My understanding is that pollution is a much bigger threat to polar bears than global warming, specifically from mercury and other heavy metals. Since polar bears are an apex predator that subsist primarily on fish and fish eating mammals they get very high exposures to these metals in their diets and it may be affecting the survival rate of their cubs. Their populations don't seem to be in any eminent danger though, so it's no crisis but rather just something that deserves further study and observation.
In the 19th Century Arctic Explorers found that eating polar bear livers could be poisonous because of the Vitamin A concentrations.