The ocean regulates Earth’s climate by absorbing, storing and redistributing much of the excess heat trapped by human-caused warming. That heat is delayed, not erased: it can later warm the atmosphere, drive evaporation, expand seawater or affect marine ecosystems.
Why the ocean absorbs so much excess heat
Rising greenhouse-gas concentrations reduce the rate at which heat escapes from Earth to space, creating an energy imbalance. The ocean takes up much of the resulting excess energy because water has a high heat capacity: it can absorb substantial heat with a smaller temperature change than air. Its vast area also puts it in contact with the atmosphere across most of the planet.
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NOAA Climate.gov reports that the oceans have absorbed more than 90 percent of the excess heat trapped in the Earth system due to human-caused warming. This describes the share of excess heat, not 90 percent of warming in every possible sense. NOAA Ocean Exploration summarizes the ocean’s broader role this way: “The ocean influences weather and climate by storing solar radiation, distributing heat and moisture around the globe, and driving weather systems.”
How ocean circulation stores and moves heat
Wind, waves, tides and mixing move heat both across the ocean and down through its depths. Currents transport warm and cold water between regions, influencing local weather and longer-term climate patterns. The Atlantic Meridional Overturning Circulation (AMOC), for example, is one circulation system whose changes could have climate feedbacks; the evidence supports potential effects, not a simple prediction that it will stop.
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Heat absorbed by the ocean does not vanish. As NOAA Climate.gov puts it, “Heat absorbed by the ocean is moved from one place to another, but it doesn’t disappear.” Some energy returns to the atmosphere through evaporation, which transfers both water vapor and heat. The ocean can also transfer energy to ice shelves, contributing to melting. By taking up heat in the meantime, the ocean moderates the pace of atmospheric warming without permanently shielding the climate from that energy.
What ocean heat content measures—and what it does not
Ocean heat content (OHC) is an estimate of the heat energy stored in a specified volume or depth range of ocean, often expressed in joules or as an anomaly relative to a reference climatology. It is not the same as sea-surface temperature (SST), which measures temperature at the surface. A surface reading cannot, on its own, describe how much heat is stored through the upper ocean or at depth.
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OHC estimates use in-water temperature observations and analysis products. To interpret a figure, check its depth range, geographic coverage, time period, reference baseline and uncertainty. NOAA’s climate data products include records for different depths, basins and time resolutions; historical coverage and uncertainty differ, especially in older and deeper records.
One concrete measure comes from NASA’s Ocean Warming indicator: the upper 2,000 meters gained 372 ± 2 zettajoules of heat since 1955, with the latest measurement dated December 2024. NASA states, “The year 2024 was the ocean’s warmest year on record.” That annual ranking and the cumulative heat-content figure describe different aspects of ocean warming.
How stored ocean heat affects sea level and ecosystems
Sea-level rise
When seawater warms, it expands. This thermal expansion contributes to sea-level rise, alongside other processes such as melting land ice. Heat stored below the surface matters because warming is not limited to the thin layer measured by SST.
Marine heat waves and coral reefs
Higher ocean heat content can stress marine ecosystems and is associated with marine heat waves and coral bleaching. The ecological effect depends on where and when heat accumulates, how intense the warming is and how long it lasts; one global OHC figure does not describe conditions at every reef or habitat.
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Weather and regional climate
Ocean heat and moisture influence weather systems and regional climate. Evaporation supplies water vapor to the atmosphere, while circulation shifts heat between regions. Changes in circulation can alter regional patterns, but they should not be reduced to a single forecast based on global heat content alone.
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Why short-term variation does not cancel the long-term buildup
El Niño and La Niña affect shorter-term ocean and atmospheric conditions, so individual years can differ from the underlying accumulation trend. A single year’s surface-temperature ranking is not interchangeable with a multidecadal OHC record.
Keep the measurement and period attached to every number. 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. NASA’s 372 ± 2 zettajoule figure instead measures cumulative change in the upper 2,000 meters since 1955. They have different time windows, depth definitions and units, so they should not be combined as though they shared a baseline.
For a specific climate question, choose the measure that matches it: SST for surface conditions, or OHC for accumulated heat within a stated depth range. Use longer records to assess the persistent trend, and shorter records to understand variability.
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