TUCSON — From the early 1700s until the 1960s, the fast moving river of wind known as the North Atlantic Jet Stream, which drives weather extremes over Europe, was pretty steady on its course. Then it became less predictable. But instrument data alone can’t tell the jet stream’s movements for comparison over the centuries, given that scientists began keeping records of weather events via instruments only in the late 19th century. The rings of trees, however, offer a far more complete historical picture of climate variations. …In recent years, the techniques for extracting information from tree rings has been honed and expanded. …The field “has exploded,” Edward Cook, director of the tree ring lab at Lamont-Doherty Earth Observatory, said. The Laboratory of Tree-Ring Research here at the University of Arizona was founded in the 1930s.