Earth's End

It was not so much as one natural disaster, but many happening throughout the world.
As the climate heated, seas continued to rise. Places that were ordinarily lush got grew so heated that they were no longer useful for growing crops. Entire islands were wiped out from rising waters and hurricanes. This caused the first round of climate refugees who moved inland around the world. Places that were normally rural, filled with the climate refugees. And cities became so overcrowded that the air became unbreathable. There was a scramble to fix the mistakes in the past. Many believed that it was too late and so started looking to the skies.
Those who were wealthy took the first trips to the moon and mars in order to began colonizing the places. Others grew out their bunkers and created exclusive underground cities. However, this opinions were open to few. As automation increased, jobs disappeared, particularly for those in the working class. And without money, people could not leave. Over time, however, even the wealthy realized that these places would be unsustainable over the long run.
Tensions grew. Conflicts broke out. It was only a matter of time before The Last World War happened. When it did, Earth's End was only a matter of time. Bombs were dropped; lands were razed. And at the end, there was no more Asia. At least not where humans could thrive.
Those who managed to survive packed into what places of the world was left. Lack of arable land led to a worldwide food crisis. Meat became a rarity and before long, most people lived on a soy and artificial protein diet. Humans continued to pack into space; there were so many space ships that it was impossible to see the night stars.
And so, it was then the world's leaders (or what was left of them) gathered. Knowing it was the Earth's End, they joined together to find a way to escape the Earth and start anew. That is when they discovered the pocket in space. And through that pocket in Space, humanity created a new home.