The magnitudes of millennial- and orbital-scale climatic change in eastern North America during the Late Quaternary

Temporal differences among fossil pollen samples from eastern North America provide a measure of the amount of climatic change that occurred on a broad spatial scale since the last glacial maximum (21,000 cal yr BP). Square-chord distances (SCDs) quantify the difference between pollen samples, and, thus, represent the potential magnitude of climatic change underlying the pollen record. The magnitude varied significantly among time scales with small changes common at centennial to millennial scales and large changes common at multi-millennial (i.e. orbital) scales. SCDs measured across 3000-yr intervals averaged 0.20, and often exceeded the maximum difference expected from samples collected within the same biome (0.15). SCDs across individual millennia were smaller and averaged 0.08. SCDs across the millennia at the beginning (13,000–12,000 cal yr BP) and end (12,000–11,000 cal yr BP) of the Younger Dryas chronozone (12,900–11,600 cal yr BP), however, averaged 0.20 and 0.18, respectively. These rapid step changes, large at sub-millennial scales, equal about 5–25% of the total glacial–interglacial transition. Large magnitude progressive changes in insolation, ice sheet extent, and atmospheric composition parallel the dominant trends in the SCD data, which show that progressive change comprises the first-order climatic pattern of the Holocene rather than stable or oscillatory patterns.