1. Tree rings (Dendrochronology)

2. Dendrochronology
Dendrochronology is the study of climate change as recorded by tree growth rings.
Each year, trees add a layer of growth between the older wood and the bark. This layer, or ring as seen in cross section, can be wide, recording a wet season, or narrow, recording a dry growing season.
Because the rings are basically recording a good growing season or a bad growing season, they are indirectly recording more than just moisture. They also document temperature and cloud cover as they impact tree growth as well.

4. Limitations
Trees in the temperate zone only record the growing season, so the winter season, no matter how dramatic, will not be seen in the ring record.
Trees in tropical regions grow year round and therefore show no real obvious annual growth rings.
Trees do not grow in all places on Earth, therefore we don’t have a tree ring record of climate change for each region and ecologic niche globally. (No trees in polar regions, high in the mountains, in the ocean!!!)

6. Studying tree rings https://www.youtube.com/watch?v=MXVHrBDgeIo
Dendrochronology