World’s Oceans Absorbed A Record Amount Of Heat — What Happens Next Could Reshape Coastlines
A Warmer Ocean, A Higher Coastline Risk
Record Heat Beneath The Waves
The ocean is storing more heat than ever, and the consequences will not remain offshore.
The upper 2,000 metres of the world’s oceans gained an estimated 23 zettajoules of heat in 2025 compared with 2024. One zettajoule is one sextillion joules. The estimate carries an uncertainty of about plus or minus eight zettajoules, but several independent datasets point in the same direction: ocean heat content reached another record.
That matters because the ocean absorbs more than 90 per cent of the excess heat building up in Earth’s climate system. Air temperature is what people feel. Ocean heat is where much of the warming is stored.
The new record does not mean the sea suddenly became uniformly hotter everywhere. Nor does it mean that 23 zettajoules translates directly into a fixed number of centimetres of sea-level rise. What it does show is that Earth remains out of energy balance. More heat is entering the climate system than escaping back into space.
That imbalance is the engine beneath long-term global warming.
What 23 Zettajoules Actually Means
Twenty-three zettajoules is 23,000,000,000,000,000,000,000 joules of additional heat.
The figure is so large that everyday comparisons quickly become absurd. The important point is not the metaphor. It is the trend.
Ocean heat content has been climbing for decades, and the rate of warming in the upper ocean has accelerated. The 2025 increase came even though average sea-surface temperatures were slightly below the extraordinary level recorded in 2024.
That distinction matters.
Sea-surface temperature can move sharply from year to year as El Niño, La Niña, winds and currents redistribute heat. Ocean heat content looks deeper. It measures the accumulated energy stored through a much larger volume of water and is therefore a stronger indicator of the long-term planetary energy imbalance.
About a third of the global ocean area was among its three warmest years on record in 2025. More than half was among its five warmest.
The tropical and South Atlantic, Mediterranean, North Indian Ocean and parts of the Southern Ocean were among the regions showing exceptional heat.
This is why global warming is better understood through several independent measures rather than surface temperature alone. The atmosphere, oceans, glaciers, sea ice and sea level are all parts of the same system.
What This Means For Global Warming
The record does not mean global air temperature will immediately jump by a predictable amount next year.
Ocean heat works on longer timescales.
When greenhouse gases reduce the amount of heat escaping to space, the climate system warms until a new balance is reached. The ocean absorbs most of that extra energy. Some remains near the surface. Some is carried deeper by currents and mixing.
That stored heat can influence climate for years or decades.
It can raise the baseline from which marine heatwaves develop. It can affect evaporation and atmospheric moisture. It can alter large-scale circulation. It can also return some heat to the atmosphere during natural climate cycles.
This is one reason El Niño can push global temperatures sharply higher even though the underlying human-driven warming trend develops over much longer periods.
The crucial fact is that continued record ocean heat means warming has not stopped merely because a particular month or year is cooler at the surface.
If greenhouse-gas concentrations keep rising, the ocean will continue absorbing enormous amounts of energy.
Current policies still place the world on a path of roughly 2.8°C of warming over this century compared with pre-industrial levels, according to the latest United Nations assessment. Full implementation of national climate pledges would lower that projected range, but not enough to make the present ocean heat trend irrelevant.
The exact future remains a choice between pathways, not a single predetermined temperature.
Why Warmer Oceans Raise Sea Level
There are two major reasons a warmer climate raises the sea.
The first is thermal expansion.
Water expands as it warms. When heat penetrates the ocean, the same mass of seawater occupies slightly more volume. Spread across the entire ocean, small changes become significant.
The second is melting land ice.
Warmer air and warmer ocean conditions contribute to the loss of mountain glaciers and the Greenland and Antarctic ice sheets. Water that had been stored on land then enters the ocean.
Both processes matter.
Recent ocean assessments put global mean sea-level rise at about 4.2 millimetres a year over 2012 to 2025. That is faster than the longer-term rate measured since the end of the twentieth century.
Four millimetres sounds trivial until it accumulates year after year and combines with tides, waves and storm surges.
A coastline does not experience the average sea level in isolation. It experiences high tides, storms, wave run-up, river flooding and sometimes sinking land on top of that higher baseline.
A small permanent rise can therefore create a much larger increase in the frequency of extreme coastal water levels.
What Happens To Coastlines
The first major effect is more frequent flooding.
A storm surge that once peaked just below a sea wall can begin overtopping it. A high tide that once remained in a drainage channel can reach roads or buildings. Flood events that were historically rare can become much more common as the starting sea level rises.
The second effect is erosion.
Higher water allows waves to attack beaches, dunes and cliffs further inland. The response varies from place to place. Some coastlines can shift naturally. Others are trapped by roads, houses, sea walls or ports.
The third effect is saltwater intrusion.
As sea level rises, salt water can penetrate further into estuaries, groundwater and low-lying farmland. That can affect drinking-water supplies, agriculture and coastal ecosystems even before permanent inundation occurs.
The fourth effect is the growing cost of protection.
Major cities can build barriers, raise infrastructure and improve drainage. Smaller communities may face harder choices between repeated repairs, new defences and managed retreat.
