Say one of these B83s went off in Moscow, because President Trump lost a Twitter war with President Putin and everything escalated rather quickly. If it detonated at the surface, it would leave a crater 420 meters (1,378 feet) across and 92 meters (300 feet) deep, according to NukeMap by nuclear historian Alex Wellerstein.
Almost instantly upon detonation, a gigantic fireball would appear, 5.7 square kilometers (2.2 square miles) in size and reaching temperatures up to 83.3 million degrees Celsius (150 million degrees Fahrenheit).
Using up 50 percent of the entire warhead’s energy, it would also be accompanied by a huge pressure wave. All buildings within a 16.8 square kilometer (6.5 square mile) area would be totally flattened.
Thanks to the thermal radiation – which uses 35 percent of the explosive's energy – everyone within a 420 square kilometer (162 square mile) region will receive third-degree burns, which will only be painful for a fraction of a second as their nerve endings will be completely destroyed.
Then there’s the ionizing and fallout radiation. Assuming there’s no wind at the time, we can assume that an area of 20.6 square kilometers (8 square miles) will be so heavily irradiated that 50 to 90 percent of people in it will die from radiation sickness.
A nuclear weapon detonation - test Baker - conducted by the US military just offshore Bikini Island. US Army
We’ll Meet Again
Ok, so now let’s destroy the world.
In order to get a very rough explosive yield for all the world’s nukes, we’ll only include the US and Russia’s, but assume they are each as powerful as the B83. That’s 13,800 thermonuclear bombs, altogether producing about as much energy as the entire US does in an entire year.
Each of these devices will hit land and detonate at the surface. Assuming they’re evenly spaced out across the world’s cities and towns, and maybe a village or two, this will annihilate 94 kilometers (23 cubic miles) of land immediately – but that’s nothing compared to what happens next.
232,000 square kilometers (90,000 square miles) of infrastructure will be blown away by the air blast. That’s about 295 metropolises the size of New York City turned to dust.
A fireball 79,000 square kilometers (31,000 square miles) will vaporize literally anything it touches, and anyone within a 5.8 million square kilometer (2.2 million square miles) area would get third-degree burns. So everyone in the same space as 3,700 cities the size of London would be scorched.
Lastly, the fallout and ionizing radiation would contaminate an area of the world about 284,000 square kilometers (110,000 square miles) in size and give most of the initial survivors radiation sickness. Of course, a lot of this fallout would reach the lower atmosphere and spread across the world, so casualties would be far higher in the long term.
So at the very least, hundreds of millions, perhaps billions, will die within the first hour. This is awful enough as it is, but what happens next?