What was done
Working with dead tree trunks located above the current treeline on tephra-covered slopes of Whitewing Mountain and San Joaquin Ridge south of Mono Lake just east of the Inyo Craters in the eastern Sierra Nevada range of California (USA), the authors identified the species to which the tree remains belonged, dated them, and (using contemporary distributions of the species in relation to contemporary temperature and precipitation) reconstructed paleoclimate during the time they grew there.

What was learned
Millar et al. report that "the range of dates for the deadwood samples, AD 815-1350, coincides with the period identified from multiple [our italics] proxies in the Sierra Nevada and western Great Basin as the Medieval Climate Anomaly," among which are tree-ring reconstructions indicating "increased temperature relative to present (Graumlich, 1993; Scuderi, 1993) and higher treelines (Graumlich and Lloyd, 1996; Lloyd and Graumlich, 1997), and pollen reconstructions [that] show greater abundance of fir in high-elevation communities than at present (Anderson, 1990)."

Focusing on other of their findings, the five researchers say "the Medieval forest on Whitewing was growing under mild, favorable conditions (warm with adequate moisture)," as indicated by "extremely low mean sensitivities [to stress] and large average ring widths." More specifically, they conclude, as reported in their abstract, that annual minimum temperatures during the Medieval Climatic Anomaly in the region they studied were "significantly warmer" (+3.2°C) "than present," while in their final paragraph they say their results "closely compare to climate projections for California in AD 2070-2099 (Hayhoe et al., 2004)," in which "average temperature increases of 2.3-5.8°C were projected and slight increases or decreases in precipitation."

What it means
Once again, we have another example of a paleoclimate study in which the Medieval Warm Period is determined to have been significantly warmer than it is currently, which is something that occurs quite often in the Medieval Warm Period Record of the Week section of CO2 Science.

References
Anderson, R.S. 1990. Holocene forest development and paleoclimates within the central Sierra Nevada, California. Journal of Ecology 78: 470-489.

Graumlich, L.J. 1993. A 1000-yr record of temperature and precipitation in the Sierra Nevada. Quaternary Research 39: 249-255.

Graumlich, L.J. and Lloyd, A.H. 1996. Dendroclimatic, ecological, and geomorphological evidence for long-term climatic change in the Sierra Nevada, USA. In: Dean, J.S., Meko, D.M. and Swetnam, D.W. (Eds.), Proceedings of the International Conference on Tree Rings, Environment and Humanity, pp. 51-59.

Hayhoe, K., Cayan, D. and Field, C.B. 2004. Emissions pathways, climate change, and impacts on California. Proceedings of the National Academy of Science USA 101: 12,422-12,427.

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

Scuderi, L. 1993. A 2,000-year record of annual temperatures in the sierra Nevada Mountains. Science 259: 1433-1436.