Background
The authors write that "one of the key questions in the reconstruction of late Holocene climate is whether or not the 20th-century warming is unusual over the past two millennia," noting that "a clear answer to this question is crucial for the assessment of the relative contribution[s] of human activities and natural processes to the observed warming."

What was done
As their contribution to this important quest for knowledge, Wu et al., developed a bi-decadal-resolution record of sea surface temperature (SST) in the Southern Okinawa Trough that covered the past 2700 years. This they did by analyzing tetraether lipids of planktonic archaea in the ODP Hole 1202B (24°48'N, 122°30'E), which they describe as "a site under the strong influence of the Kuroshio Current and East Asian monsoon."

What was learned
The five Chinese scientists report finding SST anomalies that "generally coincided with previously reported late Holocene climate events, including the Roman Warm Period [120 BC-AD 400], Sui-Tang Dynasty Warm Period [AD 550-790], Medieval Warm Period [AD 900-1300], Current Warm Period [AD 1850-present], Dark Age Cold Period [AD 400-550] and Little Ice Age [AD 1300-1850]," and that "despite an increase since AD 1850, the mean SST in the 20th century is still within the range of natural variability during the past 2700 years." In addition, they state that climate records from East China (Ge et al., 2004), the North Icelandic Shelf (Patterson et al., 2010) and Greenland (Kobashi et al., 2011) also exhibit "centennial-scale warm periods during the first millennia AD, comparable to or even warmer than mean 20th-century conditions."

What it means
Wu et al. thus answer their own question about whether or not 20th century warming is unusual over the past two millennia," finding - for their site (and several others) - that it is not. And this answer, in turn, suggests that modern warming cannot be distinguished from warming induced by what they call "natural processes," which ultimately suggests there is no compelling reason to attribute modern warming to anthropogenic CO2 emissions.

References
Ge, Q.S., Zheng, J.Y., Man, Z.M., Fang, X.Q. and Zhang, P.Y. 2004. Key points on temperature change of the past 2000 years in China. Progress in Natural Science 14: 730-737.

Kobashi, T., Kawamura, K., Severinghaus, J.P., Barnola, J.M., Nakaegawa, T., Vinther, B.M., Johnsen, S.J. and Box, J.E. 2011. High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters 38: 10.1029/2011GL049444.

Patterson, W.P., Dietrich, K.A., Holmden, C. and Andrews, J.T. 2010. Two millennia of North Atlantic seasonality and implications for Norse colonies. Proceedings of the National Academy of Sciences USA 107: 5306-5310.