For most of the past 2000 years the average temperature of the world didn't change much. There were some times where the temperatures were a little warmer or cooler in some places. One of the most famous warm times was the Medieval Warm Period and one of the most famous cool times was the Little Ice Age (not really an ice age). Other proxy measurements like the temperature measured in deep holes mostly agree with the tree rings. Tree rings and bore holes can only help scientists work out the temperature back to about 1000 years ago. Ice cores are also used to find out the temperature back to about half a million years ago.