[wolfram74]

Even without read-the-thermometer records, tree corings and lake bed sediment samples and a number of other go-out-and-look methods can get pretty useful atmospheric data from the past. I don't know what the resolution would be, but it might fill in some gaps.

[danielbigham]

One challenge with using things like trees to estimate long-ago temperatures is that you have to build a complex mathematical model to map to temperature, and I'm aware of at least one famous study that got the math wrong, to the point of being potentially suspicious. (But I haven't studied this stuff enough to know whether or not other proper studies exist)

[dragonwriter]

> One challenge with using things like trees to estimate long-ago temperatures is that you have to build a complex mathematical model to map to temperature

As it turns out, that's what you have to do with fairly recent temperatures measured with thermometers that aren't located in the same places, too.

[southern_cross]

It has been pointed out that tree rings respond mostly to changes in precipitation rather than temperature. If you can properly compensate for that, and for various other potentially confounding factors, then maybe you can derive some proxy temperature data from them. Other proxies have their own problems too, of course.