How we track the heat stored in the ocean

What the dashboard shows, where the numbers come from, and what they can’t tell you.

The short answer

About 90% of the extra heat trapped by greenhouse gases ends up in the ocean; the rest warms the land, the air and the ice. So the heat stored in the ocean is the most complete measure of how fast the planet is gaining energy. Air temperature, the number usually quoted, is only a small part of it.

That stored heat has set a new record every year since 2017. In 2025 the top two kilometres of ocean gained about 23 zettajoules (a zettajoule is 10²¹ joules), according to the yearly assessment led by China’s Institute of Atmospheric Physics. Carbon Brief worked out that this is about 39 times the energy all of humanity used in 2023.

How we know

The main series comes from the US National Oceanic and Atmospheric Administration’s National Centers for Environmental Information (NCEI), which maintains the ocean heat record first assembled by Sydney Levitus and colleagues. It covers the top 700 metres since 1955, and the top 2,000 metres in yearly values since 2005.

Most of today’s measurements come from Argo, an international fleet of about 4,000 robotic floats. Each drifts at around 1,000 metres, sinks to 2,000 metres every ten days, then rises to the surface measuring temperature and salinity on the way up, sends its readings by satellite and dives again. The first floats went out around 2000, and the fleet covered most of the open ocean by the mid-2000s, which is why the deeper series starts then. Earlier decades rely on ship measurements, which were sparse, especially in the Southern Hemisphere, so those values are less certain.

Two independent groups, China’s Institute of Atmospheric Physics and the Japan Meteorological Agency, process the same kinds of measurements their own way. The chart shows their estimates as dashed and dotted lines, shifted to the same 2005–2009 starting point, so you can see whether the groups agree on how fast heat is building up.

Reading the dashboard

Two depths. The total since 1955 uses the top 700 metres, because that record is the longest. The year-by-year chart uses the top 2,000 metres from 2005, because it includes the deeper warming that Argo can now measure.

The comparisons. Numbers this large are hard to picture, so the dashboard puts them next to familiar ones: the world’s yearly energy use, the energy the United States has used since 1955, and the Hiroshima bomb (about 6.3 × 10¹³ joules). The bomb comparison is striking but can mislead: a bomb releases its energy in an instant and destroys a city, while heat in the ocean spreads out and shows up over decades as rising seas, stronger storms and dying reefs.

How current each part is

• NOAA series: yearly values, published a few months after each year ends; we check every night.
• Institute of Atmospheric Physics and Japan Meteorological Agency series: checked every night; each updates on its own schedule.
• The 2025 headline figures (23 zettajoules, 39 times human energy use): from the January 2026 assessment and Carbon Brief, updated when the next yearly assessment appears.
• News list: recent articles, collected every night.

What the dashboard can’t tell you

It is not about the water temperature at a particular beach; that is sea-surface temperature, a different measurement. For heat extremes at sea, see the Marine Heatwave Tracker; for the sea-level rise that ocean warming drives, the Sea Level Rise Monitor.

Year-to-year changes are noisy. A year with a smaller gain does not mean warming has paused; heat moves between depths and between the ocean and the air, notably during El Niño. The trend over decades is what counts.

And it does not cover the deep ocean below 2,000 metres. A smaller fleet of floats that can dive to 6,000 metres, Deep Argo, is starting to measure it.

Further reading

• Pan, Cheng et al. (2026), Advances in Atmospheric Sciences: ocean heat content sets another record in 2025 (23 ± 8 zettajoules). doi.org/10.1007/s00376-026-5876-0
• Cheng et al. (2024), Advances in Atmospheric Sciences: the same assessment for 2023. doi.org/10.1007/s00376-024-3378-5
• Levitus et al. (2012), Geophysical Research Letters: world ocean heat content since 1955, the record NOAA’s series builds on. doi.org/10.1029/2012GL051106
• von Schuckmann et al. (2023), Earth System Science Data: where the heat in the Earth system goes, and how much ends up in the ocean. doi.org/10.5194/essd-15-1675-2023
• Carbon Brief: state of the climate in 2025, the source of the 39-times comparison. carbonbrief.org
• Argo: the float programme, with maps and data. argo.ucsd.edu (data: doi.org/10.17882/42182)

Credits

The ocean heat record exists because of generations of oceanographers and ship crews, the international teams who build, deploy and replace Argo’s floats, NOAA’s NCEI, which curates and publishes the main series, and the Institute of Atmospheric Physics and Japan Meteorological Agency teams who produce independent estimates. This dashboard collects and presents their work.