Earth’s surface would plunge below freezing within about a week if the sun suddenly vanished, and average global temperatures would fall to roughly −70°C (−100°F) within a year. Most humans outdoors without shelter would die of hypothermia in hours once temperatures crashed, but the scenario is not instant extinction. Pockets of humanity huddled around geothermal heat sources, nuclear reactors, and deep underground shelters could plausibly survive for months or even a few years, depending on how well they managed fuel, food, and their own psychology.
How Fast the Cold Arrives
The sun delivers about 1,360 watts of energy per square meter to the top of Earth’s atmosphere, and roughly half of that reaches the surface after filtering through clouds and air. Remove that input and the planet becomes a body radiating heat into space with nothing to replace it. Earth’s atmosphere holds some thermal inertia, particularly the oceans, which store enormous amounts of heat. The first few days would feel like an extended night with temperatures sliding steadily. Within a week, most land surfaces would be below 0°C. Within a couple of months, even equatorial regions would experience temperatures well below freezing. After a year, the global average surface temperature would settle somewhere around −70°C to −100°C, depending on how quickly the oceans ice over and trap their remaining warmth beneath an insulating crust.
The oceans would freeze from the top down, but this process is slower than you might expect. Seawater freezes at about −1.8°C, and as the surface layer freezes, the ice itself acts as an insulating blanket that slows further heat loss from the water underneath. Ice growth in polar seas already follows this pattern, where new ice formation in open water areas is thickened by dynamic processes and thermodynamic growth beneath existing cover.1Annals of Glaciology. Enhanced thermodynamic ice growth by sea-ice deformation Without the sun, the entire ocean surface would eventually freeze over, but the deep ocean would remain liquid for thousands of years thanks to residual heat, geothermal heating from the seafloor, and the insulating ice above. The deep sea would become the planet’s last significant reservoir of liquid water and warmth.
What Happens to Your Body in Extreme Cold
When your skin senses falling temperatures, your body’s first line of defense is to constrict blood vessels near the surface, reducing blood flow to your skin and extremities to keep your core warm. If that is not enough, you start shivering, which generates metabolic heat through rapid involuntary muscle contractions.2PubMed. Human physiological responses to cold exposure: Acute responses and acclimatization to prolonged exposure These defenses buy time, but they are limited. Once your deep core temperature drops to about 30°C (roughly 4°C below normal), survival models treat that as the threshold where death becomes likely.3PubMed. Predicting survival time for cold exposure Below that point the heart becomes dangerously unstable and organs begin failing.
How quickly you reach that threshold depends on clothing, shelter, wind, and moisture. A person standing outdoors in light clothing at −40°C could reach lethal hypothermia in under an hour. With proper cold-weather gear and no wind, survival extends to many hours. With insulated shelter and a heat source, you can survive indefinitely at those temperatures, which is exactly what people in Arctic communities do every winter. The question in a sunless world is not whether cold kills, but whether humanity can maintain enough sheltered, heated space to keep a population alive while everything else falls apart.
The Food Problem
Losing sunlight does not just mean losing warmth. It means losing photosynthesis, and with it the foundation of almost every food chain on the planet. Within days of going dark, plants begin dying. Crops in fields would freeze and die even faster. The world’s grain reserves, stockpiled in silos and warehouses, would buy some time. Global grain stocks typically amount to a few months of consumption. Canned and preserved foods would last longer, but distribution would become nearly impossible as transportation infrastructure froze and fuel supplies dwindled.
Livestock would be among the first casualties. Farm animals need feed, water, and temperatures above certain thresholds to survive. Without heated barns and continuing feed supplies, cattle, poultry, and pigs would die within days to weeks. Wild animals in cold-adapted ecosystems would fare somewhat better initially, since many Arctic and sub-Arctic species have biological adaptations for extreme cold and darkness. But even they depend on food webs rooted in photosynthesis. Herbivores would starve as plant matter disappeared under ice. Predators and scavengers would follow. The timeline for most terrestrial ecosystems to collapse is measured in weeks to months.
Any surviving human community would need to grow food without sunlight. Artificial lighting powered by nuclear or geothermal energy could theoretically support indoor agriculture. Even modest hydroponic operations can produce leafy greens and some root vegetables under LED lights. The challenge is scale. Feeding even a small community year-round requires substantial energy input and infrastructure that would be extraordinarily difficult to build and maintain during a global freeze.
