Polar bears are affected by greenhouse gases because the world is over heating. 

Global warming is causing sea ice to melt. Polar bears live on the sea ice, and when they swim and hunt for food, they need a place to rest, but if they don't have a place to rest they are gonna drown and die. 

Melting sea ice causes it to be more harder to hunt seals. Sea ice would make a obstacle to swim around for seals, allowing the polar bear to catch it. Now without sea ice the seals can escape more easily, meaning the polar bear has less food.