For decades, the oceans have absorbed the heat of human-made climate change. Now, that stored energy may be returning with compound interest
By
A few months ago, a team of scientists from the Chinese Academy of Sciences published a study carrying an unfathomable figure. In 2025 alone, they revealed, the world’s oceans absorbed an additional 23 zettajoules of heat energy.
One joule is a unit of energy equal to one 3,600th of a watt-hour. But 23 zettajoules is beyond human comprehension. Written out, the number is 23 followed by 21 zeros – 23 billion trillion joules. It is equivalent to roughly 200 times the electricity consumed by human civilisation in an entire year. Or, if you prefer a cruder image, to the energy released by 12 Hiroshima-scale nuclear bombs detonating in the ocean every single second for 12 months.
The climate system, the paper’s authors wrote, is ‘out of thermal equilibrium and accumulating heat’ at an even faster pace – the previous year’s record was 16 zettajoules.
Enjoying this article? Check out our related reads…
Oceans have been doing this job on our behalf for decades. And we barely noticed. The warming we are now experiencing is not the warming we have caused, but rather a fraction of it.
More than 90 per cent of the excess heat trapped by humanity’s greenhouse gas emissions has been locked away by the vast thermal capacity of saltwater. Acting as a planetary buffer, the oceans bought us decades of moderated surface temperatures. Last year, the Mediterranean, the North Indian Ocean, the tropical and South Atlantic, and the Southern Ocean all set heat records.
The deferred bill may arrive soon. And, most likely, with compound interest.
The oceans release stored heat through the El Niño mechanism – the alternating warming of the tropical Pacific that can reshape monsoons, intensify storms and drive surface temperatures upward. The current evidence suggests that Earth is approaching not merely an El Niño, but what some climatologists call a Super El Niño: an event so intense it may reshape regional climates for years.
NOAA places a 61 per cent probability on El Niño emerging by July, with a one-in-four chance of reaching very strong status, defined by Pacific surface temperatures rising more than 2°C above average.
Other forecasts point to a strong event by midsummer. A recent study published in the journal Ocean-Land-Atmosphere Research has found an extraordinary ‘spring annular warming pattern’ across the tropical Pacific.
Instead of building up along the Equator as usual, heat has been accumulating in a ring-shaped fashion around the entire Pacific Ocean. Though not unprecedented, it is already the largest such pattern in 40 years of satellite observation.
While the appearance of a Super El Niño is not 100 per cent certain, the implications are not hypothetical. There have been only a handful of such events since 1950. When they peak, the stored heat of the ocean spills into the atmosphere, driving global surface temperatures to new records. Several forecasters predict that 2027 will be the hottest year in the 176-year record.
What makes all of this more alarming still – this time over a longer period of time – is that the oceans’ capacity to moderate extremes is itself being undermined, most dramatically in the behaviour of the Atlantic Meridional Overturning Circulation, or AMOC.
This vast system of ocean currents carries warm surface water northward through the Atlantic and returns cold, dense water southward in deep ocean currents – a conveyor belt that has regulated the climate of entire continents for millennia. It is why London, sitting at the latitude of Canada’s Newfoundland, does not share the same winters.
The conveyor appears to be losing power. Greenland’s accelerating ice melt is pouring enormous volumes of fresh water into the Atlantic, diluting the salty, dense water whose sinking drives the whole system.
Another study, published in April in the journal Science Advances, found that the AMOC is on course to slow by more than 50 per cent by the end of this century – a figure considerably worse than previous climate models had projected. Its weakening has already been measured simultaneously at four separate latitudes across the North Atlantic.
The oceans did not ask to be our buffer. They simply did the job, by virtue of their physics and an immensity we took for granted. Yet, when a debt of 23 zettajoules a year comes due, the reckoning will likely be anything but gentle.
