A Millennial‐Scale Oscillation in Latitudinal Temperature Gradients Along the Western North Atlantic During the Mid‐Holocene

Changes in vegetation in North America indicate Holocene shifts in the latitudinal temperature gradient along the western margin of the North Atlantic. Tree taxa such as oak (Quercus) and birch (Betula) experienced opposing directions of change across different latitudes consistent with changes in temperature gradient steepness. Pollen‐inferred temperatures from 34 sites quantify the gradient changes and reconstruct a long‐term northward steepening in summer and southward steepening in winter. From 4.8 to 3.8 ka, an oscillation in tree distributions interrupted the long‐term trends as a steep temperature gradient developed north of 43.5°N. The shift likely limited cold outbreaks to the south, producing anomalously high summer temperatures at 42–43.5°N, and enabling a northward expansion of oak forests. The forest and temperature gradient changes appear consistent with orbital and ice sheet forcing as well as millennial variability in the North Atlantic pressure field analogous to the North Atlantic Oscillation on interannual time scales. Plain Language Summary In the Northern Hemisphere, average temperatures decline with latitude as climates cool toward the pole. Changes in this temperature pattern have significant consequences for weather systems and ecosystems. Past changes likely impacted the geographic extent of summer warmth and winter freezing that can determine where tree species can grow. In this study, fossil evidence of ancient tree distributions revealed how the poleward temperature gradient near the Atlantic coast of North America fluctuated over the past 11,700 years. Long‐term forest and temperature changes developed in response to the deglaciation of Canada and Earth's slow orbital changes. However, a particularly striking change 4,800–3,800 years ago briefly expanded oak‐hickory forests into mid‐latitude highlands otherwise dominated by cold‐tolerant northern tree species. The forest histories demonstrate the potential for ecosystems to change rapidly in the future, but also how fossil records can reveal poorly understood patterns and processes of millennial‐scale climate variability. Temperature patterns detected here may be analogous to patterns of climate variability in the North Atlantic region at monthly‐to‐annual scales. Key Points A major ecotone in eastern North America, which is linked to the latitudinal temperature gradient, shifted repeatedly during the Holocene Pollen‐inferred temperatures from different latitudes across the eco