A full-scale nuclear war between major powers would kill hundreds of millions of people in the first hours, then billions more over the following months and years from famine, radiation, and environmental collapse. The immediate blasts and firestorms would be catastrophic, but the longer-term effects on climate, food production, and the atmosphere would cause far more death than the weapons themselves. Here’s what the science projects would happen, step by step.
The First Hours: Blast, Heat, and Radiation
Modern nuclear warheads are primarily targeted at military installations, government centers, and infrastructure. Each detonation produces a fireball hot enough to ignite everything within several miles, a pressure wave that flattens buildings, and an initial pulse of radiation lethal to anyone nearby without heavy shielding. In a full exchange between the United States and Russia, which together hold roughly 90% of the world’s nuclear weapons, hundreds of warheads would detonate across both countries and allied nations within the span of an hour or two.
People within the immediate blast zones would die instantly. Those farther out would face severe burns, collapsed structures, and radiation exposure. At doses above 6 gray (a unit measuring absorbed radiation), symptoms begin within 30 minutes: vomiting, severe diarrhea, high fever, and headache. Without advanced medical care, doses this high are almost always fatal within a week. At lower doses of 1 to 4 gray, survival is possible with treatment, but in a post-nuclear environment, hospitals and supply chains would be destroyed across wide areas. Most people with serious radiation injuries would go untreated.
Fallout: The Invisible Threat That Follows
Within minutes of each detonation, irradiated debris is pulled up into the mushroom cloud and begins drifting downwind as fine, radioactive dust. This fallout can spread hundreds of miles, coating surfaces and contaminating water and food supplies. The danger is highest in the first 48 hours. A rule of thumb used in civil defense planning, called the 7:10 rule, helps illustrate the decay: for every sevenfold increase in time after detonation, the radiation exposure rate drops by a factor of ten. So if fallout produces 1,000 units of radiation per hour at one hour after the blast, it drops to about 100 at seven hours and 10 at 49 hours.
This means sheltering in a well-shielded building (basements, concrete structures, interior rooms away from windows) for at least the first two days dramatically reduces exposure. After two weeks, outdoor radiation levels from fallout drop to roughly 1% of their initial intensity. The practical problem in a full-scale war is that fallout zones would overlap across entire regions, and millions of people would have no access to adequate shelter, clean water, or uncontaminated food.
Nuclear Winter: How the Climate Shifts
The firestorms ignited by hundreds of nuclear detonations would loft massive quantities of soot and smoke into the upper atmosphere. In a full-scale war, models estimate roughly 150 to 180 million tons of fine soot reaching altitudes of four to nine kilometers, where rain can’t wash it out. This black layer would absorb sunlight before it reaches the surface, cooling the planet dramatically.
Climate models suggest that in the northern temperate zone (where most of North America, Europe, and Russia sit), surface temperatures could drop by around 20°C during the weeks following a summer exchange. That’s a plunge from normal summer temperatures into conditions well below freezing. Smaller but still significant temperature drops of several degrees could reach the tropics and even the southern hemisphere, persisting for months to years depending on how much soot remains aloft. The term “nuclear winter” isn’t hyperbole. Growing seasons across most of the planet’s agricultural regions would effectively disappear.
Global Famine: The Largest Killer
The collapse of agriculture would be the deadliest consequence of nuclear war by a wide margin. Modeling by researchers at Rutgers University, published in the journal Nature Food, traced the effects of reduced sunlight and temperature on global harvests of corn, rice, wheat, soybeans, and fish. Their findings are staggering: a few years after a full-scale war between the U.S., its allies, and Russia, global calorie production would drop by roughly 90%. An estimated 5 billion people would die from the resulting famine, far exceeding the hundreds of millions killed directly by the weapons.
Even a smaller regional nuclear conflict would cause global food crises. A war between India and Pakistan, involving a fraction of the world’s arsenals, could cut global calorie production by 50% and cause an estimated 2 billion deaths. Corn yields alone could fall by 7% after a regional exchange or by 80% after a large-scale global war, according to modeling from Penn State. These aren’t localized shortages. Because modern agriculture depends on stable climate patterns and international trade, disruptions to growing seasons in one hemisphere ripple across the entire food system.
Stockpiles of grain and preserved food would buy some time, but global reserves typically cover only a few months of consumption. Countries that depend on food imports would face crisis almost immediately. Even nations with domestic food production would struggle as cold temperatures, reduced sunlight, and disrupted supply chains made farming nearly impossible for years.
Ozone Destruction and Ultraviolet Radiation
The soot injected into the stratosphere doesn’t just block sunlight. It also heats the upper atmosphere and triggers chemical reactions that destroy the ozone layer. NASA researchers modeled this process and found that a full-scale nuclear war could reduce the global ozone column by 75%, with a peak loss of 65% even in the tropics. Recovery would take roughly 15 years.
In a cruel twist of timing, the thick soot layer would initially shield the surface from ultraviolet radiation. But as the soot gradually settled out over the following years, sunlight would return to a planet with drastically thinned ozone protection. UV Index values, which normally peak around 11 or 12 in the sunniest tropical locations, could exceed 35 in the tropics for four years running, with readings above 45 in southern polar regions during summer. At those levels, unprotected skin would burn in minutes. The damage to plant life, crops attempting to regrow, and ecosystems already stressed by cold and darkness would be severe.
Even a regional nuclear war would reduce global ozone by about 25%, with recovery taking 12 years. This means that survivors attempting to rebuild agriculture outdoors would face dangerous UV exposure for over a decade.
Ocean Ecosystem Collapse
The oceans wouldn’t escape the damage. Global cooling would dissolve more atmospheric carbon dioxide into surface waters, driving a measurable decrease in the chemical conditions that marine shellfish, corals, and plankton need to build their shells and skeletons. Research published in Geophysical Research Letters found that this shift would persist for approximately 10 years, compounding the existing threat of ocean acidification.
Marine food chains depend heavily on these shell-building organisms at their base. Plankton form the foundation of ocean ecosystems, feeding the fish populations that billions of people rely on for protein. With surface waters turning more corrosive, reduced sunlight slowing photosynthesis, and ocean temperatures dropping, fisheries that survived the initial disruption would decline sharply. For coastal communities that might otherwise have turned to the sea as farmland failed, this would close off one of the last remaining food sources.
What the World Looks Like After
The combined picture is one of cascading systems failure. The initial explosions destroy cities and infrastructure. Fallout contaminates vast stretches of land. Soot plunges the planet into years of cold and darkness. Agriculture collapses. The ozone layer thins to a fraction of its normal protection. Oceans become more hostile to the organisms at the base of the food chain.
Survivors in the southern hemisphere, farther from the primary targets, would have the best chances, but they would still face dramatically reduced temperatures, crop failures, dangerous UV exposure, and the collapse of global trade. There would be no imports of fuel, medicine, fertilizer, or machine parts. Electrical grids, communications networks, and transportation systems in affected regions would be destroyed or inoperable. The world that emerged after a decade of recovery would support a fraction of its pre-war population, with most of the infrastructure of modern civilization gone.
The science consistently points to one conclusion: in a full-scale nuclear war, the weapons are just the beginning. The climate and ecological aftermath would reshape the planet for a generation, and the vast majority of casualties would come not from explosions or radiation, but from starvation in the years of darkness that followed.