When Fatman exploded over HIroshima 146,000 people died. The bomb released the equivalent energy of 16 kilotons of TNT. The radius of total destruction was 1 mile, with fires across 4.4 square miles.

70,000–80,000 people, 30% of the population of Hiroshima, were killed by the blast and firestorm, and another 70,000 injured. The output of energy from the fuel in the city, lofted a stratospheric firestorm-cloud with 1000 times the energy of the bomb.

Soot-filled black rain fell on survivors.  100 of these firestorm-clouds would create a "nuclear winter", producing 1-2 degrees Celsius of global cooling. At last… a means to mitigate climate change! 

The United States has 14 Ohio-class ballistic missile submarines, each with  24 Trident II SLBMs. Each missile carries twelve multiple independently targetable reentry vehicles (MIRV)s… for a total of 4,032 warheads.

The W88 Mk-5 Warhead/Reentry Vehicle Package Yields 475 Kilotons per warhead for 1,915,200 total kilotons of destruction.

The AGM-86 ALCM is the US Air Force's current nuclear-armed air-launched cruise missile. The B-52 Stratofortress can carry 20 missiles. There are 93 deployed nuclear B-52’s, with a total of 1860 warheads. 

The W80 is a small thermonuclear warhead fusion weapon. with a variable yield of between 5 and 200 kt of TNT.. or a maximum yield of 372,000 kilotons.

We also have drones, ICBM’s, nuclear artillery and suitcase bombs. We are not short of nuclear weapons.

With just two arms of the nuclear triad, the US could deliver over 2,287,200 kilotons, or over 143,000 times the destruction leveled on Hiroshima, and nearly a million and a half times the firestorm ….yielding a world nuclear winter.

But you wouldn’t want to fire all these weapons. That would be crazy.

Even if you just fired a few…the explosions would send radioactive particles flying through the air and into the water. Wind would carry the particles around the world.

The smoke from the blasts would block out sunlight and hinder the ability of all life forms that depend on photosynthesis to survive. The exposure to radiation would cause severe health problems, otherwise known as death. 

Radioactivity damages cells in humans, animals, and plants by causing changes in their genes. The changes would lead to crippling mutations in future generations  …if there were any… 

The strike would contaminate air, soil, and water. More than 60 years after the United States tested atomic bombs near Bikini Atoll, the island remains “unlivable”.  Chernobyl is unlivable. Fukakjima is unlivable.

So, we would have a nuclear winter, with an unlivable earth surface, contaminated air, water and soil. Drifting radiation clouds would cover the earth, creating havoc and triggering “Dead Hand”, the Russian Domesday machine that reads the radiation in the atmosphere, considers a certain level of radiation a strike on the Russian homeland and automatically launches the full Russian nuclear strike arsenal.

There are a number of fission products with various half-lives.

Plutonium has a half life of 20,000 years. If there is enough plutonium in the atmosphere to kill you 10,000 times, in 20,000 years there would only be enough to kill you 5,000 times. In 40,000 years you would only be killed 2500 times. And 60,000 years 1250 times. And so on…

In a million years, something else could crawl out of tide pools and look around. They would find nothing of us. As we found nothing… All our history would have disappeared; everything we knew, learned, felt, saw, loved, believed in, remembered…all disappeared, as if it had never existed.

The earth is  four and a half billion years old. How many times have we done this?

Dead Hand (Система «Периметр», Systema "Perimetr", "Perimeter" also known as Perimeter, was a Cold War-era automatic nuclear-control system used by the Soviet Union. 

General speculation from insiders alleges that the system remains in use in the post-Soviet Russian Federation as well. An example of fail-deadly and mutual assured destruction deterrence, it can automatically trigger the launch of the Russian intercontinental ballistic missiles (ICBMs) by sending a pre-entered highest-authority order from the General Staff of the Armed Forces, Strategic Missile Force Management to command posts and individual silos. 

if a nuclear strike is detected by seismic, light, radioactivity, and overpressure sensors even with the commanding elements fully destroyed. By most accounts, it is normally switched off and is supposed to be activated during dangerous crises only; however, it is said to remain fully functional and able to serve its purpose whenever it may be needed.

 A similar system existed in the U.S. known as the AN/DRC-8 Emergency Rocket Communications System (ERCS).