That is why the phrase “cities lost to climate change” can be misleading if taken literally. The more realistic risk for many places is a gradual loss of affordability, insurability, infrastructure reliability and habitable land before a city is ever physically submerged. Some of the world’s most exposed coastal cities already face that calculation.
How Much Could Sea Level Rise By 2050?
The near-term range is narrower than the end-of-century range because much of the warming and ice response over the next few decades is already built into the system.
The Intergovernmental Panel on Climate Change projects a likely global mean sea-level rise by 2050 of roughly 15 to 23 centimetres under a very-low-emissions pathway and about 20 to 29 centimetres under a very-high-emissions pathway, relative to the 1995–2014 average.
The difference between emissions scenarios becomes much larger later in the century.
That means coastal adaptation cannot simply wait for perfect certainty about 2100. Much of the next few decades of sea-level rise is already difficult to avoid.
The practical question is how much additional rise is added after that.
What Are The Forecasts For 2100?
Under the IPCC’s very-low-emissions scenario, global mean sea level is projected to rise by around 28 to 55 centimetres by 2100 relative to 1995–2014.
Under an intermediate pathway, the likely range is about 44 to 76 centimetres.
Under a very-high-emissions scenario, the likely range rises to around 63 centimetres to 1.01 metres.
Those are global averages.
Local sea level can rise faster or slower because of ocean circulation, gravity, changing ice masses and vertical land movement. A sinking delta can experience far greater relative sea-level rise than the global mean. Land still rebounding after the last ice age can partly offset it.
The IPCC also warns that higher outcomes cannot be ruled out if poorly understood ice-sheet processes accelerate. Those possibilities are less likely, but their consequences would be much larger.
This is where climate forecasting becomes risk management rather than a simple best-guess number.
What About The UK?
Britain will not experience sea-level rise evenly.
The south of the country is generally more exposed to relative sea-level rise than parts of Scotland because the land itself is moving differently.
UK climate projections give a broad illustration of the range.
For London, projected sea-level rise by 2100 relative to the 1981–2000 average spans roughly 29 to 70 centimetres under a low-emissions scenario and 53 centimetres to 1.15 metres under a high-emissions scenario.
For Cardiff, the corresponding high-emissions range reaches about 51 centimetres to 1.13 metres.
For Edinburgh and Belfast, the projected ranges are lower because land movement partially offsets some of the global rise.
The numbers do not mean those cities will simply be underwater at the stated level. Flood barriers, tidal geometry, local elevation and future adaptation all matter.
But higher mean sea level raises the height from which every future storm tide begins.
That is the important part for planning.
Could Coastlines Move Dramatically This Century?
In some places, yes.
Low-lying islands, river deltas, eroding soft cliffs and subsiding coastal cities can see major physical change long before the global average rise approaches a metre.
Elsewhere, engineered defences may hold the coastline close to its present position for decades.
The distinction is economic as much as geological.
It is technically possible to defend many valuable urban areas against significant sea-level rise. The question is whether governments continue funding those defences, whether they can be upgraded fast enough, and what happens to less economically valuable stretches of coast.
The world is unlikely to wake up one morning to find coastlines suddenly redrawn.
The more plausible future is a long retreat in some places, heavy engineering in others, and increasingly frequent disputes over which communities receive protection.
Why The New Ocean Heat Record Matters
The 23-zettajoule increase is not a standalone disaster forecast.
It is something more fundamental.
It shows that the planet is still accumulating heat at extraordinary scale.
That heat expands seawater. It raises the energy available to marine heatwaves. It places additional stress on coral reefs and ecosystems. It interacts with the atmosphere. It remains stored in the climate system long after a temporary cool spell passes across the surface.
Most importantly for coastlines, ocean warming helps guarantee that sea-level rise does not stop when air temperatures fluctuate.
Even if global temperature eventually stabilises, sea level can continue rising for centuries as the deep ocean adjusts and ice sheets respond.
That is why the climate argument increasingly centres on how much warming is still avoided, rather than whether all further change can be stopped immediately.
The future coastline is not fixed.
But every additional increment of warming pushes the starting line further inland.
Sources
Advances in Atmospheric Sciences — Ocean Heat Content Sets Another Record in 2025 — Peer-reviewed estimate that upper-2,000-metre ocean heat content increased by about 23 ± 8 zettajoules in 2025.
Intergovernmental Panel on Climate Change — Chapter 9: Ocean, Cryosphere and Sea Level Change — Global sea-level projections to 2050, 2100 and beyond, including uncertainty from ice-sheet processes.
Mercator Ocean International — Starfish Barometer 2026 Press Release — Recent ocean indicators including the 2012–2025 sea-level rise rate and marine heat stress.
Next Reads
Global Warming Explained: Evidence, Failed Predictions, Politics and the Future — A wider evidence-led guide to what climate science shows, where uncertainty remains and how future warming is projected.
10 Cities That Could Be Lost to Climate Change – And What Their Disappearance Would Mean — A closer look at coastal exposure, subsidence, flood protection and the cities most vulnerable to rising seas.
El Niño and Climate Change — What It Means for the World — Explains how natural Pacific climate cycles interact with a warmer ocean and atmosphere.
