upper) Global upper 2000 m OHC from 1958 through 2020. The histogram presents annual anomalies relative to a 1981-2010 baseline, with positive anomalies shown as red bars and negative anomalies as blue.
Ocean heat budget from 1960 to 2020

Fig. 2. Ocean heat budget from 1960 to 2020 based on IAP analysis data from 0 to 2000 m, and from Purkey and Johnson (2010) for deep ocean change below 2000 m (units: ZJ). Figure updated from Cheng et al. (2017). The anomalies are relative to 1958-62 baseline, and the time series are smoothed by LOWESS (locally weighted scatterplot smoothing) with span width of 24 months. The gray dashed lines are the 95% confidence interval of the total ocean heat budget.
Regional observed upper 2000 m ocean heat content change from 1955 through 2020

Fig. 4. Regional observed upper 2000 m OHC change from 1955 through 2020 relative to 1981-2010 baseline. The time series (black) are smoothed by LOWESS (locally weighted scatterplot smoothing) with span width of 24 months. The blue shadings are the 95% confidence interval. [Data updated from Cheng et al. (2017)].
Table 2.2 | The assessed remaining carbon budget and its uncertainties.

Shaded blue horizontal bands illustrate the uncertainty in historical temperature increase from the 1850–1900 base period until the 2006–2015 period as estimated from global near-surface air temperatures, which impacts the additional arming until a specific temperature limit like 1.5°C or 2°C relative to the 1850–1900 period. Shaded grey cells indicate values for when historical temperature increase is estimated from a blend of near-surface air temperatures over land and sea ice regions and sea-surface temperatures over oceans.
World Ocean Heat Content 0-700 m - 1955-2010

Global Upper Ocean Heat Content is Rising