The nuclear winter refers to a period of global darkness, cooling, and extreme weather caused by stratospheric clouds of smoke, ash, and radioactive materials which lasted from 2054 until approximately 2068. It was triggered by the strategic nuclear exchange that marked the end of the Great War.
The Winter is estimated to have caused around six billion deaths - between 70% and 80% of the Earth's population in the mid-2050s. The primary reason behind this extreme death rate is widespread crop failure, resulting in global famine, exacerbated by social collapse.
The nuclear winter, along with the nuclear war which caused it, are seen as the direct cause of the geopolitical order and social situation extant in the world today.
Its latter stages are known for their particularly brilliant sunsets caused by the scattering of light in the stratosphere, often turning the entire sky a shade of orange.
The Great War began in 2050 following a period of extreme and escalating global tensions (amplified by climate change and the rise of Russia, China, and Europe as superpowers), and after several uncertain years rapidly shifted in favour of the Western powers. The use of tactical nuclear weapons became a common feature of the battlefield, particularly in China, where US forces deployed them to overcome Chinese numerical superiority.
Though it is unknown which power decided to use strategic nuclear weapons first, it is suspected to have been the Russian Federation, as the exchange occured only two weeks before Western forces were expected to take Moscow.
The winter resulted in extreme weather (such as rad-storms, dust storms, frequent electrical storms, blizzards, cold waves, etc) across much of the world.
Global temperatures plummeted, with areas of permafrost reaching far south.
Between 2054 and 2056, extremely little light reached the Earth's surface; every following year, light levels steadily returned to normal, though in 2061 they were still at only 60% of pre-war levels.
A lack of heat and light resulted in enormous die-offs of vegetation and wildlife, and desertification occured worldwide to varying degrees.
In addition, the surface of the oceans (and, by extension, the water cycle) became irradiated (by 2100, most of this radioactive material had sunk to the seabed), resulting in a massive die-off of oceanic life and the seas becoming an environment in which mutation was particularly commonplace.
The peak of population decline occured in the years immediately following the nuclear exchange; two gigadeaths are estimated to have occured in 2055 alone, dropping to one gigadeath per year by 2057, and to a mere 100 megadeaths per year by 2060. The large majority of deaths occured due to drought and starvation, although factors such as radiation hazards and fallout (especially from salted bombs) were also non-insignificant contributors.
Desertification and "wasteland-isation"
Vegetation die-off, rad-storms, and deposition of stratospheric dust caused some previously habitable areas of the planet to undergo "wasteland-isation", in which they were transformed into uninhabitable and desolate landscapes. Many of them remain so today.