Paper Reviewed
Carrara, P.E. and McGeehin, J.P. 2015. Evidence of a higher late-Holocene treeline along the Continental Divide in central Colorado. The Holocene 25: 1829-1837.

In a recent treeline study from central Colorado, Carrara and McGeehin (2015) employed a combination of 23 radiocarbon ages and annual ring counts from 18 Rocky Mountain bristlecone pine (Pinus aristata) remnants found above the local present-day limits of such trees near the Continental Divide in central Colorado, which work revealed that the majority of the tree remnants "were established above the present-day limit of bristlecone pine from prior to 2700 cal. years BP to no later than about 1200 cal. years BP."

Other such evidence for the occurrence of warmer-than-current temperatures in the mid to late Holocene in the western United States has also been found and described in a number of other studies cited by Carrara and McGeehin, namely, those of LaMarche and Mooney (1967), LaMarche and Mooney (1972), LaMarche (1973), Petersen and Mehringer (1976), Scuderi (1987), Carrara et al. (1992), Fall (1997), Lloyd and Graumlich (1997), Doerner (2007), Benedict et al. (2008), Carrara (2011), Madole (2012), Lee and Benedict (2012), Lee et al. (2014) and Morgan et al. (2014).

Clearly, therefore, there exists a wealth of real-world data from the western United States that testifies of the fact that there is nothing unusual, unnatural or unprecedented about that region's current mean yearly maximum and minimum temperatures, which further suggests that the same is likely to be the case for the rest of the country ... and maybe even the rest of the world.

References
Benedict, J.B., Benedict, R.J., Lee, C.M. et al. 2008. Spruce trees from a melting ice patch: Evidence for Holocene climate change in the Colorado Rocky Mountains, US.A. The Holocene 18: 1067-1076.

Carrara, P.E. 2011. Deglaciation and Postglacial Treeline Fluctuation in the Northern San Juan Mountains, Colorado. US. Geological Survey professional paper 1782. Denver, CO: U.S. Geological Survey.

Carrara, P.E., Grisson, J., Lawrence, D. et al. 1992. Recent Treeline Migration on Rogers Peak, Colorado. Final report, third annual dendrochronological fieldweek, Mountain Research Station, University of Colorado, 31 July-8 August, 10 pp.

Doerner, J.P. 2007. Late Quaternary prehistoric environments of the Colorado Front Range. In: Brunswig, R.H. and Publado, B.L. (Eds.) Frontiers in Colorado Paleoindian Archaeology: From the Dent Site to the Rocky Mountains. Boulder, CO: University Press of Colorado, pp. 11-38.

Fall, 1997. Timberline fluctuations and late Quaternary paleoclimates in the southern Rock Mountains, Colorado. Geological Society of America Bulletin 109: 1306-1320.

LaMarche, V.C. 1973. Holocene climatic variations inferred from treeline fluctuations in the White Mountains, California. Quaternary Research 3: 632-660.

LaMarche, V.C. and Mooney, H.A. 1967. Altithermal timberline advance in western United States. Nature 213: 980-982.

LaMarche, V.C. and Mooney, H.A. 1972. Recent climatic change and development of the bristlecone pine (P. Longaeva Bailey) krummholz zone, Mt. Washington, Nevada. Arctic and Alpine Research 4: 61-72.

Lee, C.M. and Benedict, J.B. 2012. Ice bison, frozen forests and the search for archaeology in Colorado Front Range ice patches. Colorado Archaeology 78: 41-46.

Lee, C.M., Pederson, G., Dibenedetto, J. et al. 2014. Ice Patches and Relict Wood: A Paleoclimate Proxy for the Rocky Mountain West. Geological Society of America Abstracts with Programs, vol. 46. Boulder, CO: Geological Society of America.

Lloyd, A.H. and Graumlich, L.J. 1997. Holocene dynamics of treeline forests in the Sierra Nevada. Ecology 78: 1199-1210.

Madole, R.F. 2012. Holocene alluvial stratigraphy and response to climate change in the Roaring River valley, Front Range, Colorado. Quaternary Research 78: 197-208.

Morgan, C., Losey, A. and Trout, L. 2014. Late-Holocene paleoclimate and treeline fluctuation in Wyoming's Wind River Range, U.S.A. The Holocene 24: 209-219.

Petersen, K.L. and Mehringer, P.J. Jr. 1976. Postglacial timberline fluctuations, La Plata Mountains, southwestern Colorado. Arctic and Alpine Research 8: 275-288.

Scuderi, L.A. 1987. Late-Holocene upper timberline variation in the southern Sierra Nevada. Nature 325: 242-244.