Holocene Climatic Change in the North American Great Plains
Nordt, L., von Fischer, J., Tieszen, L. and Tubbs, J. 2008. Coherent changes in relative C4 plant productivity and climate during the late Quaternary in the North American Great Plains. Quaternary Science Reviews 27: 1600-1611.
Nordt et al.'s data suggest that their region of study was slightly warmer than it has yet to be in modern times during parts of both the Medieval and Roman Warm Periods, and that it was significantly warmer during a sizeable portion the mid-Holocene Thermal Maximum or Climatic Optimum, as it is sometimes called.
It therefore appears that for a broad swath of the midsection of the United States stretching from the center of Texas all the way to the U.S. border with Canada (and probably some distance beyond), the supposedly unprecedented warming of the 20th century (according to claims of the world's climate alarmists) was not unprecedented at all, having likely been surpassed one thousand, two thousand and four to five thousand years ago, when there was much less CO2 in the air than there is today. This observation thus begs the question of what was the cause of those earlier warmer-than-present periods. The answer of Nordt et al. is that "these warm intervals ... exhibit a strong correlation to increases in solar irradiance," as per the irradiance reconstruction of Perry and Hsu (2000).
Nordt, L., von Fischer, J. and Tieszen, L. 2007. Late Quaternary temperature record from buried soils of the North American Great Plains. Geology 35: 159-162.
Perry, C.A. and Hsu, K.J. 2000. Geophysical, archaeological, and historical evidence support a solar-output model for climate change. Proceedings of the National Academy of Sciences 97: 12,433-12,438.
von Fischer, J.C., Tieszen, L.L. and Schimel, D.S. 2008. Climate controls on C3 vs. C4 productivity in North American grasslands from carbon isotope composition of soil organic matter. Global Change Biology 14: 1-15.