Just a few years from now advances in physics lead to a full understanding of gravity, and our ability to manipulate gravitational fields. Humanity being what it is, the first commercial application was the anti-gravity brassiere.
As with all new technology the long-term implications were poorly understood, and less well handled. Disposal of old bras in landfill led to enormous clumps of waste floating up into the air over the cities of the world. Despite the stench these were soon colonised by the homeless, the poor, economic refugees, and social rebels. At first they lived by recycling waste, then, as the landfill rotted to compost, by growing cash crops.
Over the years the floating waste heaps became islands of bucolic farmland with comfortable homes, windmills and solar farms, each one a mini techno-paradise with a substantial real estate value. Rich people bought their own islands, and everybody aspired to live on one.
Within a fifty years the human race had abandoned the surface of the earth, which returned to its natural state. Mankind floated nationless across land and sea, navigating the trade winds with the seasons, or mooring for holidays and exploration. Soon after, a few brave islands covered their surface in a high hemispherical membrane, and set out to explore the stars.