Ocean Heat Content Hits a Record: Ocean Absorbs 90% of Warming Heat

What Is Ocean Heat Content? The Planet's "Heat Savings Account"

Definition of ocean heat content (OHC)

Ocean heat content (OHC) is the total thermal energy stored in seawater down to a given depth. It is measured in joules (J); for the whole ocean the numbers reach 10²¹ joules, or zettajoules (ZJ). If sea surface temperature is the ocean's "skin temperature," OHC shows how much heat has accumulated in the ocean as a whole, calculated by integrating temperature changes from the surface down to several hundred or 2,000 meters.

Air and sea surface temperatures swing widely from year to year because of El Niño and volcanic eruptions. OHC integrates heat through the deep layers, so short-term fluctuations are small and it steadily reflects which way Earth's energy balance is tilting. Climate scientists therefore regard it as one of the most reliable indicators of global warming.

Why does heat accumulate in the ocean?

As greenhouse gases increase, Earth absorbs more heat than it radiates to space, creating an energy imbalance. Most of this excess heat goes into the ocean. Water has a much larger specific heat than air, and the ocean covers about 70% of the planet with an average depth of roughly 3,700 meters. The heat contained in the entire atmosphere is said to be about the same as in just the top few meters of the ocean, which makes the sea an enormous heat reservoir.

About 90% of the Extra Heat Goes into the Ocean

The IPCC's breakdown of the heat

The IPCC Sixth Assessment Report (AR6), Working Group I, breaks down where the extra heat accumulated in the Earth system during 1971–2018 went: about 91% in the ocean, about 5% on land, about 3% in ice (ice sheets, sea ice and glaciers) and about 1% in the atmosphere. The rise in air temperature we feel is only a small part of the stored heat; most of it is quietly absorbed by the sea.

Assessment by the Japan Meteorological Agency

The Japan Meteorological Agency (JMA) explains that about 90% of the heat added by global warming is taken up by the ocean. Its 2020 announcement, based on an analysis down to 2,000 m depth, showed a large increase in ocean heat content between 1955 and 2019, with the pace accelerating since the mid-1990s. Converted into an average temperature rise of the whole layer the change looks small, but because the mass of the ocean is enormous, a slight rise in water temperature corresponds to a vast amount of heat.

The weight of "0.1°C"

Even a 0.1°C rise in the average temperature of the top 2,000 m of seawater requires heat equal to many tens of times the primary energy humanity consumes in a year. That is why a rise of just a fraction of a degree in the ocean must never be taken lightly.

A New Record in 2025: Recent Trends

Record highs for years in a row

A study published in January 2026 in Advances in Atmospheric Sciences reported that ocean heat content (down to 2,000 m) in 2025 set a new record. The increase over 2024 was about 23 zettajoules, described as the largest annual rise since 2017. Press coverage reports that about 16% of the ocean area recorded its highest heat content on record and about 33% ranked among the top three. The result combines several observational datasets and a reanalysis, and independent research groups have confirmed a similar trend.

Ocean heat content has risen almost continuously since the 1950s, and record-breaking has become nearly annual in recent years. Unlike air temperature, there is little room to say "this year was cooler"; it clearly shows that warming has not stopped.

The acceleration

JMA's analysis also shows that the increase in ocean heat content has accelerated since the mid-1990s. This is consistent with the growing energy imbalance as greenhouse gas concentrations rise. Because estimates differ slightly between research institutions, the figures in this article should be taken as reported estimates.

How Is It Measured? Argo Floats and Ocean Observation

How the Argo program works

The main players in measuring the ocean's heat are the floats of the international Argo program. About 4,000 autonomous robots, called Argo floats, are deployed across the world's oceans. Roughly every 10 days each float dives to about 2,000 meters and then rises to the surface, recording temperature and salinity along the way. At the surface it transmits the data by satellite, and the data are freely available to everyone. Global deployment in the early 2000s let humanity monitor deep-ocean temperature on a global scale in near real time for the first time. According to the WMO, Argo has achieved 2 million profiles in about 20 years.

Observations before Argo and their limits

Before Argo, observations relied mainly on expendable bathythermographs (XBTs) dropped from ships and on research vessels. These were biased toward shipping lanes and certain areas, and coverage of the Southern Hemisphere and of winter seas was thin. Estimates of past ocean heat content therefore carry large uncertainties, and corrections and statistical gap-filling are still used for long-term estimates.

