The oceans are the great invisible reservoir of the climate crisis, absorbing almost all of the warming caused by human activities. In 2025, this silent role has reached a new critical point: last year, the planet’s seas accumulated more heat than any other year ever measured, according to a scientific analysis published in Advances in Atmospheric Sciences.
The most reliable thermometer of the climate crisis
Unlike air temperature, which is subject to fluctuations linked to climatic phenomena such as El Niño and La Niña, the heat accumulated in the oceans tells the underlying trend of the climate system. The measurements, based on data collected by buoys, satellites and oceanographic instruments, focus on the top 2,000 meters of water, where most of the excess energy is absorbed.
“Every year the planet warms: setting new records has become a broken record,” John Abraham, a climate scientist at the University of St Thomas and co-author of the analysis, told the Guardian. “Global warming is warming of the oceans. If you want to know how much the Earth has already warmed, the answer is in the oceans.”
More heat, more disasters
In detail, the study quantifies the increase in heat accumulated by the oceans in 2025 at approximately 23 zettajoules compared to the previous year. A zettajoule is a unit of energy measurement equal to 10²¹ joules, a number that is difficult to guess in everyday life. To give an order of magnitude, the researchers explain that the heat absorbed each year by the oceans is equivalent to over 200 times the electricity consumed globally by humanity.
The analysis also shows that the rate of ocean warming has significantly accelerated in the last two decades: today the oceans are warming more than double compared to the average of the second half of the twentieth century, a sign of persistent energy imbalance that is not compensated even in the “coolest” climate phases, such as the recent transition towards La Niña.
This surplus energy does not remain confined beneath the surface. Instead, it fuels increasingly violent weather events. Hurricanes and typhoons become more intense, rainfall more extreme, and floods more frequent. Ocean heat also contributes to sea level rise, through the thermal expansion of water, putting billions of people living in coastal areas at risk.
Another direct effect is the increase and prolongation of marine heat waves, which are causing real havoc in marine ecosystems. Corals, kelp forests and fish species struggle to survive persistently high temperatures, with consequences that impact fishing and food security.
From Antarctica to the Mediterranean
Ocean warming is not uniform. In 2025 the most affected areas include the tropical and southern Atlantic, the North Pacific and the Southern Ocean. The latter, which surrounds Antarctica, particularly worries scientists due to the recent collapse of winter sea ice.
Basins closer to Europe are also changing rapidly. The North Atlantic and the Mediterranean are not only warmer, but also more saline, more acidic and less oxygenated. According to researchers, this is a profound transformation of the state of the ocean, which makes ecosystems and the human activities that depend on them increasingly vulnerable.
Reliable observations date back to the mid-twentieth century, but reconstructions indicate that the oceans could now be at the highest heat levels in at least a thousand years, with a pace of warming unprecedented in the last two thousand years.
“As long as the Earth’s heat continues to increase, the heat content of the oceans will increase and records will continue to fall,” Abraham further explained to the Guardian. The unknown, he concludes, is not scientific but human: it depends on how quickly we manage to reduce the emissions that are transforming the sea into the largest, and most dangerous, heat accumulator on the planet.
