Background: California, along with the rest of the western United States, has been experiencing a severe drought for the last three year, and it is being described as the driest period in California history. The people saying that, however, did not ask the trees. Dendrochronologists, scientists who study tree rings, believe there is more to the story. By looking at the size of tree rings, they are able to estimate the amount of rainfall an area received much farther back than recorded history.

The graph below uses tree ring data to estimate the percentage of the western United States was under drought conditions for the last 1200 years. The red indicates an above average area experiencing drought, and the blue indicates below average. 

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