What was done
Noting, among other things, that current "concerns over anthropogenic climate change" emphasize the importance of understanding natural climatic variability in the U.S. Rocky Mountain West, and that "the hydroclimate of Yellowstone National Park (YNP) is of "particular interest" in light of its role as the "headwaters for both the Columbia and Missouri Rivers," the authors developed "a new tree-ring-based reconstruction of annual YNP precipitation spanning AD 1173-1998," working with "133 limber pine (Pinus flexilis James) and Douglas fir (Pseudotsuga menziesii (Mirbel) Franco) at four sites … in the Yellowstone National Park region."

What was learned
Gray et al. found that "extreme wet and dry years within the instrumental period fall within the range of past variability," and that "the magnitude of the worst-case droughts of the 20th century (AD 1930s and 1950s) was likely equaled or exceeded on numerous occasions [our italics] before AD 1900," or as they say in another place in their paper, "they were likely equaled or exceeded at least thirty times in the preceding six centuries [our italics]." In addition, they report that their precipitation reconstruction shows "significant decadal to multidecadal variability that can produce strong regime-like behavior in regional precipitation, with the potential for rapid, high-amplitude switching between persistent wet and dry conditions," adding that comparisons with the results of other studies "suggest that YNP droughts and wet periods over multiple time scales are often part of spatially complex, extra-regional to sub-continental precipitation events."

What it means
The world's climate alarmists continue to proclaim that (1) global warming leads to more extremes of both dry and wet (drought and flood) conditions, and that (2) 20th-century global warming has produced a climatic state that is warmer than anything earth has experienced over the prior one (Mann et al., 1998, 1999) to two (Mann and Jones, 2003) millennia - or much longer (Hansen et al., 2006). However, proxy precipitation data from the USA's Yellowstone National Park (which are well correlated with larger data sets of extra-regional to sub-continental scale), augmented by data from all around the world (see Precipitation in our Subject Index), suggest that one or both of these claims simply cannot be correct.

References
Hansen, J., Sato, M., Ruedy, R., Lo, K., Lea, D.W. and Medina-Elizade, M. 2006. Global temperature change. Proceedings of the National Academy of Sciences USA 103: 14,288-14,293.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.

Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.

Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.