Global Warming

It's so rare these days to develop an international consensus on anything, I'm almost willing to take anything we can get, including the general agreement that burning fossil fuels is causing global warming and leading to all sorts of impending doom. But a news article the other day caught my eye.
It seems that melting ice cap in Norway has revealed farmland that's been covered with ice since the 1700's. Well, there were no automobiles in the 1700, and no coal burning power plants. Somehow the polar bears survived to this day, as did the coral reefs.
So what gives?