Question

There are a lot of ways to collect historical temperature data. Even the "accurate temperature data" you mentioned can't be used directly.

The data is collected from fixed stations, but over the years, the environments in which those stations are situated changes. Cities grow up around them, and cities are always warmer than non-cities, so when they look at the data they have to correct for those effects. The result is that any temperature data is only a proxy for a more complicated underlying model.

So anything that varies as a result of mean temperatures can serve as a dataset. Trees grow at different rates as the temperature changes (and also rainfall and such), so tree-ring measurements show information about temperatures. Ice forms and thaws in different amounts when the temperatures change, so cores drilled from glaciers show temperature data. Both of these examples can go much farther back in time than simple thermometer data, though the thermometer data is often still used to calibrate and align the other methods.