The Ocean Is Absorbing Most of the Planet's Excess Heat

Most discussions of global warming focus on the atmosphere. Average surface temperatures, annual records, degrees above pre-industrial baselines. These are real measurements and they matter. But they describe the smaller part of the story. Since 1971, the world's oceans have absorbed more than 90 percent of the excess heat that human activity has added to the climate system. Without that absorption, the warming that land surfaces and the atmosphere have experienced would have arrived faster and more severely than it has. The ocean has been moderating the rate of change the whole time, and most of the change itself is happening out of sight, underwater.

The ocean's capacity to absorb heat is a function of water's physical properties. Water holds far more heat per unit of mass than air does, and the ocean is vast. Since 1971, the world's oceans have absorbed approximately 367 zettajoules of heat down to 2,000 metres of depth, a figure that continues to grow by roughly 6.8 zettajoules per year. A zettajoule is a unit most people have no intuitive feel for, which is part of why the scale of ocean warming is so difficult to communicate. One way to approach it: the heat the ocean has absorbed since 1971 is equivalent to detonating multiple Hiroshima-scale nuclear weapons every second for fifty years. The comparison is imperfect but it gives the number a human dimension that the raw figure does not.

This absorption is not passive. It reflects the ocean's thermal inertia, its tendency to take up heat slowly, hold it, and release it slowly. That inertia is why the planet has not warmed as fast as it would if the atmosphere were absorbing the same energy directly. The ocean is acting as a buffer, moderating the rate of atmospheric warming in ways that have made the consequences of greenhouse gas emissions less immediately visible than they would otherwise be. That moderation is real and it has bought time. It has also deferred consequences rather than eliminated them. Heat stored in the ocean does not disappear. It enters circulation patterns, influences weather systems, and eventually returns to the surface in ways that play out across decades rather than years.

The thermocline is central to understanding how this works. Below the warm surface layer of the ocean lies a zone of rapidly dropping temperature that acts as a boundary between the sun-warmed upper ocean and the cold deep water below. Heat absorbed at the surface does not move freely through this boundary. In most parts of the ocean, it accumulates in the upper layers where circulation, evaporation, and atmospheric exchange are most active. Where deep water formation occurs, particularly in the North Atlantic and around Antarctica, surface water does descend and carry heat with it into the deep ocean. These circulation pathways are slow, operating on timescales of centuries, and they mean that heat entering the ocean today will influence deep ocean temperatures long after the surface conditions that drove it have changed.

Marine heatwaves are one of the more visible consequences of the heat the ocean has already absorbed. Since 1982, the frequency of marine heatwaves has very likely doubled. They are also becoming longer, more intense, and more geographically extensive. The 2023 global ocean surface temperature reached 20.80 degrees Celsius, a record at the time, before 2024 exceeded it at 20.87 degrees, 0.51 degrees above the 1991 to 2020 average. These anomalies are not statistical abstractions. Marine heatwaves bleach coral reefs when temperatures exceed the thermal tolerance of the symbiotic algae that give corals their colour and their nutrition. They shift the distribution of fish species as populations follow the thermal ranges they require. They alter the productivity of upwelling zones that support some of the world's most important fisheries. They accelerate ice melt in polar regions where the relationship between ocean temperature and ice stability is direct and consequential.

The concept of committed warming sits underneath all of this. Because of the ocean's thermal inertia, some degree of additional warming is already locked in regardless of what happens to emissions in the near term. The heat that has entered the ocean system will continue influencing surface temperatures, weather patterns, and sea levels for decades. The relationship between current emissions decisions and future temperature outcomes is not immediate. Actions taken now affect conditions that will manifest across timescales that extend well beyond normal planning horizons in finance, infrastructure, or policy. That lag is one of the more important features of the climate system for anyone thinking seriously about long-term risk.

The fisheries consequences are already being documented in Canadian waters. Species that were not historically present on the Atlantic and Pacific coasts are appearing with greater frequency as thermal ranges shift northward. Lobster distributions have moved, cod recovery is complicated by changing prey availability and ocean conditions, and the timing of salmon migration is shifting in ways that affect both Indigenous food fisheries and commercial operations. These are not predictions. They are observations from the past decade that reflect conditions driven partly by ocean heat that was absorbed years or decades earlier.

The insurance and infrastructure implications follow the same logic. Coastal assets are designed around historical sea level, storm intensity, and temperature ranges. As ocean heat content continues to rise, those historical ranges become less reliable as planning assumptions. Storm systems draw energy from warm ocean surfaces, and the intensification of tropical storms over unusually warm water is one of the better-documented consequences of increased ocean heat content. Infrastructure built to withstand historical conditions is not necessarily built to withstand the conditions that the ocean's current heat trajectory will produce.

None of this means the situation is beyond response. The ocean's role as a heat buffer has moderated warming in ways that matter enormously. It also means that the atmosphere and surface conditions people experience today reflect only a portion of the energy that has entered the climate system. The rest is in the water, moving through circulation patterns, influencing the systems that fishing communities, coastal cities, and marine ecosystems depend on. Accounting for that properly is not a technical refinement. It is the difference between understanding the system and misreading it.