Challenges in the deep sea and polar regions

Most Argo floats observe down to 2,000 m. Deep Argo floats for greater depths are being developed, but observation points are still limited. In ice-covered polar regions floats cannot surface, so data are hard to obtain. Filling these "observation gaps" is a future task.

In Japan, JAMSTEC (Japan Agency for Marine-Earth Science and Technology) and JMA take part in the Argo program, contributing float deployment and data analysis.

What Happens When the Ocean Stores Heat? The Big Picture

Sea level rise: thermal expansion

Water expands when warmed. Thermal expansion of seawater is a major cause of sea level rise, and the IPCC assesses that about half of the 1971–2018 rise was due to it. In recent years the contribution of melting ice sheets and glaciers has also grown, but as long as heat keeps accumulating in the ocean, thermal expansion will continue. Heat that reaches deep layers does not return easily, so sea level is expected to keep rising for centuries even if emissions stopped now.

Marine heatwaves and ecosystems

As the ocean stores more heat, marine heatwaves, short-term high-temperature events, become more likely. Coral bleaching, loss of seaweed beds and shifts in fish distribution are typical examples. See also our related article What Is a Marine Heatwave?.

Fisheries and our daily lives

Rising water temperatures affect the distribution and catch of Japan's main fishery resources such as Pacific saury and salmon. See Ocean Warming and Fisheries for details. Warm seas also fuel typhoons and heavy rain, raising the risk of weather disasters.

Effects on the ocean's absorption capacity

As water warms, the amount of carbon dioxide and oxygen it can hold decreases. The ocean's role in softening warming (carbon uptake) could weaken; read this together with our article The Ocean's Role as a Carbon Sink.

What Is Happening in the Seas Around Japan

Rising water temperatures near Japan

According to JMA's long-term monitoring, sea surface temperature around Japan has risen by more than about 1°C per 100 years, faster than the global average. The rise is especially marked in the central Sea of Japan and in areas influenced by the Kuroshio. This trend directly affects fisheries, coastal ecosystems and weather.

The Kuroshio large meander and heat transport

The Kuroshio, flowing along Japan's south coast, is a "heat conveyor" carrying tropical heat northward. When it persists in a large meander, coastal temperature distributions change, affecting fishing conditions and weather. For the relationship between currents and temperature, see Ocean Warming and Changing Currents.

Effects on the coast

When sea surface temperatures are high, thermal expansion adds to sea level rise along Japan's coasts. See Sea Level Rise on Japan's Coasts for details.

Why Warming Sometimes Seems to "Stop"

The hiatus debate and the ocean

From the 2000s to the early 2010s there was a period when the rise in global average temperature seemed to slow temporarily, called the "global warming hiatus." Even then, Argo data showed that ocean heat content kept increasing. The heat had not vanished; it had moved into deeper layers.

El Niño and the redistribution of heat

During El Niño, Pacific heat is released into the atmosphere and global average temperature temporarily rises. During La Niña, heat tends to go down into the ocean. What matters is that even when air temperature rises and falls, the total heat in the ocean steadily increases.

What We Can Do: Understanding and Action

Cutting emissions and adapting

To curb the increase in ocean heat content, the fundamental answer is to reduce greenhouse gas emissions and narrow Earth's energy imbalance. At the same time, adaptation measures for unavoidable sea level rise and marine heatwaves (coastal protection, fisheries transition, coral reef conservation) are important.

Supporting observation

Observation networks such as Argo rely on international cooperation and continuing funding. Their data are used in weather forecasting, fishery forecasts and climate model validation, which are directly tied to our lives. Understanding the value of observation is itself a form of support.

How to read the information

• A cooler year does not necessarily mean warming has stopped
• The latest OHC values differ slightly between research institutions
• When reading news figures, check the unit (ZJ) and the depth covered

Summary: The Ocean Is the Front Line of Warming

The ocean takes in about 90% of the heat added by global warming, and ocean heat content was reported to set another record in 2025. Thanks to observation networks such as Argo floats, we can now follow hard-to-see changes inside the ocean with numbers. Stored ocean heat comes back to our lives in many ways: sea level rise, marine heatwaves, fisheries and weather. Let's take the first step to protect the ocean through everyday actions and an accurate understanding of the facts.