The world’s oceans have once again broken heat absorption records, marking the eighth consecutive year of increasing thermal energy retention. In 2025, they absorbed an unprecedented 23 zettajoules of heat – a figure that surpasses even the prior record set in 2024. This continued warming trend underscores the planet’s accelerating climate crisis, even if surface temperatures have not risen as dramatically.
The Scale of Ocean Warming
A zettajoule represents an almost unimaginable quantity of energy: one sextillion joules. To put this into perspective, the 23 zettajoules absorbed in 2025 are equivalent to the explosive yield of 12 Hiroshima-sized atomic bombs detonating underwater. Scientists also calculate this as enough energy to boil over two billion Olympic swimming pools, or 200 times the electrical consumption of the entire human population in a single year.
The research, published in Advances in Atmospheric Science, was conducted by over 50 scientists from the United States, Europe, and China. It highlights that oceans absorb over 90% of excess heat trapped in the atmosphere, acting as the planet’s primary heat sink.
Why Deep Ocean Heat Matters
While global temperature measurements often focus on surface readings, the study emphasizes the critical importance of tracking heat content at depth. The deep ocean is warming at a slower rate but stores vastly more thermal energy. This is because heat slowly circulates downwards, aided by currents and ocean circulation patterns.
As Zeke Hausfather, a research scientist at Berkeley Earth, explains: “Ocean heat content is in many ways the most reliable thermostat of the planet. That’s where all the heat is going—and that’s the reason why almost every year we set a new record for ocean heat content, because there’s so much heat being absorbed by the ocean.”
The long-term implications are severe: even if fossil fuel emissions were halted immediately, it would take centuries for this stored heat to cycle through the ocean. As Raphael Kudela, a professor of ocean science at UC Santa Cruz, puts it: “What people often don’t grasp is that it’s taken 100 years to get the oceans that warm at depth…We’re going to pay this cost for a very, very long time, because we’ve already put the heat in the ocean.”
How We Measure Ocean Heat
Monitoring ocean temperatures has evolved over time. Early data collection dates back to Benjamin Franklin’s transatlantic voyages in the 18th century and the HMS Challenger expedition in the 1870s. However, consistent measurement of deep-sea temperatures is a relatively recent development.
Today, scientists rely on a combination of:
- Argo floats: A global network of over 3,500 robotic buoys that collect temperature data from various depths.
- Ship-based sensors: Instruments mounted on hulls and buoys.
- Satellite measurements: Remote sensing of sea surface temperatures.
- Animal-borne sensors: Devices attached to marine mammals to gather data in inaccessible regions.
- Mathematical models: Algorithmic predictions trained on existing datasets.
This multi-faceted approach ensures that the data is consistent, despite the challenges of monitoring such a vast environment.
The Inevitable Long-Term Effects
The continued absorption of heat by the oceans guarantees significant, long-lasting consequences for climate stability and marine ecosystems. Even if emissions were reduced drastically, the heat already stored will continue to drive warming for centuries. The current trends demonstrate that the planet’s oceans are not just warming—they are becoming a thermal reservoir that will dictate climate patterns for generations to come.
