Hudson bay, canada — on a typical day at the top of the world, a polar bear ventures onto a platform of arctic sea ice to hunt for seals. seals are a great source of blubber, which polar bears need to sustain their energy in the harsh, cold environment of the frozen tundra. in recent years, however, life for polar bears at the southern edge of their habitat near the hudson bay has become anything but typical. here, sea ice disappears in the summer, forcing polar bears onshore, where they eat little or no other food until the sea ice returns in the fall and provides their hunting platform.

the problem is that over the last few years, the sea ice has disappeared earlier in the summer and returned later in the fall. in september 2012, for example, the amount of sea ice present in the arctic hit a record low. the polar bears typically arrive to start venturing across the sea ice in search of food in early november. in 2012, however, the sea ice that the bears depend on for survival did not form until the first week in december.

experts blame global warming. global warming refers to rising temperatures on earth, which scientists believe has been accelerated due to the accumulation of greenhouse gases in the earth's atmosphere. these gases trap and radiate heat from the sun. some experts say that temperatures in arctic regions have risen by as much as 5 degrees celsius over the past century.

as a result of global warming, the hudson bay polar bears experience shortened hunting seasons and less time to prepare for their lengthy stay on shore. as a result, these bears cannot increase their body weight enough to remain healthy until the sea ice returns. in addition, they must go without food for longer stretches. malnourishment among the bears has resulted in smaller litter sizes and even the inability to adequately nurse their young. that means fewer polar bears born overall and fewer young polar bears that survive into adulthood. scientists at the us geological survey suggest that if such trends continue, two-thirds of the world's polar population could disappear by 2050.

how has a rise on global temperature impacted the life of the polar bear?
.a rise in temperatures has disrupted the arctic food chain common to polar bears.

key to maintaining a healthy weight and insulative fat layer is a diet of seals.

the us geological survey suggest that if such trends continue, two-thirds of the world's arctic ice could disappear by 2050.

the accumulation of greenhouse gases in the earth's atmosphere has been substantially increased by the combustion of fossil fuels.

scientists predict that malnourishment among the bears resulting in smaller litter sizes will result in their extinction in several decades.

as a result of melting sea ice, there should be more vegetation on-shore for the polar bears to eat.