Oceans Sound Alarm: Eight Consecutive Years of Record-Breaking Heat Absorption
The world’s oceans, our planet’s most vital heat sink, are sounding an unprecedented alarm. For an astonishing eighth year in a row, global oceans have absorbed a record-breaking amount of heat, signaling a profound and accelerating shift in Earth’s climate system. New research, published in the journal Advances in Atmospheric Science, reveals that 2025 saw an additional 23 zettajoules of heat absorbed – the highest annual increase since modern measurements began in the 1960s, and a significant jump from the 16 zettajoules recorded in 2024.
Understanding the Unfathomable: Zettajoules and Atomic Bombs
To grasp the sheer scale of this warming, one must first comprehend a “zettajoule.” While a single joule powers a tiny lightbulb for a second, a zettajoule is one sextillion joules – a staggering 23,000,000,000,000,000,000,000 joules absorbed in a single year. John Abraham, a professor of thermal science at the University of St. Thomas and a co-author of the study, often struggles to contextualize such immense figures for the public.
“Last year was a bonkers, crazy warming year—that’s the technical term,” Abraham quipped, adding, “The peer-reviewed scientific term is ‘bonkers’.” His most striking analogy compares the energy stored in the ocean to the destructive power of atomic bombs: the 2025 warming is energetically equivalent to 12 Hiroshima bombs detonating in the ocean every second, every day, for an entire year. Other calculations paint an equally stark picture, equating this energy to boiling 2 billion Olympic swimming pools or more than 200 times the annual electrical consumption of everyone on the planet.
The Ocean’s True Thermostat: Beyond Surface Temperatures
As the planet’s largest heat sink, oceans absorb over 90 percent of the excess heat trapped by greenhouse gases in the atmosphere. This heat doesn’t just linger at the surface; aided by currents and circulation, it slowly penetrates into the deeper, darker reaches of the ocean. This crucial distinction highlights a critical flaw in relying solely on surface temperature measurements, which are often used to determine “hottest years on record.”
The study notes that overall sea surface temperatures in 2025 were slightly lower than in 2024 (which holds the record for the hottest surface year). Meteorological phenomena like El Niño and La Niña can significantly influence surface temperatures, sometimes masking the true extent of overall ocean warming. For instance, a strong El Niño in 2024 contributed to higher surface temperatures, while a weak La Niña at the end of 2025 saw a massive surge in deep ocean heat content.
Zeke Hausfather, a research scientist at Berkeley Earth and a co-author, explains, “If the whole world was covered by a shallow ocean that was only a couple feet deep, it would warm up more or less at the same speed as the land. But because so much of that heat is going down in the deep ocean, we see generally slower warming of sea surface temperatures [than those on land].”
Why Deep Ocean Heat Content Matters
While surface temperatures directly impact weather patterns and marine life we interact with, the heat stored in the deep ocean is the most reliable indicator of climate change’s long-term effects. “Ocean heat content is in many ways the most reliable thermostat of the planet,” Hausfather asserts. “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.”
This relentless absorption has profound implications for sea-level rise (as water expands when heated), marine ecosystems, and the intensity of extreme weather events. The hidden heat accumulating beneath the waves represents a massive energy imbalance that will continue to drive climate impacts for decades to come, even if greenhouse gas emissions were to cease today.
Pioneering the Depths: A History of Ocean Measurement
Tracking ocean temperatures is not a new endeavor; Benjamin Franklin famously recorded sea temperatures during his transatlantic voyages. The HMS Challenger expedition in the 1870s pioneered deeper measurements, laying the groundwork for modern oceanography. However, consistent, widespread measurement of temperatures substantially below the surface is a relatively recent development, with the study’s earliest data extending back to the 1960s when navies began collecting such information.
A true revolution in understanding deep ocean temperatures came with the deployment of the international network of Argo floats. Since the early 2000s, over 3,500 robotic buoys have continuously collected invaluable data from oceans worldwide, providing the comprehensive picture that allows scientists to track these critical changes with unprecedented accuracy.
The Unseen Crisis: A Call to Action
The continuous record-breaking absorption of heat by our oceans serves as a stark reminder of the escalating climate crisis. It underscores that even when surface temperatures fluctuate, the planet’s fundamental energy balance is being profoundly altered. The “bonkers” warming of our oceans is not just a scientific curiosity; it is a critical warning that demands urgent and decisive action to mitigate greenhouse gas emissions and safeguard the future of our planet.
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