A hundred and fifty years ago, there came the Accommodators. With the Accommodators, came the Innocence. With the Innocence, came the Cataclysm. With the Cataclysm, came the Great Flood. And with the Great Flood, came the largest-scaled rebellion mankind had ever seen.
For those who don't care for all the TL;DR:
§ Those compatible with Innocence - Accommodators - came to Earth with hopes of finding a new planet to settle.
§ The sudden influx of Innocence in the foreign atmosphere, however, caused a global cataclysm, resulting in what is now known as the Great Flood. An estimated *90% of the Earth's native population was wiped clean.
§ When the waters receded, *85% of the Earth's surface was left in a barren state; a desert. *10% of what isn't desert remained prosperous enough to farm; that percentage is in control of the last few governments clinging to the last shreds of power.
§ The other *5% of farmable land is in control of the Noah, fenced off and guarded by Akuma, and is maintained by the few pieces of Innocence that they've managed to gather.
§ It's now an arms race between the Accommodators gathering Innocence to restore the planet, and the Noah gathering it for their personal goals.
(*) denotes inaccurate statistics due to the difficulty of retrieving precise data. It's possible the numbers are closer to 73%, 60%, 22%, and 6%, respectively.