US-Russia war causes extinction.

One 2008 study looked at a Russia-US nuclear war scenario, where Russia would target 2,200 weapons on Western countries and the US would target 1,100 weapons each on China and Russia. In total, therefore, 4,400 warheads detonate, equivalent to roughly half the current inventories held each by Russia and the US.

Nuclear weapons held by other states were not used in this scenario, which has a 440-Mt explosive yield, equivalent to about 150 times all the bombs detonated in World War II. This full-scale nuclear war was estimated to cause 770 million direct deaths and generate 180 Tg of soot from burning cities and forests. In the US, about half the population would be within 5km of a ground zero, and a fifth of the country’s citizens would be killed outright.

A subsequent study, published in 2019, looked at a comparable but slightly lower 150 Tg atmospheric soot injection following an equivalent scale nuclear war. The devastation causes so much smoke that only 30-40 percent of sunlight reaches the Earth’s surface for the subsequent six months.

A massive drop in temperature follows, with the weather staying below freezing throughout the subsequent Northern Hemisphere summer. In Iowa, for example, the model shows temperatures staying below 0°C for 730 days straight. There is no growing season. This is a true nuclear winter.

Nor is it just a short blip. Temperatures still drop below freezing in summer for several years thereafter, and global precipitation falls by half by years three and four. It takes over a decade for anything like climatic normality to return to the planet.

By this time, most of Earth’s human population will be long dead. The world’s food production would crash by more than 90 percent, causing global famine that would kill billions by starvation. In most countries less than a quarter of the population survives by the end of year two in this scenario. Global fish stocks are decimated and the ozone layer collapses.

The models are eerily specific. In the 4,400 warhead/150 Tg soot nuclear war scenario, averaged over the subsequent five years, China sees a reduction in food calories of 97.2 percent, France by 97.5 percent, Russia by 99.7 percent, the UK by 99.5 percent and the US by 98.9 percent. In all these countries, virtually everyone who survived the initial blasts would subsequently starve.