Summary of consequences: U.S.-Russian war producing 150 million tons of smoke
2600 U.S. and Russian strategic nuclear weapons on high-alert are launched (in 2 to 3 minutes) at targets in the U.S., Europe and Russia (and perhaps at other targets which are considered to have strategic value).
Some fraction of the remaining 7600 deployed and operational U.S. and Russian strategic nuclear warheads/weapons are also launched and detonated in retaliation for the initial attacks.
Hundreds of large cities in the U.S., Europe and Russia are engulfed in massive firestorms which burn urban areas of tens or hundreds of thousands of square miles/kilometers.
150 million tons of smoke from nuclear fires rises above cloud level, into the stratosphere, where it quickly spreads around the world and forms a dense stratospheric cloud layer. The smoke will remain there for many years to block and absorb sunlight.
The smoke blocks up to 70% of the sunlight from reaching the Earth's surface in the Northern Hemisphere, and up to 35% of the sunlight is also blocked in the Southern Hemisphere.
In the absence of warming sunlight, surface temperatures on Earth become as cold or colder than they were 18,000 years ago at the height of the last Ice Age
There would be rapid cooling of more than 20°C over large areas of North America and of more than 30°C over much of Eurasia, including all agricultural regions
150 million tons of smoke in the stratosphere would cause minimum daily temperatures in the largest agricultural regions of the Northern Hemisphere to drop below freezing for 1 to 3 years. Nightly killing frosts would occur and prevent food from being grown.
Average global precipitation would be reduced by 45% due to the prolonged cold.
Growing seasons would be virtually eliminated for many years.
Massive destruction of the protective ozone layer would also occur, allowing intense levels of dangerous UV light to penetrate the atmosphere and reach the surface of the Earth.
Massive amounts of radioactive fallout would be generated and spread both locally and globally. The targeting of nuclear reactors would significantly increase fallout of long-lived isotopes.
Gigantic ground-hugging clouds of toxic smoke would be released from the fires; enormous quantities of industrial chemicals would also enter the environment.
It would be impossible for many living things to survive the extreme rapidity and degree of changes in temperature and precipitation, combined with drastic increases in UV light, massive radioactive fallout, and massive releases of toxins and industrial chemicals.
Already stressed land and marine ecosystems would collapse.
Unable to grow food, most humans would starve to death.
A mass extinction event would occur, similar to what happened 65 million years ago, when the dinosaurs were wiped out following a large asteroid impact with Earth (70% of species became extinct, including all animals greater than 25 kilograms in weight).
Even humans living in shelters equipped with many years worth of food, water, energy, and medical supplies would probably not survive in the hostile post-war environment.