That's an old story. This might help you catch up on what's going on with the bears lately.
BBC NEWS | Science & Environment | 'Stress' is shrinking polar bears
Here's another bit of news.
NERI/AU - Polar bears have become smaller - new AU research in international media
Besides, this patch of warming is happening quicker than the others. That's a new variable. We don't know what will happen to specific species because of it. On the cold scale it appears that rapid cooling killed off an awful lot of critters back about 65 million years ago, or else it might have been the pollution that did.
NERI/AU - Polar bears have become smaller - new AU research in international media
So the bears are becoming smaller. Yes I heard those stories. Why are they becoming smaller? Speculate, blame it on the effects of global warming. Everthing gets blamed on global warming these days. You still take that seriously?
Are their populations shrinking? No. Some are shrinking and some are increasing, with the net effect being an increase. Generally predator populations increase when their food availability increases. I've heard there's been a considerable increase in seal populations too, so that might be the reason for so many bears. And body size generally decreases when food isn't as available. It seems most likely that they've bred themselves out of a place at the table, they've outgrown their food supply and there's an adjustment happening. Also, it's likely that they were larger before for the same reason their populations were increasing, abundant food. THis shrinkage may just be them reverting to pre-population explosion sizes.
Actually this patch of warming isn't quicker than others. Some have been slower and some have been faster. And you're correct about cold killing off critters. Somewhere I read that cooling is the main culprit of all mass extinctions. Warming, to the contrary, results in an expansion of both diversity and quantity of life.