I don't see that scenario as particularly relevant. There was no widespread fossil fuel burning in those days so the cicumstances were different. Yes there was some natural variation, but the big difference is that now the effect of human greenhouse gases is overlaid on top of that natural variability. I could also argue that (I presume) the Rhine river has not frozen over since then - so that proves man made global warming is happening, but I won't because we can't easily attribute individual events without a vast amount of research, like the conributors to Nature did for the recent research paper on the England and Wales floods in 2000.
