The Toba Humanitarian Crisis took began in 2041 and resulted in the birth of the UNPA. It also resulted in the deaths of an estimated 800 million people. Ultimately, this also caused the eruption of the Great World War.
The Toba Humanitarian Crisis actually began in 2039. In 2039 the world was 4 degrees centigrade warmer than it was in 1990 due to global warming and many parts of the globe were quickly turning into deserts, resulting in waves of climate refugees travelling north and south from the equator. Something needed to be done.
So in 2039 governments decided to begin putting sulphur dioxide into the stratosphere to cool the planet. The idea was that the sulphur dioxide should linger for many weeks at such altitudes and dissipate throughout the atmosphere, reacting with water vapour to produce microscopic droplets of sulphuric acid, which would reflect some sunlight away before it reached the ground, causing a global cooling effect that would counter global warming.
Sulphur deposits were discretely mixed in with aviation fuel so that it would enter the stratosphere while aircraft were in flight. This was discrete enough to prevent too much attention from the public, as geoengineering was still a much debated topic at the time.
The project was a success and within a few months the global climate had cooled by an average of 2 degrees to be just 2 degrees centigrade above the average climate temperature in 1990; continuing the project would maintain these temperatures. However, in 2041 Mount Toba erupted, throwing hundreds of cubic kilometres of ash and dust into the atmosphere.
The Mount Toba eruption lowered the average temperature by another 2 degrees, causing temperatures to return to what they were in 1990. However, most farms now used GM crops that were altered to withstand the warmer climate, and these quickly failed due to the sudden cooling.
World grain production dropped by a third and in a single year nearly 400 million of the 9 billion people in the world starved to death. Food imports and exports were nearly non-existant as even arable countries struggled to feed their population; rich countries withdrew aid and borders were closed against climate refugees. By 2042 over a third of the world's population were living in failed states.
Despite the temperature of the world climate returing to 4 degrees above 1990 levels in 2042 (due to an immediate cancellation of all geoengineering projects), by the official end of the crisis in 2047 a further 400 million had been killed due to social unrest that was a direct consequence of the Toba eruption.