Somewhere beneath your feet, roughly 5,150 kilometres down, sits a ball of iron and nickel the size of the Moon. It's about 5,400°C. That's hotter than the surface of the sun, which feels like an aggressive design choice for the inside of a planet people are trying to live on.
But that heat is the only reason you're alive.
The Earth's core does two things that matter. It drives convection currents in the liquid outer core, which generates the magnetic field. And it keeps the mantle warm enough to flow, which powers plate tectonics. Lose the core's heat and you lose both. What follows is a slow, quiet apocalypse.
The magnetic field goes first
Earth's magnetic field exists because the liquid outer core is churning. Molten iron and nickel, driven by heat from the solid inner core below, circulates in enormous convection cells. This motion generates electric currents, and those currents produce the magnetosphere. It's called a geodynamo, and it's been running for at least 3.45 billion years.
Cool the core, and the outer core stops circulating. No circulation, no dynamo. No dynamo, no magnetic field.
This doesn't happen overnight. The core has staggering thermal inertia. Even if you could magically switch off the heat source, the outer core would take millions of years to solidify completely. But the weakening would begin within centuries. The field would become patchy, inconsistent, flipping polarity erratically before fading altogether.
We've actually seen the early signs of this. The South Atlantic Anomaly is a region where the magnetic field is already significantly weaker than expected. Satellites passing through it report increased radiation interference. It's a hole in the armour, and it's been growing.
What the magnetic field actually blocks
The sun is constantly throwing charged particles at us. Protons, electrons, alpha particles, all travelling at several hundred kilometres per second. This is the solar wind, and on a calm day it carries about a million tonnes of matter per second past our planet.
The magnetosphere deflects almost all of it. Without it, that stream hits the upper atmosphere directly.
First, the charged particles strip away atmospheric molecules. Not dramatically, not in a single blast, but steadily. Hydrogen goes first because it's lightest. Then helium. Over geological time, heavier molecules follow. The atmosphere thins. Air pressure drops.
There's a planet that's already been through this process. Mars.
Mars is the worked example
Mars once had a magnetic field. It had a liquid core, convection currents, the whole setup. About four billion years ago, the core cooled enough for the dynamo to shut down. We know this because the Martian crust still carries magnetic signatures from that era, frozen into ancient volcanic rocks like a geological fossil of a field that no longer exists.
After the field collapsed, the solar wind went to work. Mars's atmosphere, which scientists believe was once thick enough to support liquid water on the surface, was gradually stripped away. Today the atmospheric pressure on Mars is about 0.6% of Earth's. You can't have liquid water at that pressure. You can't breathe. You can barely call it an atmosphere at all.
The water evaporated, broke down into hydrogen and oxygen in the upper atmosphere, and the hydrogen was carried away by solar wind. Some water is still there, locked in ice at the poles and possibly underground, but the surface is bone-dry.
That's Earth's future without a core. Not tomorrow, not next century. But eventually.
Plate tectonics stops
Convection in the mantle drives the movement of tectonic plates. The mantle itself isn't liquid (a common misconception), but over millions of years it behaves like an extremely viscous fluid. Hot material rises from near the core, spreads along the base of the crust, cools, and sinks back down. This convection is what pulls oceanic plates apart at mid-ocean ridges and pushes them together at subduction zones.
Cool the core, cool the mantle. Cool the mantle, and this circulation slows and eventually stops.
No plate tectonics means no new mountains. No volcanic activity, which means no fresh minerals brought to the surface. No subduction, which means the carbon cycle breaks. Carbon dioxide that gets absorbed into rocks through weathering normally gets recycled back into the atmosphere through volcanic emissions. Without that return path, atmospheric CO₂ drops over millions of years. Less CO₂ means less greenhouse warming.
The planet cools from the outside too.
The carbon problem compounds
This is where it gets properly grim. Without volcanism recycling carbon, the long-term carbon cycle (which operates on timescales of 100,000 to 500,000 years) breaks down entirely. Chemical weathering of silicate rocks continues to pull CO₂ from the air, but nothing puts it back.
Atmospheric CO₂ concentration drops. Global temperatures follow. Ice sheets advance. More ice means more sunlight reflected back into space, which means more cooling, which means more ice. This is a positive feedback loop, and without volcanic CO₂ emissions to act as a thermostat, there's no natural mechanism to stop it.
Earth has been through something like this before. The Snowball Earth events, around 700 million years ago, saw ice sheets extending to the equator. The planet recovered because volcanic activity eventually pumped enough CO₂ into the atmosphere to trigger warming. Without that safety valve, a Snowball Earth becomes permanent.
Could life survive any of this?
Surface life would be in serious trouble long before the atmosphere fully stripped away. The combination of increased radiation (no magnetic field), falling temperatures (no volcanic CO₂ recycling), and reduced atmospheric pressure creates conditions that most complex organisms simply can't handle.
But life underground might persist for a remarkably long time. Deep subsurface ecosystems, the kind we've found in mines and boreholes kilometres below the surface, don't rely on sunlight or atmospheric oxygen. They run on chemosynthesis, feeding on chemical reactions between water and rock. Some of these organisms have been living in isolation for millions of years already.
As long as there's liquid water somewhere below the surface, and enough residual heat to keep it liquid, these microbial communities could carry on. They'd outlast everything on the surface by aeons. The last living things on a dying Earth would be bacteria in rock cracks, completely unaware that the sky had disappeared.
How long do we actually have?
The honest answer is that Earth's core is cooling. It has been since the planet formed. The inner core is growing by roughly a millimetre per year as the outer core solidifies onto it. A 2022 study from ETH Zurich found that the core may be cooling faster than previously estimated, based on thermal conductivity measurements of bridgmanite, the most abundant mineral in the lower mantle.
But "faster than expected" in geological terms is still absurdly slow by human standards. The core has enough thermal energy to keep the dynamo running for at least another billion years. Probably longer. The sun will become a red giant and swallow the Earth in about five billion years, so the core cooling is unlikely to be what gets us.
Still. It's a strange comfort to know that the thing keeping you alive right now is a slowly cooling ball of iron you'll never see, generating an invisible shield you'll never feel, against radiation you can't detect. And it won't last forever. Nothing powered by leftover heat ever does.