The global warmists look in many places to find the temperatures in times gone by. The thermometer wasn't invented until 1654, and the Farenheit temperature scale wasn't defined until 1724. In consequence we only have thermometer data going back a few hundred years. For temperatures before the thermometer it is necessary to look at other indications, types of pollen in sediments, various kinds of isotope analysis, date of grape harvest, accounts of the freezing of lakes and rivers, accounts of the extent of alpine glaciers.
To my surprise, the global warmists now look at tree rings as an indication of temperature. Trees lay down thicker layers of new wood in good years and thinner layers in bad years. The patterns of thick and thin rings are distinctive and have been used for dating for more than 50 years. It's possible to match up the ring patterns of living trees with the patterns in trees long dead and in this fashion extend the tree ring dating back many thousands of years. Timber from ancient Indian pueblos was dated in the 1930's. Timbers from a sunken Viking vessel were both dated and located by tree rings. The ring patterns indicated the vessel was built of Irish oak.
The global warmists looked at tree ring data (there is quite a bit of it) and decided that ring width was controlled by the temperature, wide rings in warm years, thin rings in cold years.
There is a problem with this approach. Moisture, rain fall, is much more important to trees than temperature. Nice moist years, even cool moist years, are good years from a tree's point of view. In short, the width of tree rings has little or nothing to due with the temperature and everything to do with the amount of rainfall.
The global warmists noticed that their tree ring data didn't show a warming trend in modern times. So, they merely dropped the tree ring data for the last 50 years from their graphs.
The hacked Climate Research Unit files show us the use of questionable tree ring data, and even more reprehensible, the editing of the already questionable tree ring data to make the hockey stick graphs look more alarming.
In short, don't use tree ring data to indicate ancient temperature, 'cause the width of tree rings doesn't vary with temperature.