I haven't read up much on historic global temperature estimate methods, but:
Surely, any methods which rely on geological processes and markers to estimate temperature are going to show a significant "flattening" of the peaks and troughs. A quick scan of wikipedia shows the "current" ice age to have started 2.58 million years ago, with glacials occuring on 40,000 and 100,000 year cycles. I doubt very much that temperature estimate methods that cover such large periods of time can show variations accurately to within 1,000 years, yet alone 100 years.
Perhaps, had we directly measured global temperature 100,000 years ago (or 1 million etc), we'd be seeing equally great variations in temperature that we've seen in the last 100-150 years. Perhaps we also need to ask what we consider to be "normal" climatic conditions.
I think it's fair to say that whilst there is consensus amongst climatologists regarding climate change, our level of understanding of the climate is still very much in its infancy.
Dave
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