Abstract
The instrumental temperature record is of insufficient length to fully express the natural variability of past temperature. High elevation tree-ring widths from Great Basin bristlecone pine (Pinus longaeva) are a particularly useful proxy to infer temperatures prior to the instrumental record in that the tree-rings are annually dated and extend for millennia. From ring-width measurements integrated with past treeline elevation data we infer decadal- to millennial-scale temperature variability over the past 4,500 years for the Great Basin, USA. We find that twentieth century treeline advances are greater than in at least 4,000 years. There is also evidence for substantial volcanic forcing of climate in the preindustrial record and considerable covariation between high elevation tree-ring widths and temperature estimates from an atmosphere–ocean general circulation model over much of the last millennium. A long-term temperature decline of ~−1.1 °C since the mid-Holocene underlies substantial volcanic forcing of climate in the preindustrial record.
Highlights
Multiple lines of evidence, including comparisons with instrumental climate data over the last century, show that patterns in the ring widths of upper treeline bristlecone pine are strongly influenced by temperature variability, at decadal to centennial scales (LaMarche 1974; Hughes and Funkhouser 2003; Salzer and Kipfmueller 2005; Salzer and Hughes 2007; Salzer et al 2009; Kipfmueller and Salzer 2010; Towlinski-Ward et al 2010; Bunn et al 2011)
Using bristlecone pine ring widths to infer paleotemperature has been complicated by the discovery that trees somewhat below, yet still very near the upper treeline environment, as close as 150 vertical m, do not contain the same temperature signal as trees within *100 m of treeline
Past treeline estimates from the SHP site extend into the mid-Holocene (Fig. 2a)
Summary
Multiple lines of evidence, including comparisons with instrumental climate data over the last century, show that patterns in the ring widths of upper treeline bristlecone pine are strongly influenced by temperature variability, at decadal to centennial scales (LaMarche 1974; Hughes and Funkhouser 2003; Salzer and Kipfmueller 2005; Salzer and Hughes 2007; Salzer et al 2009; Kipfmueller and Salzer 2010; Towlinski-Ward et al 2010; Bunn et al 2011). It has been argued that microtopographical features such as cold air pooling, for example, could complicate defining temperature-sensitive trees based solely on elevation or distance from treeline (Bunn et al 2011) In spite of these challenges, we suggest the most accurate estimates of past temperature will come from ring widths formed in trees within *100 m of the species upper elevation ecotonal boundary. Old deadwood samples taken near modern upper treeline elevation or just below are avoided, as these rings would have been below the 100 m cold zone during their time of formation, and are inappropriate for paleotemperature inference This line of reasoning is based on the assumption that the same, or a very similar, temperature-growth threshold relationship existed in past eras as exists currently. This standardization approach is designed to retain decadal to multi-centennial scale variability and to minimize the
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