Holocene paleoclimate data from the Arctic:: testing models of global climate change

To evaluate the spatial variability of Arctic climate change during the present interglacial, CAFE Project Members compiled well-dated terrestrial, marine, and ice-core paleoenvironmental records spanning the past 10-12 thousand years (ka). Six tundra biomes of increasing summer temperature requirements were defined based on regionally coherent pollen assemblages. Using a rule-based approach, pollen spectra were converted to tundra, forrst/tundra, or Forest biomes ranked by their average growing season requirements. Marine sea-surface reconstructions were based on proxy data following a similar rule-based approach. From these data-based reconstructions. departures in summer temperatures from modern normals were calculated in 1 ka time slices through the Holocene. To test predictive models, data-based summer temperature reconstructions were compared with general circulation model (GCM) simulations for 10 ka and 6 ka ago. Paleodata and model results both show that warming occurred earlier across Beringia and Asia relative to lands adjacent to the North Atlantic, and that Late Holocene cooling was most apparent in the North Atlantic region. However, the GCM over-predicts the magnitude of Mid-Holocene warming over northern Asia and underestimates the intensification of the North Atlantic drift in the early Holocene. Strong spatial variability in environmental response during the Holocene, despite symmetric (insolation) forcing, suggests that any future changes, whether caused by anthropogenic or natural factors, are unlikely to result in a uniform change across the Arctic, adding additional complexity to forecasts of global impacts. (C) 2001 Elsevier Science Ltd. All rights reserved.