“Lisa J. Graumlich, who examines the ring patterns of foxtail pine trees and western junipers in the Sierra Nevada, has compiled a detailed record of the year-to-year variation in temperature and precipitation over the last thousand years.

She has seen in the North American trees the feathery but unmistakable signatures of the Medieval Warm Period, a era from 1100 to 1375 A.D. when, according to European writers of the time and other sources, the climate was so balmy that wine grapes flourished in Britain and the Vikings farmed the now-frozen expanse of Greenland; and the Little Ice Age, a stretch of abnormally frigid weather lasting roughly from 1450 to 1850. A Crucial Question

“We can now see that these were global climate phenomena, not regional temperature variations,” she said. “The question is, how did we get those warmer temperatures during pre-industrial times, and what can we learn from those conditions about what is going on today?”

By her analysis, the 20th century has seen more exaggerated swings in moisture than during any comparable period in the last millennium. She and others at the laboratory are particularly concerned because the tree rings clearly declare that in the Western United States, the years 1937 through 1986 have been abnormally wet compared with past centuries — and these are the years of the greatest immigration to the supposedly golden coastal states.”

In Unexpected Places, Clues to Ancient and Future Climate; Warming? Tree Rings Say Not Yet – The New York Times