"Hide the decline"

And what it means in relation to tree ring proxies, by Hrynyshyn, reviewing a book.

That excerpt appears immediately above a graph that shows how temperatures inferred from tree-ring records since about 1850 (the “proxies”) are a pretty good match for actual temperature records derived from thermometers right up until the 1980s. After that, the tree-ring data begin to show lower temperatures than were actually recorded.

Just why tree rings no longer provide useful proxy data for temperatures is not known. There are several theories, many of which suggest that climate change itself is the problem. Trees no longer grow as they once did before the climate started changing so rapidly. But the point is, there is no question that tree-ring growth rates of the past — before we had thermometers — can serve as useful proxies for historical temperature data. They are much less useful now, but that doesn’t matter so much because we have actual temperature records. All of this was sorted out back in 1998. It’s not new, nor even particularly interesting, to anyone familiar with the science.

It makes sense but at the same time I’m a bit baffled by this – how do you calibrate then? If you assume the trees are behaving nowadays than before…

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