New paper finds Medieval Warm Period in Tibet was warmer than the present, solar control of climate

A recent paper published in Quaternary Science Reviews reconstructs temperatures in Tibet over the past 2,500 years and finds the Medieval Warm Period was warmer than the current warm period. According to the authors, their data indicate "warmer climatic conditions during the MWP than the current warm period."

The authors find "the natural climate variability on the northern Tibetan Plateau during the late Holocene might be linked to solar irradiance changes on multi-decadal and centennial timescales." The paper demonstrates there is nothing unusual, unnatural or unprecedented about either the timing or the degree of warming experienced by the northern Tibetan Plateau over the past century or so.

From the latest edition of the NIPCC Report:
A 2500-Year Temperature History of the Northern Tibetan Plateau

Reference:

He, Y., Zhao, C., Wang, Z., Wang, H., Song, M., Liu, W. and Liu, Z. 2013. Late Holocene coupled moisture and temperature changes on the northern Tibetan Plateau. Quaternary Science Reviews 80: 47-57.

In the words of He et al. (2013), "understanding of climate variability under natural conditions could help improve projections of future climate change," while noting that the majority of temperature records from around the globe "show more or less similar patterns" that "can be roughly divided into a few warm and cold periods, such as the Current Warm Period [CWP] after AD 1850, the Little Ice Age (LIA) between AD 1850 and AD 1400, and the Medieval Warm Period (MWP) or Medieval Climate Anomaly (MCA) between AD 1400 and AD 800 (e.g. Esper et al., 2002; Mann and Jones, 2003; Moberg et al., 2005; Mann et al., 2009; Ge et al., 2010)," while still longer records "show another warm/cold oscillation, namely the Dark Ages Cold Period (DACP) and the Roman Warm Period (RWP, Lamb, 1985)."

To see how well the temperature history of the northern Tibetan Plateau might mimic these other records from around the globe, He et al. used alkenone indices U^K₃₇ and %_C37:4 - which they derived from sediment cores they extracted from two lakes in the plateau's Qaidam Basin - to reconstruct high-resolution histories of temperature and moisture changes over the last ~2500 years.

In addition to meshing well with the temperature histories noted above, the seven Chinese scientists report that on multi-decadal and centennial timescales, the temperature changes they derived "appear to correspond to solar irradiance changes as inferred from residual Δ¹⁴C and ¹⁰Be records (Bard et al., 2000; Reimer et al., 2004)." And they add that "within chronological uncertainty, potentially corresponding temperature and hydrological changes could be identified for the Oort, Wolf, Sporer, Maunder and Dalton solar minimums, as well as the Medieval and modern solar maximums," implying "the natural climate variability on the northern Tibetan Plateau during the late Holocene might be linked to solar irradiance changes on multi-decadal and centennial timescales." Last of all, they say their data also indicate "warmer climatic conditions during the MWP than the current warm period."

What all this means is that there is nothing unusual, unnatural or unprecedented about either the timing or the degree of warming experienced by either the northern Tibetan Plateau or the world over the past century or so, which leads to the logical conclusion that the increase in the air's CO₂ content over this period need not have been responsible for any of the warming of this period anywhere.

Additional References
Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus 52B: 985-992.

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Ge, Q.S., Zheng, J.-Y., Hao, Z.-X., Shao, X.-M., Wang, W.-C. and Luterbacher, J. 2010. Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference. Geophysical Research Letters 37: 10.1029/2009GL041281.

Lamb, H.H. 1985. Climatic History and the Future. Princeton University Press, New Jersey, USA.

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

Mann, M.E., Zhang, Z., Rutherford, S., Bradley, R.S., Hughes, M.K., Shindell, D., Ammann, C., Faluvegi, G. and Ni, F. 2009. Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly. Science 326: 1256-1260.

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.

Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Bertrand, C.J.H., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Hughen, K.A., Kromer, B., McCormac, G., Manning, S., Bronk Ramsey, C., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., van der Plicht, J. and Weyhenmeyer, C.E. 2004. IntCal04 terrestrial radiocarbon age calibration, 0-26 Cal kyr BP. Radiocarbon 46: 1029-1058.