SCIENCE · WHAT IF · PART 3 OF 3

What Survives a Year Without the Sun?

October 2026

Also on X: watch the briefing on X.

A frozen museum — but the air still breathes

First, a correction to the obvious mental image. A year into the dark, the surface would be brutally cold — the commonly cited estimate is an average near −100°F — but the air itself would not have frozen. That detail matters, and the physics is simple: liquefying nitrogen and oxygen takes temperatures far below anything the first year delivers. Air freezing out of the sky belongs to a stable state millions of years away, at something like −400°F, in Caltech's modelling. Within the year of this scenario, the atmosphere stays an atmosphere. What freezes is the surface — and what survives gathers where the cold can't reach.

The world that never saw the sun

Drop two kilometres down to the ridges in the middle of the ocean, where the seafloor splits and volcanic vents — black smokers — pour out water that can reach 400 degrees Celsius. They were discovered only in 1977, by scientists in the submersible Alvin at the Galápagos Rift, and what they found rewrote biology: whole ecosystems with no connection to sunlight at all. Bacteria there live by oxidising chemicals such as hydrogen sulphide — a process called chemosynthesis — and those bacteria feed tube worms, clams and shrimp in the black water around the vents. Photosynthesis's total shutdown, the catastrophe of Parts 1 and 2, is simply not an event in that world. It never ran on the sun. It would not even notice the dark.

The patient survivors

Land keeps its own reserves. Microbes living deep in Earth's crust would survive, in ground that stays warm long after the surface freezes, because heat escapes the interior only slowly. Seeds sleep in the soil, built by evolution to wait out bad years — though a year is at the far edge of the design. And the great trees hold the most surprising line in the whole scenario: thanks to slow metabolism and substantial stores of sugar, large trees could survive for several decades without sunlight, standing in the dark long after the forests around them became a museum. Survival, it turns out, mostly belongs to whatever learned to wait.

Our lifeboat is the same fire

And humans? The experts who model this scenario point to the same resource the vent worlds use: Earth's internal heat. Nuclear- and geothermal-powered habitats are the survival options they name, and the prototype already exists. Iceland heats roughly nine in ten of its homes with geothermal energy and draws about 30% of its electricity from it — a working demonstration that a society can run on the planet's fire instead of the sun's. In the dark, places like Iceland stop being picturesque and become arks. The sun built almost everything on the surface. But Earth's own fire was always running a second, quieter world underneath — and for one year, that world is the one that lasts.

The takeaway

• After a year, estimates put the average surface temperature near minus 100 degrees Fahrenheit — yet the air itself would not freeze; that takes millions of years.
• Deep-sea hydrothermal vents, discovered in 1977, host ecosystems built on chemosynthesis — wholly independent of sunlight, untouched by the blackout.
• Vent fluid can reach 400 degrees Celsius, and the tube worms, clams and shrimp around the black smokers would carry on unchanged.
• On land: microbes in the warm crust, seeds in the soil, and great trees that could live for decades on stored sugar.
• Humanity's best refuge is geothermal — Iceland already heats nine in ten homes with the Earth's own heat.

So tell us — the sun comes back after exactly one year. Does humanity recover, or never again?

The full trilogy: Part 1 — What If the Sun Went Out? and Part 2 — The Year Earth Froze Start at the eight-minute warning, end at the ice lid that saved the sea — then come back and tell us whether humanity makes it.