Ocean heat content

• 1. Introduction

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  Ocean heat content is a measure of the energy absorbed and stored by the world's oceans, and what it has been revealing over recent decades is striking. In 2024, the upper 2000 meters of the global ocean reached a new heat record, exceeding the 2023 value by 16 plus or minus 8 zettajoules. That record broke the one set the year before, which itself broke the one before that. The past eight years have each set a new annual record. What is driving this relentless accumulation of heat? Where is it going? And what does it mean for the planet's future? Those are the questions this documentary sets out to answer.

• 2. The Oceans as a Heat Bank

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  Oceans cover 70% of Earth's surface and hold a thermal reservoir unlike anything else on the planet. Their enormous heat capacity lets them absorb vast quantities of energy with only a modest rise in temperature, which is why they have absorbed more than 90% of the excess planetary heat generated by global warming between 1971 and 2018. That outsized share is not incidental. Most of the extra energy that enters through the atmosphere is taken up and held by seawater far more effectively than by land or ice, which have lower heat-transfer coefficients and respond more quickly to surface temperature swings. A large-ensemble reanalysis published in 2024 estimated a warming trend for the period 1961-2022 of 0.43 plus or minus 0.08 watts per square metre, with a statistically significant acceleration of about 0.15 plus or minus 0.04 watts per square metre per decade. Human-caused increases in greenhouse gas emissions are identified as the principal driver by reducing the outgoing infrared radiation from Earth's atmosphere and creating a persistent imbalance in the planet's energy budget.

• 3. Measuring a Hidden Ocean

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  Since before 1960, research vessels and ocean stations have sampled sea temperatures at the surface and at depth around the world. Those early surveys provided a long baseline but were limited by poor spatial coverage and variable data quality, which made it difficult to distinguish long-term warming trends from natural climate variability such as El Nino-Southern Oscillation swings or the temporary cooling effects of major volcanic eruptions. The picture sharpened dramatically after the year 2000 when the Argo program launched an internationally coordinated network of robotic profiling floats. Argo's initial complement of roughly 3000 units had expanded to nearly 4000 by 2020. Each float follows a precise 10-day cycle: it descends to 1000 metres, drifts with the current for nine days, then drops to 2000 metres and measures temperature, salinity, and pressure on its ascent to the surface, where it transmits its data via satellite relay. Since 1992, the TOPEX/Poseidon and subsequent Jason satellite series have measured sea level rise with altimeters, capturing ocean heat content from orbit. Since 2002, the GRACE and GRACE-FO missions have added gravitational measurements of ocean mass changes. The combination of Argo floats and satellite observations has steadily closed the gaps in ocean heat accounting.

• 4. Where the Heat Is Going

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  By 2020, about one-third of the total accumulated heat energy had propagated to depths below 700 metres, a fact that surprised many researchers given how slowly heat diffuses through still water. The five highest ocean heat readings at a depth of up to 2000 metres all occurred within the period 2020-2024. The upper ocean, defined as 0-700 metres, has warmed consistently since 1971. Warming in the intermediate layer from 700-2000 metres is considered very likely, and temperature increases in the deep ocean below 2000 metres are considered likely. The Southern Ocean has taken up a disproportionately large share of global heat compared with other ocean basins, while the subpolar North Atlantic has warmed more slowly. A 2022 study found that 62% of the warming recorded in the North Atlantic along 25 degrees north between 1850 and 2018 is stored in the water column below 700 metres. Pacific trade winds also play a role in redistributing heat vertically: during La Nina years, roughly 30% more heat is transported from the upper ocean into deeper layers, an effect linked to changes in the subtropical overturning circulation. A 2015 study concluded that Pacific ocean heat gains during certain periods were partly offset by an abrupt redistribution of heat into the Indian Ocean.

• 5. Consequences Reach the Surface

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  Thermal expansion of warming seawater has accounted for 30-40% of global sea-level rise from 1900 to 2020. Elevated ocean heat content is also an accelerator of sea ice, iceberg, and tidewater glacier melt; ice retreat in the Arctic and within northern fjords such as those of Greenland and Canada has been rapid and widespread. Breakup of the Thwaites Ice Shelf and its West Antarctic neighbours contributed about 10% of sea-level rise in 2020. As ice retreats, polar albedo drops, amplifying both regional and global energy imbalances in a self-reinforcing cycle. High sea surface temperatures help drive tropical cyclones, atmospheric rivers, and heat waves that can reach far inland. Marine heat waves create regions of life-threatening and persistently elevated water temperature, contributing to coral bleaching and shifts in the range of marine species. The rate at which the ocean absorbs anthropogenic carbon dioxide has approximately tripled from the early 1960s to the late 2010s, proportional to rising atmospheric concentrations of the gas. That absorption causes ocean acidification, which reduces growth and calcification rates in shell-forming organisms, lowers the capacity for acid-base regulation in bivalves, and disrupts the metabolic pathways of organisms in ways that cut their usable energy. Warming of the deep ocean also carries the potential to destabilise the vast frozen methane hydrate deposits that have accumulated there, releasing a powerful greenhouse gas into the water column and eventually the atmosphere.

• 6. An Essentially Irreversible Signal

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  Scientists say with very high confidence that the increase in ocean heat content driven by anthropogenic carbon dioxide emissions is essentially irreversible on human time scales. The net rate of change in the top 2000 metres from 2003 to 2018 was a gain averaging 9.3 zettajoules per year. Planetary heat uptake for the well-observed 2005-2019 period is thought to exceed measurement uncertainties, meaning the signal is real and detectable above noise. Years with increased ocean heat uptake have been associated with negative phases of the interdecadal Pacific oscillation, a pattern that climate scientists use to interpret longer cycles of warming and temporary heat redistribution. The international standard for defining and computing ocean heat content, known as TEOS-10, was approved in 2010 by the Intergovernmental Oceanographic Commission, giving researchers around the world a common framework for comparing results. That standardisation, combined with the ongoing Argo float network and the CERES instruments monitoring Earth's radiation budget from orbit, means the ocean heat record is now one of the most carefully watched single indicators of the planet's energy state.