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The ocean regulates Earth’s climate by absorbing, storing and redistributing much of the excess heat trapped by human-caused warming. NOAA Climate.gov says the oceans have absorbed more than 90 percent of that excess heat. This slows some short-term atmospheric warming, but the energy is stored—not erased—and it continues to affect sea level, weather and marine life.
Why the ocean absorbs so much heat
As greenhouse-gas concentrations rise, less heat escapes from Earth to space, creating an energy imbalance. The ocean takes up a large share of the resulting excess energy. Water has a high heat capacity: it can absorb substantial energy before its temperature rises as much as the temperature of air would. The ocean’s vast extent reinforces its role as Earth’s main heat reservoir.
NOAA Climate.gov estimates that more than 90 percent of the excess heat trapped in the Earth system due to human-caused warming has been absorbed by the oceans. That figure refers to excess heat, not to every possible measure of global warming. NOAA also describes the ocean as “the largest solar energy collector on Earth.” NOAA Climate.gov’s ocean heat content explainer gives the estimate and explains the distinction.
How stored heat moves—and why it does not disappear
Winds, waves, tides and mixing move heat through the ocean, both across regions and down through the water column. Currents redistribute energy between latitudes and basins, helping shape regional climate. As NOAA Ocean Exploration puts it, “The ocean influences weather and climate by storing solar radiation, distributing heat and moisture around the globe, and driving weather systems.” NOAA Ocean Exploration explains this connection between the ocean, weather and climate on land.
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Heat can later leave the ocean or affect other parts of the climate system. Evaporation transfers energy along with water vapor into the atmosphere; stored ocean heat can also contribute to melting ice shelves and can warm the air. In NOAA Climate.gov’s words, “Heat absorbed by the ocean is moved from one place to another, but it doesn’t disappear.” The ocean delays some atmospheric warming, but it is not a permanent buffer against climate change.
What ocean heat content measures
Ocean heat content (OHC) is an estimate of the energy stored in a specified volume or depth layer of the ocean, often reported in joules or as an anomaly relative to a stated reference period. It is not the same as sea-surface temperature (SST): SST describes conditions at the surface, whereas OHC can include heat accumulated far below it.
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Scientists estimate OHC from ocean temperature observations and data products. The meaning of a reported value depends on its depth range, geographic coverage, time window and reference baseline. NOAA’s Global Ocean Heat Content data product provides records for different depths, basins and time resolutions. Older and deeper-ocean estimates have varying observational coverage and uncertainty, so values from different products or layers should not be treated as interchangeable.
Two measures with different time windows
NASA’s Ocean Warming indicator reports that the upper 2,000 meters gained 372 ± 2 zettajoules of heat since 1955; its latest measurement is dated December 2024. NASA also reports that 2024 was the ocean’s warmest year on record. See NASA’s Ocean Warming indicator for the measurement and its context.
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Separately, NOAA Climate.gov estimates a full-depth ocean heat gain rate of about 0.66–0.74 watts per square meter, averaged over Earth’s surface, for 1993–2024. This is a rate over a different period and for a different depth scope than NASA’s cumulative upper-2,000-meter total; the numbers should not be combined as though they shared a baseline. NOAA’s page also gives estimates for depth bands, with its 2,000–6,000-meter component covering January 1988 to October 2014.
What ocean warming changes
Sea level
As seawater warms, it expands. This thermal expansion adds to sea-level rise, alongside other contributors such as melting land ice. The ocean’s role as a heat reservoir therefore has consequences for coastlines even when the warming is not immediately apparent at the surface.
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Marine ecosystems
Higher OHC can stress marine ecosystems and is associated with marine heat waves and coral bleaching. A surface-temperature reading alone cannot describe all the heat stored below, but surface conditions can still be important to organisms and habitats. NOAA’s archived explainer discusses these effects and the distinction between surface temperature and accumulated heat: Ocean heat content, NOAA Climate.gov.
Regional climate patterns
Ocean circulation helps distribute heat and can influence weather and climate patterns in particular regions. Changes in circulation, including potential changes involving the Atlantic Meridional Overturning Circulation (AMOC), may have climate feedbacks. The U.S. National Climate Assessment’s ocean and coastal chapter discusses these possibilities; it does not support treating a specific outcome, such as a named current stopping, as a simple prediction.
Why yearly conditions can vary while ocean heat accumulates
Ocean heat content reflects accumulated energy over a defined layer, while weather and surface temperatures can vary from year to year. El Niño and La Niña influence shorter-term conditions, so a single year’s surface-temperature ranking need not tell the whole story about the ocean’s longer-term heat accumulation. NOAA’s discussion of ocean heat content and climate variability notes the importance of longer records for assessing the underlying trend.
To interpret an ocean-warming number, check what it measures: surface temperature or integrated heat content; which depth and region it covers; what time period and anomaly baseline it uses; and what observational coverage and uncertainty apply. Those details determine whether two figures can be compared meaningfully.
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