Earth was in a complete mess. People, in their selfishness, has built factories for creating various products and processing electricity.
They also cut trees to make wood products and create more farms.
This led to higher concentration of CO2 in the atmosphere as many of the forests, a major carbon reservoir, were destroyed which decreased the total amount of carbon dioxide being consumed and because the newly-built factories released ever more CO2 in the atmosphere.
Because of this, Antarctica melted almost wholly,
Increasing sea levels, which caused many coastline cities to sink in water; many people lost their houses.