Analysis of tree rings of California redwoods (giant sequoias)
Giant sequoias are one of the longest living species in the world. According to Los Angeles Times, the oldest coastal redwood is 2,520 years old and the oldest giant sequoia is about 3,200 years old. The tree growth would keep a faithful record of long-term climate in a large geogological span, and for this reason they are called the "living fossils". They can be an ideal proxy to study climate change or historical weather anomaly.
-- Data source: International Tree Ring Data Bank