An Environmental History of Yellowstone National Park
Whitlock, C., Dean, W., Rosenbaum, J., Stevens, L., Fritz, S., Bracht, B. and Power, M. 2008. A 2650-year-long record of environmental change from northern Yellowstone National Park based on a comparison of multiple proxy data. Quaternary International 188: 126-138.
Whitlock et al. report that their many datasets "are consistent with overall warmer/drier conditions during the Medieval Climate Anomaly," which they say has been variously dated between AD 650 and 1300 in the western US and Great Plains. More specifically, they note that "the Crevice Lake data suggest a warm interval with dry winters between AD 600 and 850, followed by less dry but still warm conditions between AD 850 and 1100." In addition, they write that "other studies in YNP indicate that trees grew above present-day treeline and fires were more frequent in the Lamar and Soda Butte drainages between AD 750 and 1150," citing Meyer et al. (1995).
As for the modern period, the seven researchers say their data indicate that "the last 150 years of environmental history since the formation of YNP have not been anomalous within the range of variability of the last 2650 years, and many of the proxy indicators suggest that 19th and 20th century variability at Crevice Lake was moderate compared with earlier extremes." In fact, they note that with the possible exception of the charcoal record, "all of the data show greater variability in the range of ecosystem conditions prior to the establishment of the YNP in 1872."
The many parameters measured by Whitlock et al. testify to the non-uniqueness of YNP's 20th-century climate and the significant warmth of the Medieval Warm Period, which allowed trees in some parts of the Park to grow at higher elevations than they do presently, which in turn suggests that much of the Medieval Warm Period was significantly warmer than the Current Warm Period has been to date.
Meyer, G.A., Wells, S.G. and Jull, A.J.T. 1995. Fire and alluvial chronology in Yellowstone National Park: climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107: 1211-1230.