The first assumption is that global warming has caused, and will cause, a predictable reduction in sea ice. The second assumption holds that polar bear populations will dwindle because they are dependent on sea ice to hunt for prey. But each of these assumptions is fraught with problems.

But the IPCC projections are based on the assumption that Arctic ice melting is the result of global warming caused by greenhouse gas emissions--an assumption that was recently shown to be of dubious merit by NASA, which is hardly a hotbed of climate skepticism.

Son Nghiem explained that the warming of recent years was, in fact, caused by a change in wind patterns.

Thus, to determine whether polar bears will be endangered by the disappearance of sea ice, trends in population are predicted using something called population viability analysis (PVA)

PVA can be a useful tool in policy cost-benefit analysis, but its results are only as accurate as the data and the model assumptions that go into it.