Vitamin D and the Biology of Permanent Darkness
Beyond cold and hunger, the absence of sunlight would attack human health in subtler ways. Your skin produces vitamin D when exposed to ultraviolet B radiation from the sun, and that vitamin plays a central role in calcium and phosphate metabolism, bone health, and immune function. Research has linked vitamin D deficiency with increased risk of autoimmune diseases, certain cancers, cardiovascular disease, and infectious illness.4PubMed Central. Sunlight and Vitamin D: A global perspective for health In a world without sunlight, every human would become vitamin D deficient unless they had access to supplements or fortified foods. Existing supplement stockpiles would not last forever, and manufacturing new ones would require functioning chemical industry, an unlikely luxury in a frozen world.
Children would be hit hardest. Without adequate vitamin D, growing bones do not mineralize properly, leading to rickets and skeletal deformities. Adults would face osteomalacia (softening of bones) and increased susceptibility to fractures and infections. Over the span of months, a surviving population without supplementation would become progressively weaker and more vulnerable to disease.
Psychological Toll of Endless Night
Humans are not just physically dependent on sunlight. Our circadian rhythms, mood regulation, and sleep architecture are all tuned to the daily cycle of light and dark. We have direct evidence of what prolonged darkness does to people from polar research stations, where winter brings months of continuous night. At China’s Zhongshan Station in Antarctica, researchers found that expeditioners experienced delayed circadian rhythms, shifted sleep phases, and increased rates of subsyndromal seasonal affective disorder during the polar winter.5PubMed. Circadian Rhythm and Sleep During Prolonged Antarctic Residence at Chinese Zhongshan Station
At a Polish research station on Svalbard, mission duration showed a moderate influence on hostility and depression among crew members, with both measures tending to increase over the course of the stay.6Springer Polar Sciences. Well-Being at the Polish Polar Station, Svalbard: Adaptation to Extreme Environments These findings come from small, well-supported teams who know their isolation is temporary and who have reliable heat, food, and communication with the outside world. Strip away those comforts and multiply the stress by existential uncertainty, and the psychological deterioration would be far more severe. Depression, insomnia, interpersonal conflict, and cognitive decline would become serious threats to group cohesion and decision-making, precisely when clear thinking matters most.
Artificial lighting can partially substitute for sunlight in regulating circadian rhythms, particularly bright full-spectrum light timed to simulate a normal day-night cycle. Submarines and space stations use this approach. But maintaining such systems requires consistent electrical power and functioning equipment, another link in the chain that would need to hold for survivors to stay mentally functional.
Where Survivors Might Hold Out
Not every spot on Earth would become equally inhospitable. The most promising refuges for long-term survival would cluster around two types of energy source: nuclear and geothermal.
Nuclear power plants, if kept operational, could provide electricity and heat for surrounding communities for years. A typical reactor contains enough fuel for 18 to 24 months of operation before refueling. The world has several hundred operational reactors. The catch is that nuclear plants require skilled operators, cooling water, and functioning control systems. Many plants near coastlines would face problems as seawater froze and intake systems failed. Inland plants near large lakes or rivers might fare better in the early months, while liquid water remained available beneath ice.
Geothermal areas like Iceland, Yellowstone, parts of New Zealand, and the East African Rift would become humanity’s most naturally heated real estate. In Iceland, geothermal energy already heats about 90% of homes and provides electricity. That infrastructure would keep working without the sun, since the heat comes from Earth’s interior, not from solar input. Communities in these areas would have warmth and power, though they would still need to solve the food problem through artificial agriculture and would still face vitamin D deficiency and psychological strain.
Deep underground facilities, mine shafts, and bunkers would also retain above-freezing temperatures, since rock a few hundred meters below the surface stays at a relatively stable temperature regardless of surface conditions. The deeper you go, the warmer it gets, rising by roughly 25°C per kilometer of depth. A community living at depth with a nuclear or geothermal power source, stored food, and some form of indoor agriculture could theoretically persist for years.
Life That Does Not Need Sunlight
While most of Earth’s life depends on photosynthesis, there are entire ecosystems that run on chemical energy instead. Deep-sea hydrothermal vents support thriving communities of bacteria, tube worms, clams, and shrimp, all powered by the chemical energy in hot, mineral-rich fluids gushing from the seafloor. These chemosynthetic communities use compounds like hydrogen sulfide as an energy source the same way plants use sunlight. Research at sublacustrine hydrothermal vents in Yellowstone Lake has documented distinct chemosynthetic bacterial communities thriving at temperatures from 16°C to 110°C, with sulfur-oxidizing bacteria fixing carbon in complete darkness at rates that support productive microbial ecosystems.7PubMed Central. Microbial communities and chemosynthesis in yellowstone lake sublacustrine hydrothermal vent waters
In a sunless world, these chemosynthetic ecosystems would be the only ones to carry on largely unchanged. The deep ocean, kept liquid by geothermal heat beneath a kilometers-thick ice cap, would still have active hydrothermal vents pumping hot fluid and chemical energy into the water. Microbial life around those vents would continue. Whether the more complex vent animals would survive long-term is less certain, since some depend on oxygen produced by surface photosynthesis that dissolves into the deep ocean over time. Without that resupply, dissolved oxygen in the deep sea would gradually deplete over centuries, potentially limiting what could live there. But the microbes themselves, many of which are anaerobic, would persist for geological timescales.
