Earth’s core is cooling, but at a pace so slow it makes geological time feel rushed. The inner core grows as liquid iron in the outer core freezes onto it, and the best estimates put that growth rate at roughly a few millimeters per year. Full solidification of the outer core lies billions of years in the future, and the consequences of that cooling are profound: the loss of Earth’s magnetic field, the shutdown of plate tectonics, and eventually a planet far less hospitable to life. The twist is that the Sun will likely destroy Earth before the core finishes the job.
Where the Core’s Heat Comes From
Earth’s interior heat has two main origins. The first is primordial heat, left over from the planet’s formation about 4.5 billion years ago, when the energy of countless collisions and the compression of gravity heated the young Earth to extreme temperatures. The second is radiogenic heat, produced by the ongoing decay of radioactive elements like uranium, thorium, and potassium scattered throughout the mantle and possibly in the core itself. Together, these sources fuel mantle convection, plate tectonics, and the geodynamo that generates Earth’s magnetic field.1Earth and Planetary Science Letters. Quantifying Earth’s radiogenic heat budget
A third heat source matters specifically for the core: latent heat. As the outer core slowly cools, liquid iron crystallizes onto the solid inner core. That phase change releases energy, the same way water releases heat when it freezes. This latent heat, along with the gravitational energy released when heavier elements settle toward the center during crystallization, is a critical driver of convection in the outer core. In fact, modeling suggests that cooling alone may be too sluggish to drive the vigorous fluid motions needed for the geodynamo; the additional kick from latent heat release as the inner core grows is what keeps convection going.2Journal of Geodynamics. Latent-heat driven convection in the earth’s core: Numerical results
Whether the core itself contains significant amounts of radioactive potassium-40 has been debated for decades. Early thermal models of the core calculated that about 10 trillion watts of radioactive heating would be needed to power the dynamo on its own, which sits at the edge of what geochemists consider plausible.3Geophysical Journal International. Thermal evolution of the Earth’s core The current consensus is that radioactive decay in the core plays a secondary role at best, with cooling and crystallization doing most of the heavy lifting.
The Bottleneck at the Core-Mantle Boundary
The rate at which the core cools depends heavily on how fast heat can escape across the core-mantle boundary, the sharp interface roughly 2,900 kilometers below the surface where liquid iron meets solid rock. The mantle above acts like a thick, sluggish insulating blanket. Heat can only leave the core as fast as the mantle carries it away.
Estimates of the heat flowing across this boundary have increased over the past couple of decades. Current models place it somewhere between 5 and 17 terawatts, with many landing in the range of 13 to 15 terawatts.4Geophysical Journal International. Changes in core–mantle boundary heat flux patterns throughout the supercontinent cycle For context, Earth’s total surface heat flow is about 46 terawatts, so the core contributes roughly a quarter to a third of it.5Nature Geoscience. Core–mantle boundary heat flow The rest comes from radioactive decay in the mantle and crust.
Those revised-upward estimates have real implications. Higher heat flow out of the core means the core is cooling faster than older models assumed, which in turn means the inner core could be younger than previously thought, and that mantle plumes, the columns of hot rock that feed volcanic hotspots, play a bigger role in moving that deep heat to the surface.5Nature Geoscience. Core–mantle boundary heat flow
The Magnetic Field Depends on a Liquid Core
Earth’s magnetic field exists because of fluid motion in the liquid outer core. Electrically conducting molten iron, driven by convection, generates and sustains magnetic fields through a self-reinforcing process called the geodynamo.6PubMed. Earth’s core and the geodynamo The key ingredient is vigorous fluid flow. As long as the outer core remains liquid and keeps convecting, the magnetic field persists.
The relationship between core cooling and the dynamo is not straightforward, though. A certain amount of cooling is necessary: the temperature difference between the inner core and the core-mantle boundary drives the convection that powers the field. But if the core cooled so much that the outer core began to solidify significantly, the volume of convecting liquid would shrink, and eventually the dynamo would fail. The geodynamo needs a sweet spot: enough cooling to drive convection, but enough liquid iron to sustain it.
Plate tectonics also feeds back into this system. The style of tectonics on the surface affects how efficiently the mantle removes heat from the core. Models show that when thick layers of dense rock accumulate at the base of the mantle, they insulate the core, reducing heat flow and potentially weakening or killing the dynamo.7Geochemistry, Geophysics, Geosystems. Influence of plate tectonic mode on the coupled thermochemical evolution of Earth’s mantle and core Conversely, a sluggish style of tectonics in Earth’s early history may have actually helped sustain a thermally driven dynamo by keeping core heat flow in the right range.8PubMed Central. Coupled fates of Earth’s mantle and core: Early sluggish-lid tectonics and a long-lived geodynamo
How the Inner Core Got Started
The inner core has not always existed. Early in Earth’s history, the entire core was liquid. As it gradually cooled, conditions eventually allowed iron to begin crystallizing at the center, where pressures are highest. Exactly when this happened remains one of the bigger open questions in deep-Earth science.
