In 2025, Earth’s oceans were warmer than ever: What does it mean for the planet?

he Earth’s oceans absorbed more heat in 2025 than in any year since modern measurements began in the 1960s, according to a new study. They absorbed an additional 23 zettajoules’ worth of heat last year, which was significantly higher than the 16 additional zettajoules absorbed in 2024.

Energy is usually measured in joules. A zettajoule is one sextillion joules. That means the 23 zettajoules the oceans absorbed last year can be written as 23,000,000,000,000,000,000,000.

The analysis, ‘Ocean Heat Content Sets Another Record in 2025’, was published in the journal Advances in Atmospheric Sciences on Friday (January 9). It was carried out by more than 50 scientists across the United States, Europe, and China.

For their study, the researchers used temperature data collected by a range of instruments across the oceans. This data was then used to calculate the heat content of the top 2,000 metres of the oceans — where most of the heat is absorbed.

John Abraham, a professor of thermal science at the University of St Thomas (the US) and a co-author of the paper, told The Wired magazine that 23 zettajoules of energy is equivalent to 12 Hiroshima bombs exploding in the ocean.

Another way to look at the number is that one would need 23 zettajoules’ worth of heat to boil 2 billion Olympic swimming pools, or more than 200 times the electrical use of everyone on Earth, according to Abraham.

He said, “Last year was a bonkers, crazy warming year — that’s the technical term… The peer-reviewed scientific term is ‘bonkers’.”

The study also found that global average sea-surface temperatures (SSTs), where the ocean water and atmosphere meet, last year were the third highest on record. They were around 0.5 degrees Celsius above the 1981–2010 average baseline. However, some regions are warming faster than others. For instance, the South Atlantic and North Pacific Oceans, and the Southern Ocean are among the warmest regions.

Note that almost 90% of the extra heat trapped by greenhouse gases (GHGs) has been absorbed by the oceans, making them steadily warmer over the decades.

Higher ocean temperatures can have irreversible consequences for marine ecosystems. For instance, warmer oceans lead to an increase in ocean stratification — the natural separation of an ocean’s water into horizontal layers by density, with warmer, lighter, less salty, and nutrient-poor water layering on top of heavier, colder, saltier, nutrient-rich water. Usually, ocean ecosystems, currents, wind, and tides mix these layers.

The rise in temperatures, however, has made it harder for the water layers to mix with each other. Due to this, oceans are able to absorb less carbon dioxide from the atmosphere, and the oxygen absorbed is not able to mix properly with cooler ocean waters below, threatening the survival of marine life.

Nutrients are also not able to travel up to the surface of the oceans from below. This could threaten the population of phytoplankton — single-celled plants that thrive on the ocean surface and are the base of several marine food webs. Phytoplankton are eaten by zooplankton, which are consumed by other marine animals such as crabs, fish, and sea stars. Therefore, if the phytoplankton population plummets, there could be a collapse of marine ecosystems.

Warmer oceans cause marine heat waves (MHWs), which occur when the surface temperature of a particular region of the sea rises to 3 or 4 degrees Celsius above the average temperature for at least five days. Between 1982 and 2016, such heatwaves have doubled in frequency and have become longer and more intense, according to a 2021 study by the UN’s Intergovernmental Panel on Climate Change (IPCC).

MHWs are devastating for marine ecosystems as they contribute to coral bleaching, which reduces the reproductivity of corals and makes them more vulnerable to life-threatening diseases. They also impact the migration pattern of aquatic animals.

According to several studies, higher ocean temperatures may also result in more frequent and more intense storms like hurricanes and cyclones. Warmer temperatures lead to a higher rate of evaporation as well as the transfer of heat from the oceans to the air. That’s why, when storms travel across hot oceans, they gather more water vapour and heat. This results in more powerful winds, heavier rainfall, and more flooding when storms reach the land — meaning heightened devastation for humans.

Abraham told The Guardian, “As long as the Earth’s heat continues to increase, ocean heat content will continue to rise, and records will continue to fall… The biggest climate uncertainty is what humans decide to do. Together, we can reduce emissions and help safeguard a future climate where humans can thrive.”