As the average temperature of the Earth planet is slowly on the rise, it will cause the North Pole ice sheets to shrink whilst increasing the temperatures of the area simultaneously.
Assuming that global warming continues to intensify, as the North Pole increases in temperature, the polar bears with thicker fur coats will find it harder to survive due to the heat.
Having a thinner coat of fur becomes an favourable trait since it is easier to survive with and is more suitable for the changing environment. As a result, this variant will survive easier. Over generations, this variation becomes more and more common until it turns into an adaptation.
Eventually, this adaptation changes the genetic makeup of the entire population and polar bears have little to no fur.