Floral evidence for high summer temperatures in southern Scandinavia during 15–11 cal ka BP

Frederik Schenka, Ole Bennikeb, Minna Välirantac, Rachael Averya, Svante Björckd, and Barbara Wohlfartha

aBolin Centre for Climate Research & Department of Geological Sciences, Stockholm University, Sweden
bGeological Survey of Denmark and Greenland, Øster Voldgade 10, Copenhagen K, DK 1350, Denmark
cEnvironmental Change Research Unit (ECRU), Ecosystems and Environment Research Programme, University of Helsinki, PO Box 65, FI-00014, Helsinki, Finland
dDepartment of Geology, Quaternary Sciences, Lund University, Box 117, SE221-00, Lund, Sweden

Mountains and glacier. Photo by Frederik Schenk

Abstract
The global climate transition from the Lateglacial to the Early Holocene is dominated by a rapid warming trend driven by an increase in orbital summer insolation over high northern latitudes and related feedbacks. The warming trend was interrupted by several abrupt shifts between colder (stadial) and warmer (interstadial) climate states following instabilities of the Atlantic Meridional Overturning Circulation (AMOC) in response to rapidly melting ice sheets. The sequence of abrupt shifts between extreme climate states had profound impacts on ecosystems which make it challenging to reliably quantify state variables like July temperatures within a non-analogue climate envelope. For Europe, there is increasing albeit inconclusive evidence for higher stadial summer temperatures than initially thought. Here we present a comprehensive floral compilation of plant macrofossils from lake sediment cores of 15 sites from S-Scandinavia covering the period ∼15 to 11 ka BP. We find evidence for a continued presence of plant species indicating high July temperatures throughout the last deglaciation. The presence of hemiboreal plants in close vicinity to the southern margin of the Fennoscandian Ice Sheet implies a strong thermal summer forcing for the rapid ice sheet melt. Consistent with some recent studies, we do not find evidence for a general stadial summer cooling, which indicates that other reasons than summer temperatures caused drastic setbacks in proxy signals possibly driven by extreme winter cooling and/or shorter warm seasons.

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