Recent reversal of Neoglacial climate cooling trend in the Swedish Scandes as evidenced by mountain birch tree-limit rise

Abstract

An intensive study reconstructs the local late-Holocene tree-limit history ( Betula pubescens ssp. tortuosa) in the southern Swedish Scandes, as a proxy palaeoclimatic indicator. Elevational tree-limit rise by 75 m, in accordance with instrumentally recorded summer warming, took place over the past century as evidenced by a combination of historical records and modern age–structure analyses. This regionally representative development was put into long-term perspective by radiocarbon-dating of a large sample of megafossil wood remains (trunks and roots), preserved and recovered at the same location and elevation as the new and higher tree-limit. The magnitude and rate of 20th century tree-limit advance (implicitly climate warming) were without precedent for the past ca. 3500 calendar years. The course of development conforms to an increasingly emergent worldwide pattern, suggesting that warming over the past century is a fundamental reversal of a long-term (Neoglacial) cooling trend. The results are of relevance for current efforts to attribute causes of recent warming in terms of natural versus anthropogenic forcing. The highly anomalous nature of the past century, with respect to climate and ecological change, could argue that at least in part, the warming may be human-induced. However, definite conclusions have to await more global-scale data.