What was done
Based on isotopic soil carbon measurements made on 24 modern soils and 30 buried soils scattered between latitudes 48 and 32°N and longitudes 106 and 98°W, the authors developed a time series of C4 vs. C3 plant dynamics for the past 12 ka (ka = 1000 ¹⁴C yr BP) in the mixed and shortgrass prairie of the U.S. Great Plains; and because, as they describe it, the percent soil carbon derived from C4 plants "corresponds strongly with summer temperatures as reflected in the soil carbon pool (Nordt et al., 2007; von Fischer et al., 2008)," they were able to devise a history of the relative warmth of the climate of the region over this protracted period.

What was learned
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.

What it means
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).

References
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.