One complication is that nucleating a crystal in the core is surprisingly hard. Mineral physics calculations suggest you need at least 450 degrees Kelvin of supercooling below iron’s melting point to spontaneously nucleate a solid crystal. But geophysical constraints suggest the actual supercooling available was probably less than 100 K, and at most around 420 K.9Nature Reviews Earth & Environment. The formation and evolution of the Earth’s inner core That gap between what seems to be needed and what was available has pushed researchers toward models where the inner core had at least two growth phases: an initial rapid burst that froze supercooled liquid at rates of centimeters per year, followed by the slower, steady crystallization at millimeters per year that continues today.9Nature Reviews Earth & Environment. The formation and evolution of the Earth’s inner core
Most estimates place the inner core’s age somewhere between half a billion and about 1.5 billion years, though some models push it younger still. Given that Earth itself is 4.5 billion years old, the solid inner core is a relatively recent addition. The dynamo ran on thermal convection alone for billions of years before inner core crystallization began supplying the extra energy from latent heat and gravitational settling.
What Happens When the Magnetic Field Fails
A planet without a strong magnetic field is far more exposed to its star. Earth’s magnetic field deflects most of the charged particles streaming from the Sun, the solar wind, and shields the atmosphere from being gradually stripped away. Lose the field, and two things get worse in a hurry: radiation exposure at the surface and long-term atmospheric loss.
The radiation angle is more immediate. During intense solar particle events, a weakened or absent magnetic field would allow far more energetic particles to penetrate the atmosphere. Modeling shows that with no geomagnetic field, a strong solar event would dump enough reactive nitrogen compounds into the atmosphere to severely deplete the ozone layer for years. Ground-level UV radiation would spike by 20 to 25 percent, and the rate of solar-induced DNA damage would rise by 40 to 50 percent.10PubMed Central. Global impacts of an extreme solar particle event under different geomagnetic field strengths That is not a civilization-ending event from a single storm, but sustained, repeated exposure over geological time would be a different story for surface ecosystems.
The atmospheric stripping side is slower but more existential. Without a magnetic field, the solar wind can interact directly with the upper atmosphere, ionizing gas molecules and accelerating them to escape velocity. On Mars and Venus, where this process has been studied by spacecraft, oxygen ions near the boundary where solar wind meets the atmosphere get picked up and carried away.11Monthly Notices of the Royal Astronomical Society. Mass, energy, and momentum capture from stellar winds by magnetized and unmagnetized planets: implications for atmospheric erosion and habitability Over hundreds of millions of years, this kind of erosion can thin an atmosphere substantially.
The End of Plate Tectonics and the Carbon Cycle
If the core cooled enough to shut down mantle convection, plate tectonics would grind to a halt. That sounds like a geological inconvenience, but it would fundamentally alter Earth’s surface chemistry. Plate tectonics drives the long-term carbon cycle: carbon dioxide is pulled from the atmosphere by weathering and carried into the mantle at subduction zones, then returned to the atmosphere through volcanic eruptions. Current estimates put volcanic carbon outgassing from mid-ocean ridges at roughly 23 million metric tons of carbon per year, with arc volcanoes adding another 21 million metric tons.12National Science Review. Deep carbon recycling viewed from global plate tectonics
Without that recycling, atmospheric carbon dioxide would gradually be drawn down by weathering reactions until levels dropped too low to sustain a greenhouse effect. The planet would cool. Alternatively, if volcanism sputtered out while large amounts of CO₂ were still in the atmosphere with no subduction to remove it, temperatures could soar. Either way, the thermostat that has kept Earth’s climate roughly stable over billions of years would be broken. Mars and Venus both lack plate tectonics today, and their wildly different surface conditions offer a preview of what can happen without this recycling engine.
Mars Already Went Through This
Mars is the closest thing we have to a case study. It once had a global magnetic field generated by a core dynamo, but that field died early, likely somewhere between 3.7 and 4.1 billion years ago.13Nature Communications. Weak magnetism of Martian impact basins may reflect cooling in a reversing dynamo Mars is smaller than Earth, so its core cooled faster. Once the dynamo shut down, the solar wind had direct access to the Martian atmosphere.
The consequences were dramatic. Estimates derived from Mars’s modern water chemistry suggest the planet lost a global layer of water somewhere between 80 and 1,500 meters deep.13Nature Communications. Weak magnetism of Martian impact basins may reflect cooling in a reversing dynamo The thick, potentially warm atmosphere that once allowed liquid water on the surface was stripped away over hundreds of millions of years, leaving the cold, thin atmosphere Mars has today. Whether the dynamo’s shutdown was the primary cause of atmospheric loss or merely removed one layer of protection while other processes did most of the work remains debated, but the correlation between losing the field and losing the atmosphere is hard to ignore.
The exact timing of Mars’s dynamo death also matters for understanding how quickly things go wrong. A dynamo that lasted until 3.7 billion years ago would have produced a very different atmospheric escape history than one that died before 4.1 billion years ago.13Nature Communications. Weak magnetism of Martian impact basins may reflect cooling in a reversing dynamo Pinning down that date has been one of the goals of recent Mars missions and magnetism studies.
