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5,000 Years of Great Basin Temperatures Derived from Tree Rings
Reference
Salzer, M.W., Bunn, A.G., Graham, N.E. and Hughes, M.K. 2014. Five millennia of paleotemperature from tree-rings in the Great Basin, USA. Climate Dynamics 42: 1517-1526.

Background
The authors write that "the instrumental temperature record is of insufficient length to fully express the natural variability of past temperature," but they say that "high elevation tree-ring widths from Great Basin (USA) 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 [back in time] for millennia."

What was done
Working with samples of bristlecone pine living trees and remnant wood located in the upper treeline zone (within 100 m of known treelines or estimated past treelines) in three separate western North America mountain ranges - Sheep Mountain (SHP) in California, Mt. Washington (MWA) in Nevada, and the Ruby Mountains' Pearl Peak (PRL) in Nevada - Salzer et al. were able to reconstruct a 4,582-year regional ring-width index chronology spanning the period 2575 BC-AD 2006.

What was learned
Based on the results of their painstaking work, the four researchers report that "the inferred temperature of the modern period was exceeded twice in the Common Era: +1.69°C in the mid first century AD (centered on AD 33) and +1.58°C in the mid seventh century (centered on AD 634)," while "the coldest interval is in the mid to late fifteenth century AD (centered on AD 1469)." And "consistent with these results and suggesting that some of the departures from mean conditions are at least hemispheric in scale," they say that "estimates of northern Scandinavian summer temperatures indicate nearly identical dates for their warmest and coldest 30-year periods over the last ~2,000 years, at AD 21-50 and AD 1451-1480 respectively," citing Esper et al. (2012). In addition, they say that other paleotemperature proxy archives, "such as lacustrine and meadow sediment cores, packrat middens, and glacial moraine dynamics (Thompson et al., 1994; Clark and Gillespie, 1997; Smith and Betancourt, 2006; Reinemann et al., 2009) are in general agreement with our results, recording higher temperatures during the middle Holocene."

What it means
As has been demonstrated time and time again, by a variety of means employed by a host of different researchers, late 20th-century and early 21st-century temperatures were neither unusual, unnatural nor unprecedented. And the fact that there were greater hot-to- cold and cold-to-hot swings in temperature prior to the Industrial Revolution - when the air's CO2 concentration was much lower and much less variable than it has been subsequently - suggests that the increase in the atmosphere's CO2 content over the past century or so has likely had next to no effect on the planet's air temperature.

References
Clark, D.H. and Gillespie, A.R. 1997. Timing and significance of late-glacial and Holocene cirque glaciation in the Sierra Nevada, California. Quaternary International 38/39: 21-38.

Esper, J., Frank, D.C., Timonen, M., Zorita, E., Wilson, R.J.S., Luterbacher, J., Holzkamper, S., Fischer, N., Wagner, S., Nievergelt, D., Verstege, A. and Buntgen, U. 2012. Orbital forcing of tree-ring data. Nature Climate Change: 10.1038/NCLIMATE1589.

Reinemann, S.A., Porinchu, D.F., Bloom, A.M., Mark, B.G. and Box, J.E. 2009. A multi-proxy paleolimnological reconstruction of Holocene climate conditions in the Great Basin, United States. Quaternary Research 72: 347-358.

Smith, F.A. and Betancourt, J.L. 2006. Predicting woodrat (Neotoma) responses to anthropogenic warming from studies of the palaeomidden record. Journal of Biogeography 33: 2061-2076.

Thompson, R.S., Whitlock, C., Bartlein, P.J., Harrison, S.P. and Spaulding, W.G. 1994. Climatic changes in the western United States since 18,000 yr B.P. In: Wright, H.E., Kutzbach, T., Webb, T.I., Ruddiman, W.F., Street-Perrott, F.A. and Bartlein, P.J. (Eds.). Global Climates Since the Last Glacial Maximum. University of Minnesota Press, Minneapolis, Minnesota, pp. 468-513.

Reviewed 4 June 2014