Abstract
The upper treeline of Scots pine (Pinus sylvestris L.) is renowned as a sensitive indicator of climate change and variability. By use of megafossil tree remains, preserved exposed on the ground surface, treeline shift over the past millennium was investigated at multiple sites along the Scandes in northern Sweden. Difference in thermal level between the present and the Medieval period, about AD 1000-1200, is a central, although controversial, aspect concerning the detection and attribution of anthropogenic climate warming. Radiocarbon-dated megafossil pines revealed that the treeline was consistently positioned as much as 115 m higher during the Medieval period than today (AD 2000-2010), after a century of warming and substantial treeline upshift. Drawing on the last-mentioned figure, and a lapse rate of 0.6oC/100 m, it may be inferred that Medieval summer temperatures were about 0.7 oC warmer than much of the past 100 years. Extensive pine mortality and treeline descent after the Medieval warming peak reflect substantially depressed temperatures during the Little Ice Age. Warmer-than-present conditions during the Medieval period concur with temperature reconstructions from different parts of northern Fennoscandia, northwestern Russia and Greenland. Modern warming has not been sufficient to restore Medieval treelines. Against this background, there is little reason to view further modest warming as unnatural.
Highlights
The current scientific and public concern about future climate change and associated geoecological transformations draws on unvalidated General Circulation Models (IPCC 2007)
Such an effort can be accomplished by studying the history of the upper treeline, whose natural position and dynamism in principle relates to prevailing temperatures (Tranquillini 1979; Holtmeier 2003; Körner & Paulsen 2004)
Treeline rise by about 200 m (1915-2007) in the Swedish Scandes, as estimated by Kullman & Öberg (2009), is virtually what should be expected from recorded summer temperature rise by 1.4 °C and assuming a lapse rate of 0.6 °C/100 m
Summary
The current scientific and public concern about future climate change and associated geoecological transformations draws on unvalidated General Circulation Models (IPCC 2007) Their failure to convincingly account for stalling global temperature evolution since the late 1990s (Akasofu 2013; Tollefson 2014) indicates incomplete systemic understanding, which argues for more extensive inclusion of robust historical data as a prerequisite for realistic modelling of climate evolution and associated biological consequences (Humlum et al 2011). The importance of putting present-day change into a wider temporal context of previous climate and ecosystem variation cannot be overstated (cf Zwiers & Hegerl 2008) Such an effort can be accomplished by studying the history of the upper treeline, whose natural position and dynamism in principle relates to prevailing temperatures (Tranquillini 1979; Holtmeier 2003; Körner & Paulsen 2004). There is compelling evidence that the position of the upper treeline performs in a sub-centennial-scale dynamic equilibrium with climate evolution, indicative of profound effects on high-mountain biota (Kullman 2010a, 2012)
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