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. Early Eocene climates were globally warm, with ice-free conditionsat both poles. Early Eocene polar landmasses supported extensive forestecosystems of a primarily temperate biota but also with abundantthermophilic elements, such as crocodilians, and mesothermic taxodioidconifers and angiosperms. The globally warm early Eocene was punctuated bygeologically brief hyperthermals such as the Paleocene–Eocene ThermalMaximum (PETM), culminating in the Early Eocene Climatic Optimum (EECO),during which the range of thermophilic plants such as palms extended intothe Arctic. Climate models have struggled to reproduce early Eocene Arcticwarm winters and high precipitation, with models invoking a variety ofmechanisms, from atmospheric CO2 levels that are unsupported by proxyevidence to the role of an enhanced hydrological cycle, to reproduce wintersthat experienced no direct solar energy input yet remained wet and abovefreezing. Here, we provide new estimates of climate and compile existingpaleobotanical proxy data for upland and lowland midlatitude sites inBritish Columbia, Canada, and northern Washington, USA, and fromhigh-latitude lowland sites in Alaska and the Canadian Arctic to compareclimatic regimes between the middle and high latitudes of the earlyEocene – spanning the PETM to the EECO – in the northern half of NorthAmerica. In addition, these data are used to reevaluate the latitudinaltemperature gradient in North America during the early Eocene and to providerefined biome interpretations of these ancient forests based on climate andphysiognomic data.

Paleobotanical proxies for early Eocene climates and ecosystems in northern North America from middle to high latitudes