Could Humans Tap Chemosynthetic Ecosystems?
In theory, a surviving human community near a geothermal area could attempt to farm chemosynthetic organisms for food. In practice, this would be extraordinarily difficult. The bacteria that form the base of vent ecosystems are not exactly appetizing or calorie-dense, and scaling up their cultivation would require specialized bioreactors, chemical feedstocks, and engineering expertise that would be hard to maintain during civilizational collapse. It is more realistic to think of chemosynthetic ecosystems as a proof of concept, evidence that biology can persist without solar energy, rather than a practical food source for humans.
A more plausible approach would be growing conventional food crops under artificial light, supplemented by mushroom farming (fungi do not need light to grow) and possibly algae cultivation in heated tanks. Mushrooms can break down dead organic matter, so a community with access to wood, agricultural waste, or other cellulose-rich material could produce some calories through fungal cultivation even without electricity for grow lights. Algae, particularly spirulina, can be grown in relatively simple setups and provides dense nutrition including protein, vitamins, and minerals.
Rogue Planets and the Long View
Earth without the sun is not purely hypothetical in cosmic terms. The galaxy contains a large population of free-floating, or “rogue,” planets that have been ejected from their parent star systems and wander through interstellar space without any stellar warmth. Some of these planets may retain moons, and recent modeling suggests that for a meaningful fraction of such moons, tidal heating from the planet’s gravitational pull could maintain liquid water oceans beneath an ice crust. In roughly 12 to 15 percent of modeled cases, moons orbiting close to their rogue planet host experienced tidal heating comparable to what we estimate for Europa or Enceladus in our own solar system, and the eccentricity that drives that heating can persist for billions of years.8Astrobiology. Free-floating (rogue) planets are thought to be numerous in the Galaxy and may retain their moons after ejection from their natal systems.
This means that sunless worlds with liquid water are not rare freaks of the universe. They may be common. If life can arise and persist around chemical energy sources in the dark, as it clearly does at Earth’s own hydrothermal vents, then rogue planet moons represent a vast and almost entirely unexplored category of potentially habitable worlds. For the question of how long humans could survive without the sun, the broader lesson is that sunlight is essential for life as we currently live it, with open-air agriculture, natural ecosystems, and vitamin D from the sky, but it is not strictly essential for life itself.
Realistic Timelines for Human Survival
Pulling together the cascading failures, the survival timeline for different groups of humans looks roughly like this:
- Unprotected outdoors: Hours to days, depending on latitude, clothing, and local temperatures at the moment the sun disappears.
- In conventional homes: Days to weeks. Without centralized heating (most of which depends on fuel supply chains that would quickly break down), insulated houses would slowly cool toward ambient outdoor temperatures. People with firewood or other combustible fuel could hold out longer.
- Near nuclear plants or geothermal sources: Months to a few years. Heat and electricity could sustain small communities, but food supplies would become the binding constraint unless indoor agriculture could be scaled up quickly.
- In deep underground bunkers with power: Potentially several years, limited by food stocks, equipment failure, and psychological endurance.
The absolute outer edge of human survival, assuming the best-case scenario of a well-organized community near a geothermal source like Iceland with functioning greenhouses, stored food, supplement stockpiles, and strong social cohesion, might stretch to a decade or somewhat beyond. But this would require everything to go right, an unlikely proposition given the sheer scale of disruption. More realistically, the last humans would probably die within a few years, done in by some combination of starvation, equipment failure, disease in a weakened and vitamin-depleted population, or social breakdown under the relentless psychological pressure of permanent darkness and cold.
Why the Oceans Outlast Us
The irony of the scenario is that while human civilization would collapse almost immediately and our species would likely be extinct within years, the planet itself would remain biologically alive for a very long time. The deep oceans, sealed beneath a thick ice shell, would retain liquid water and geothermal energy for millennia. Chemosynthetic microbial communities at hydrothermal vents would carry on, oblivious to the catastrophe above. Earth would become something like a frozen version of Europa: a world where all the interesting biology happens beneath the ice, far from a surface that no longer supports complex life. The planet’s last living organisms would be bacteria clustered around volcanic vents on the ocean floor, doing exactly what their ancestors have done for billions of years, harvesting chemical energy in the dark.