For most of 2024, an extraordinary number hung above the world's oceans: the global average sea surface temperature sat at its highest level ever recorded for that time of year, day after day after day. The record-breaking streak continued deep into 2025. The oceans, which cover more than seventy percent of the Earth and absorb more than ninety percent of the excess heat trapped by greenhouse gases, had not simply nudged past an old mark — they had broken it by a margin that alarmed climate scientists. Average global sea surface temperatures, and even more so the heat content stored in the upper ocean, climbed into territory never seen in the modern observational record. The sea was, quite literally, the hottest it has been in recorded history.
Those two years offered a sustained, real-world experiment in what a warmer ocean does to the planet. Ocean heat is not an abstract detail; it is the fuel for marine heatwaves, the energy that feeds hurricanes, the force that expands the water and lifts sea level, and the stress that bleaches coral reefs and pushes marine life toward collapse. This article looks at what actually happened in 2024 and 2025, the forces behind the surge, the cascade of impacts it set in motion, and the lessons it carries for the years ahead.
What the Records Showed
Ocean heat is measured in two ways that both matter. The first is sea surface temperature — the temperature of the thin uppermost layer of the ocean — which shapes the weather and the marine ecosystems that live in that layer. The second, and arguably the more important, is ocean heat content: the total amount of heat stored in the volume of the ocean, down from the surface to depth, expressed in units of energy. From the end of 2023 both began climbing steeply, and each of the next two years pushed them higher.
Through much of 2024, the global average sea surface temperature ran consistently above the record set the year before, repeatedly passing the thresholds that had never been crossed in the daily global mean. The following year raised the mark again. Ocean heat content reached a new high in 2024 and then an even higher high in 2025, continuing an unbroken run of record-warm years stretching back more than a decade. Each of those years by itself would have been a headline; the striking fact is that the ocean has now been warming without pause at a record pace for years on end.
What made 2024 and 2025 unusual was not the existence of a record — records are broken somewhere almost every year in a warming world — but the size of the margin. The heat of 2023-2025 has sat far above the long-term trend of roughly one-tenth of a degree per decade, with individual years landing well outside the band that past projections had suggested. That gap is the signal scientists have described as one of the most striking departures in the observational record. Whether it eases as natural variability reasserts itself, or marks the ocean settling into a new, warmer regime, is one of the most important open questions in climate science right now.
What Drove the Surge
A set of factors combined to push the ocean this high. The first is the long-term baseline: the oceans have been absorbing the vast majority of the extra heat added to the climate system by greenhouse gases for decades, so each year's pool of stored heat is larger than the last. Built on top of that decade-by-decade accumulation, the strongest single contributor to the recent spike was the El Niño that began in mid-2023. El Niño, the warm phase of the Pacific's ENSO cycle, releases a large pulse of heat from the deep tropical Pacific into the atmosphere and reshuffles weather worldwide, and its influence spills well into the following year. Its lagged effect contributed directly to the record warmth of the ocean surface through 2024.
A Muted Cooling
Scientists also noticed that the pull-back was less than expected. When El Niño ended, the ocean did not sink back toward its trend line at the pace the models anticipated. The lingering warmth held the sea surface above most projections well into 2025, which is one reason researchers describe the recent period as sitting outside the envelope of what was forecast. The question of why the cooling was so muted — and whether it hints at a faster-than-modelled response of the ocean to the accumulating heat — has become one of the active fronts of ocean climate research.
2024-2025 Ocean Heat at a Glance
90%+: Of the excess heat from global warming that the oceans absorb
Record: Global sea surface temperature at its highest on record through 2024 and again in 2025
Years-long: Consecutive run of record global ocean heat content in the modern era
El Niño: The 2023-2024 El Niño supercharged an already record-warm ocean
Marine heatwaves: Prolonged heat across the Caribbean, Tropical Atlantic, and beyond
A Cascade of Consequences
Warmth stored in the ocean does not stay there. It moves into the atmosphere, into the storms that cross it, into the reefs and the fisheries that live in it, and into the coastlines that the rising sea washes against. The record heat of 2024-2025 rippled outward into all of these.
Marine Heatwaves and the Life Within
The record highs in the surface average did not express themselves evenly. Around them, events unfolded as punishing, long-lived marine heatwaves. In the tropical Atlantic and the Caribbean, sea surface temperatures climbed well above normal and stayed high for months, bringing on the conditions behind the widespread coral bleaching seen during both years. Reefs from the Caribbean to the Great Barrier Reef suffered large-scale coral bleaching, and the marine life that depends on corals — fish, invertebrates, and the fisheries that feed tens of millions of people — faced some of the most severe stress on record. Species that could not escape the heat either perished or slipped toward cooler water, reshaping the composition of marine ecosystems across entire ocean basins.
