A 1500-year reconstruction of annual mean temperature for temperate North America on decadal-to-multidecadal time scales

Abstract

We present two reconstructions of annual average temperature over temperate North America: a tree-ring based reconstruction at decadal resolution (1200–1980 CE) and a pollen-based reconstruction at 30year resolution that extends back to 480 CE. We maximized reconstruction length by using long but low-resolution pollen records and applied a three-tier calibration scheme for this purpose. The tree-ring-based reconstruction was calibrated against instrumental annual average temperatures on annual and decadal scale, it was then reduced to a lower resolution, and was used as a calibration target for the pollen-based reconstruction. Before the late-19th to the early-21st century, there are three prominent low-frequency periods in our extended reconstruction starting at 480 CE, notably the Dark Ages cool period (about 500–700 CE) and Little Ice Age (about 1200–1900 CE), and the warmer medieval climate anomaly (MCA; about 750–1100 CE). The 9th and the 11th century are the warmest centuries and they constitute the core of the MCA in our reconstruction, a period characterized by centennial-scale aridity in the North American West. These two warm peaks are slightly warmer than the baseline period (1904–1980), but nevertheless much cooler than temperate North American temperatures during the early-21st century.