The Sun Gets There First
Here is the part that changes the practical calculus. The Sun is a middle-aged star that will eventually exhaust its hydrogen fuel and swell into a red giant. Current stellar evolution models predict that Earth will be engulfed by the expanding Sun roughly 7.6 billion years from now, about 500,000 years before the Sun reaches the tip of its red giant phase.14Monthly Notices of the Royal Astronomical Society. Distant future of the Sun and Earth revisited Long before engulfment, increasing solar luminosity will boil off Earth’s oceans and make the surface uninhabitable, probably within one to two billion years.
The core’s cooling timeline, while uncertain, almost certainly extends well beyond when the Sun makes Earth’s surface a wasteland. The inner core has been growing for perhaps a billion years, and the outer core contains an enormous reservoir of thermal energy still being replenished by radioactive decay and released by ongoing crystallization. Projections for full solidification vary widely depending on assumptions about thermal conductivity, but most fall in the range of many billions of years. The core won’t fully cool in time for it to matter for life on Earth; the Sun’s evolution will render the question moot.
That said, the geodynamo could weaken or become unstable long before full solidification. The magnetic field is already declining, having dropped about 5 percent per century since continuous measurements began in 1840, though reconstructions suggest the field was roughly constant for the 250 years before that.15Science. Fall in Earth’s magnetic field is erratic The current decline appears to originate from patches of reversed magnetic flux in the Southern Hemisphere rather than from a global weakening of the dynamo itself. In other words, the field fluctuates and occasionally reverses, but those short-term variations are not signs that the core is running out of steam.
Why the Timeline Is Still So Uncertain
A frustrating reality about deep-Earth science is that the numbers feeding into core-cooling models remain contentious. The single most debated variable is the thermal conductivity of iron and iron alloys at the extreme pressures and temperatures found in the core. If core iron conducts heat efficiently, heat escapes faster, the inner core is younger, and the dynamo’s future is shorter. If conductivity is lower, everything runs slower.
Estimates for thermal conductivity at the core-mantle boundary range enormously. One set of experiments on iron-silicon alloys found conductivity of about 20 watts per meter per kelvin for a silicon-rich outer core composition.16Nature Communications. Low thermal conductivity of iron-silicon alloys at Earth’s core conditions with implications for the geodynamo Other experiments on pure iron and iron-silicon alloys yielded values two to three times higher, in the range of 41 to 145 watts per meter per kelvin.17Geophysical Research Letters. Electrical and thermal transport properties of iron and iron‐silicon alloy at high pressure That is not a minor discrepancy. A factor-of-three difference in conductivity feeds directly into how fast the core loses heat, how old the inner core is, and whether the dynamo could have been sustained by thermal convection alone through Earth’s early history.
Part of the disagreement traces to the experiments themselves. Measuring anything at core conditions, pressures above 100 gigapascals and temperatures above 3,000 K, requires diamond anvil cells, tiny devices that squeeze samples between the tips of two diamonds while heating them with lasers. Recent work has revealed that samples in these devices don’t behave as uniformly as assumed: they compress and expand unevenly, ending up 30 to 50 percent thinner than expected.18Geophysical Research Letters. Non‐Isotropic Contraction and Expansion of Samples in Diamond Anvil Cells: Implications for Thermal Conductivity at the Core‐Mantle Boundary Since conductivity calculations depend on sample geometry, that systematic error could explain some of the spread in published values. Until experimentalists settle on more reliable measurement techniques, the timeline for core cooling will remain fuzzy.
Magnetic Reversals and the Extinction Question
One long-standing question is whether magnetic field reversals, when the north and south magnetic poles swap, cause mass extinctions. The idea is intuitive: during a reversal, the field weakens substantially for thousands of years, leaving life more exposed to cosmic radiation. Researchers noticed correlations between the timing of some reversals and extinctions in the fossil record decades ago, but the evidence has never been convincing enough to establish a direct causal link. The radiation dose increase during a reversal is probably not large enough on its own to kill organisms, and marine creatures living under water would be shielded regardless.19GSA Bulletin. Possible Direct Causal Relation Between Geomagnetic Reversals and Biological Extinctions
More recent modeling has painted a subtler picture. A reversal combined with a powerful solar particle event could deplete ozone enough to raise UV levels significantly for years.10PubMed Central. Global impacts of an extreme solar particle event under different geomagnetic field strengths That kind of compound event, rare but not impossible over millions of years, might stress ecosystems enough to contribute to extinction pulses without being the sole cause. The field has reversed hundreds of times in Earth’s history, and life has persisted through all of them, so whatever damage reversals cause, it is not catastrophic on its own. The bigger concern would be a permanent loss of the field, which is a fundamentally different scenario from a temporary reversal.
Earth’s core, in short, is dying the slowest possible death. Every year, a thin shell of liquid iron freezes onto the inner core, the outer core convects a little less vigorously, and a few more terawatts of ancient heat leak through the mantle toward the surface. The process has been running for over four billion years and has billions more to go. For anyone wondering whether this should keep them up at night, the Sun’s expanding waistline is a far more pressing appointment on the geological calendar.