An 858-year proxy record of July temperature for east-central Idaho shows multi-decadal periods of extreme cooling centered around AD 1300, 1340, 1460, and after AD 1600. These cold intervals are interrupted by prolonged warm spells in the early 1400s, late 1500s, and in the 1930s. The spatial signature of the paleoclimate record is centered on the north-central Rockies and central Great Plains, and expands over North America following a wave-like pattern. Neither instrumental nor proxy data in Idaho northeast valleys show unusual warming during the twentieth century. Climate episodes over the last three centuries are in broad agreement with the Greenland borehole temperature history. Low-frequency patterns are consistent with other northern hemisphere tree-ring records for the late Holocene, and provide a chronology of warm and cold intervals during the Little Ice Age.