Background
The authors write that "numerous paleoclimatic studies have extended the instrumental data to the past ~2000 years in various parts of the world," confirming that "global temperatures have varied between relative cold (i.e. the Little Ice Age, LIA) and warm (i.e. the Medieval Warm Period, MWP) conditions," additionally noting that in the pre-industrial era, "natural climate variability has been mainly linked to solar irradiance changes." But they say that "substantial scientific uncertainty exists on whether temperatures at the current warm stage are equal to or higher than those during the MWP, the most recent natural warm interval before the industrial period." And, therefore, they set out to further explore this significant scientific question.

What was done
Working with sediment cores that they extracted from the centers of Lake Sugan (38°52'N, 93°75'E) and Lake Gahai (37°08'N, 97°31'E), He et al. derived a pair of decadally-resolved, alkenone-based, temperature records for the Qaidam Basin of the northern Tibetan Plateau.

What was learned
The ten Chinese researchers report that "relative warm/cold periods can be clearly identified, including the current warm period after AD 1850, the LIA between AD 1350-1850, the MWP between AD 700-1350, the Dark Ages Cold Period between AD 50-700, and the warm period before AD 50," which is often referred to as the Roman Warm Period. In addition, they state that the two temperature records are "broadly consistent" with an alkenone-based record from Lake Qinghai (Liu et al., 2006), tree-ring-based temperature reconstructions from the mid-eastern Tibetan Plateau (Liu et al., 2009) and synthesized temperature reconstructions from the northeastern Tibetan Plateau (Yang et al., 2003) and the Northern Hemisphere (Moberg et al., 2005)." What is more, they say their records show that "the regional temperatures during the MWP exceeded those in the recent warm period," noting that "the estimated warmth during the MWP optimum relative to the recent level could be bracketed by the ~1.9°C estimated from Lake Gahai and ~0.5°C from Lake Qinghai." And in concluding their paper, they say that "the temperature variability appears to correspond to solar irradiance changes, suggesting a possible link between the two, at least on the northern Tibetan Plateau, during the late Holocene."

What it means
He et al. confidently state that "the MWP warmth was probably in a climate state beyond the climate variability captured by the recent warm period, joining a growing body of such evidence elsewhere," much of which we have collected over the past several years and archived in our Medieval Warm Period Project, where it can be seen that there is nothing unusual, unnatural or unprecedented about earth's current warmth.

References
Liu, Y., An, Z.S., Linderholm, H.W., Chen, D.-L., Song, H.-M., Cai, Q.-F., Sun, J.-Y. and Tian, H. 2009. Annual temperatures during the last 2485 years in the mid-eastern Tibetan Plateau inferred from tree rings. Science in China Series D - Earth Science 52: 348-359.

Liu, Z.H., Henderson, A.C.G. and Huang, Y.S. 2006. Alkenone-based reconstruction of Late-Holocene surface temperature and salinity changes in Lake Qinghai, China. Geophysical Research Letters 33: 10.1029/2006GL026151.

Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.

Yang, B., Achim, B. and Shi, Y.F. 2003. Late Holocene temperature fluctuations on the Tibetan Plateau. Quaternary Science Reviews 22: 2335-2344.