 Polar bears survive by hunting seals. To reach their prey, they need a platform of ice floating on the sea where the seals live. Sea ice in productive hunting areas is particularly important for polar bear survival. The problem is that Arctic sea ice is melting due mainly to human cause global warming. As the planet is warmed, the Arctic region is warm the fastest of all. One reason is because the reflective ice has been replaced by dark oceans which absorb bar heat from the sun. About 70% of the Arctic sea ice has disappeared over just the past 35 years. Not all the Arctic is the same. Sea ice behaves differently in different regions. In some regions, like Canada's Hudson Bay, the sea ice is seasonal. It melts each summer and refreezes in the fall. As a result of global warming, the ice freeze seasons in these areas have gotten longer and longer. This is endangering the polar bear populations in those regions. In other regions, the sea ice is more persistent and polar bear populations are in better shape for now. In areas called divergent ice regions, sea ice retreats from the shore during the summer like a retracting bridge. Over the last few decades of warming, the sea ice in these regions has retreated further and further away from the shore. This leaves the polar bears with two options. They can come ashore and forego hunting until the ice returns in the fall or they can swim long distances to reach the remaining ice pack where there may be few seals to hunt. It's in these seasonal ice and divergent ice regions that polar bears are most endangered by global warming in the vanishing Arctic sea ice. However, there are also areas in which global warming doesn't yet threaten polar bear populations. One of these is called convergent ice regions where sea ice forms along the shore. For now, there's enough sea ice in these regions to allow polar bears to continue successfully hunting seals. However, scientists expect that even in these regions, sea ice and polar bear populations may be gone by the end of the century. Similarly, there are islands in the Arctic far enough north that sea ice stays in place along their coasts even in the summer. These are called archipelago ice regions. They provide hunting opportunities for polar bears. However, even this island ice will eventually melt if the planet warms too much. Scientists attract 19 different polar bear populations in the Arctic. The different groups live in a variety of these ice regions. Scientists found that the populations of four groups are declining, five are stable, one is increasing, and nine don't have enough data to say for sure. The declining populations tend to live in regions with seasonal or divergent ice. The stable populations usually live in regions with convergent or archipelago ice. For example, in Western Hudson Bay, where the sea ice is seasonal, the polar bear numbers have declined by 22% over the past 30 years. In the southern Beaufort Sea, a divergent ice region, the polar bear population has shrunk 40% in just six years. The population included 1,600 bears in 2004, but shrunk to 900 bears in 2010. Only two of 80 polar bear cubs tracked by scientists during that time survived while the usual survival rate is about 50%. Some polar bears in the area survived by staying on land during the summer feeding out whale carcasses. That's not a long-term solution for the local polar bear population. Polar bears need sea ice to hunt and warming melts sea ice. So the connection between human-caused global warming and the endangerment of the polar bears is crystal clear. However, one myth argues that polar bear numbers are greater now than in the 1970s so there are no danger from global warming. And it's true that in the mid-20th century, polar bear populations may have been in even worse shape than they are today. But this claim is an oversimplification. Melting sea ice is not the only thing affecting polar bear numbers. In the mid-20th century, polar bear hunting was widespread and over 1,000 bears were killed each year. As a result, polar bear populations dwindled. Fortunately, the countries where polar bears lived all enacted hunting regulations between the 1950s and 1970s. Thanks to these regulations and conservation efforts, polar bear populations recovered. We ended the previous threat to polar bear populations from hunting, but we replaced it with a new threat from human-caused global warming. This is melting the sea ice they need to hunt on for food.