Storms Supercharged
Warm water is the fuel of tropical cyclones, and a hotter ocean supercharges them. Record ocean heat gives storms more energy to work with, sustaining the conditions for rapid intensification — the process by which a storm jumps a category or two in the space of a single day. In both 2024 and 2025, the unusually warm tropical seas supported late-season storms and explosive intensification, and they fed the flood-heavy rainfall that several hurricanes dumped over land. The heat did not cause storms on its own, but it stacked the deck in the atmosphere, supplying the extra energy that turns strong events into record-breaking disasters.
Sea Level and the Long Thaw
Ocean heat and gravity raise the sea. As the water warms it expands, and thermal expansion has been responsible for a large share of the global sea-level rise observed in recent decades. The record heat of 2024-2025 added to that expansion, and it drew ice melt from the polar sheets and mountain glaciers into the sea. Each incremental degree of stored ocean heat ensures the ocean keeps creeping higher along coastlines everywhere — for decades to come, even after emissions begin to fall.
How the Ocean Absorbs Heat
The ocean does not simply hold heat; it absorbs it. The seas have taken in the lion's share of the extra energy that greenhouse gases have pushed into the climate system over the past century, and because water has a far higher heat capacity than air, they can absorb an enormous amount of heat with only a modest rise in temperature. It is precisely because of that enormous heat capacity that the ocean acts as the planet's main thermal reservoir — and why a small change in ocean temperature holds as much energy as a far larger change in the atmosphere. The consequence is a lag: the ocean stores heat and releases it slowly, which is why the sea is likely to keep warming and keep delivering marine heatwaves for years even once the atmosphere begins to stabilize.
Lessons for the Future
The record warmth of 2024-2025 is not news in kind, but it is new in degree. It showed that the seams of the climate system are less stable than expected, and it brought into focus the physical connections linking the ocean's stored heat to marine life, to the intensity of hurricanes, and to the pace of sea-level rise. It also laid bare the central message of ocean climate science: the ocean has been absorbing the world's emissions, so the long-term course of ocean heat is set by the choices the world keeps making. The heat already absorbed will keep warming the ocean and pushing up the sea; how much additional heat the ocean must take on depends on how quickly emissions are cut. The sooner they fall, the sooner the ocean can level off — and the smaller the burden of record heat, marine and storm, for the generations that follow.
Frequently Asked Questions
Was 2024 the hottest year on record for the ocean?
Yes. The global average sea surface temperature and ocean heat content each reached record highs in 2024, and the records were matched or exceeded again in 2025, continuing a multi-year run of record ocean warmth.
Why did ocean temperatures jump so high in 2024 and 2025?
Decades of heat absorbed into the ocean is the backdrop, and the surge was amplified by the 2023-2024 El Niño, which released stored tropical Pacific heat. The ocean also cooled more slowly than expected after the El Niño, keeping temperatures above projections.
How does a warmer ocean affect hurricanes?
Warm water is the fuel for tropical storms. The extra ocean heat of 2024 gave storms more energy, and it was one factor behind the rapidly intensifying storms and higher wind speeds seen in those seasons.
What does the record heat mean for marine life?
It drives marine heatwaves and coral bleaching wherever the warm water persists, and it pushes fish species toward cooler waters, reshaping marine ecosystems, fisheries, and the livelihoods that depend on them.
Will ocean temperatures keep rising?
Year-to-year swings are driven by natural variability such as El Niño, but the long-term trend is set by emissions. The ocean will keep warming, keep experiencing marine heatwaves, and keep expanding until the heat added to the climate system falls to net zero.
Related Articles
El Niño and the 2025 Global Heatwave: A Perfect Storm — The warm Pacific pattern that helped push ocean heat to record highs.
Marine Heatwaves: When the Ocean Burns — The damaging ocean heat waves of recent years as a live backdrop to the global record.
Ocean Heat: The Hidden Driver of Extreme Weather — How stored ocean heat shapes storms, heatwaves, and the climate system.
Hurricanes and Tropical Storms: The Warming Connection — The warm sea that fuels ever-stronger storms.
The El Niño–La Niña Cycle: Earth's Climate Thermostat — The natural cycle behind the 2023-2024 El Niño.