Where is the ocean heat coming from?
Something interesting is happening in the deepest parts of the ocean.
The abyssal ocean, the water ~4,000 to 6,000 meters below the surface, appears to be warming at an accelerating rate [1].
“Warming acceleration” means more than a steady rise in temperature. It means the rate of warming itself is increasing. The deepest water appears to be gaining temperature faster over time.
That creates a difficult physical question:
What process can produce this pattern so far below the surface?
The atmosphere can warm the ocean from above, and ocean circulation can carry that heat downward. The depth and shape of the observed signal still need to be explained quantitatively. A mechanism has to match the magnitude, depth, geography, and timing of the warming.
I’m working through five main possibilities.
1. Adiabatic compression
Could the deep water only appear warmer because it moved downward and got squeezed by increasing pressure?
Water warms slightly when it sinks because the pressure rises. This is called adiabatic compression. The effect does not add new heat to the water. It changes the temperature reading through reversible compression.
It’s similar to a bicycle pump warming as air is compressed. The material gets warmer even though no outside heat source has been added.
2. Redistribution and isopycnal heave
Could existing heat be shifting vertically as ocean density layers move?
The ocean is organized into layers of different density. Oceanographers often track surfaces of equal density, called isopycnals.
Those surfaces can move upward and downward. This motion is called isopycnal heave.
If you had several different density of blankets stacked on a bed, and you push one part of the stack upward or downward, a fixed point in the room may suddenly be occupied by a different blanket. The blanket at that height changed even though the blankets themselves did not necessarily gain heat.
A similar effect can happen in the ocean. A sensor at a fixed depth may record warmer water because a warmer density layer moved into that location.
This is a real redistribution mechanism, so it needs to be tested carefully.
I am tracking sigma-4 density surfaces through time, measuring their vertical displacement, and calculating the temperature change that their movement would produce at fixed depth.
The key comparison is:
temperature change at fixed pressure;
temperature change predicted from vertical heave;
temperature change while following the same density surface.
That last measurement is especially important. If the water warms even while we follow the same density surface, vertical movement cannot explain the entire signal.
3. Antarctic Bottom Water and overturning
Could changing deep-ocean circulation be carrying warmer water into the abyss?
Much of the coldest and densest water in the global ocean forms around Antarctica. This water sinks and spreads through the deepest ocean as Antarctic Bottom Water.
Changes in its formation, volume, temperature, salinity, or circulation could alter which water masses occupy the abyss.
For example, the deepest ocean could warm if:
less very cold Antarctic Bottom Water is being formed;
its upper boundary moves downward;
warmer deep water replaces some of the cold bottom water;
the overturning circulation changes its pathways or speed.
This would be more than simple up-and-down heave. It would involve changes in the amount and distribution of different water masses.
To test this, I need to track density-class occupancy, bottom-water thickness, temperature and salinity changes, and the movement of water-mass boundaries.
The salinity pattern is very important because circulation-driven replacement should leave a combined temperature, salinity, and density signature. A temperature-only match would be less convincing.
4. Geothermal and hydrothermal heating
Could some of the heat be entering the ocean from below?
Earth continuously releases internal heat through the seafloor. Heat can enter through:
ordinary geothermal conduction;
mid-ocean ridges;
hydrothermal vents;
volcanic regions;
tectonically active margins.
The main question is scale.
Hydrothermal vents can release extremely hot water locally, but the abyssal warming signal covers much larger regions. A geothermal explanation would need to provide enough total energy and reproduce the observed geography and depth pattern.
A seafloor source should also leave particular signatures. We might expect stronger warming near spreading ridges, vent fields, volcanic provinces, or regions with elevated geothermal flux.
This mechanism cannot be evaluated from temperature alone. It requires geographic comparisons, seafloor heat-flow estimates, bathymetry, ridge and vent locations, and a model of how bottom-generated heat spreads through the ocean.
The calculation must distinguish a genuine basal heat input from circulation that simply moves existing ocean heat toward or away from the seafloor.
5. Electromagnetic induction and ohmic heating
Could changing magnetic fields drive electric currents through Earth and the ocean, with some of that electrical energy turning into heat?
Seawater, ocean sediments, the crust, and the mantle all conduct electricity.
When Earth’s magnetic environment changes (e.g., during geomagnetic storms) it can induce electric fields and currents inside conducting materials.
Electrical resistance converts part of that current energy into heat. This is called ohmic heating or Joule heating.
The existence of induction is established physics. The open question is whether the amount, location, and persistence of the resulting heating are remotely large enough to matter for the abyssal ocean.
The energy could be deposited across several layers:
the ocean;
marine sediments;
the crust;
the lithosphere;
the mantle.
My test has to calculate how the induced currents vary with depth, frequency, conductivity, and geographic structure. It must then determine how much energy becomes heat, where that heat is produced, and whether any measurable fraction can reach or remain in the deep ocean.
We will need the electromagnetic forward solver I am building to complete this test.
The solver begins with a validated 1D spherical Earth model. It will track induction and ohmic heating through radially layered ocean, crust, lithosphere, and mantle structures. Later versions can add 3D conductivity variations and geographic detail.
Until that solver is complete and validated, the electromagnetic mechanism remains an open quantitative question.
One pattern, five tests
These mechanisms are not all stand-alone. Several processes could operate at the same time. Circulation could redistribute heat while density surfaces move. A changing water mass could shift both temperature and salinity. Small basal or electromagnetic inputs could exist without explaining the dominant signal.
The goal is to measure how much each mechanism can actually account for.
For every candidate, I am asking the same basic questions:
Does it produce the correct sign?
Is it large enough?
Does it occur at the correct depths?
Does it match the geography?
Does it reproduce the temperature, salinity, and density pattern?
Does it leave a large unexplained residual?
The abyssal ocean warming problem will not be solved by finding a physically possible story.
It requires a mechanism that survives testing.
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References
[1] Johnson, G. C. (2026). Observed multi-decadal acceleration of globally averaged abyssal ocean warming. Geophysical Research Letters, 53(14), e2026GL124104. https://doi.org/10.1029/2026GL124104
