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The Medieval Warm Period and Little Ice Age in Arid Central Asia
Reference
Chen, F.-H., Chen, J.-H., Holmes, J., Boomer, I., Austin, P., Gates, J.B., Wang, N.-L., Brooks, S.J. and Zhang, J.-W. 2010. Moisture changes over the last millennium in arid central Asia: A review, synthesis and comparison with monsoon region. Quaternary Science Reviews 29: 1055-1068.

Background
The authors write that arid central Asia (ACA, an inland zone in central Asia from the Caspian Sea in the west to the southern Mongolian Plateau in the east) is "a unique dry-land area whose atmospheric circulation is dominated today by the westerlies," further stating that it is "one of the specific regions that are likely to be strongly impacted by global warming," which could greatly impact its hydrologic future.

What was done
In an attempt to obtain this important knowledge, Chen et al. evaluated "spatial and temporal patterns of effective moisture variations," using seventeen different proxy records in the ACA and synthesizing a decadal-resolution moisture curve for this region over the past millennium, employing five of the seventeen records based on their having "reliable chronologies and robust proxies."

What was learned
The nine researchers report that the effective moisture (precipitation) in the ACA has a generally inverse relationship with the temperature of the Northern Hemisphere, as portrayed by Moberg et al. (2005), China, as portrayed by Yang et al. (2002), and Central Asia, as portrayed by Esper et al. (2007). That is to say, as they describe it, the "wet (dry) climate in the ACA correlates with low (high) temperature." And stating it in yet another way, they indicate that the ACA "has been characterized by a relatively dry Medieval Warm Period (MWP; the period from ~1000 to 1350 AD), a wet little Ice Age (LIA; from ~1500-1850 AD)," and "a return to arid conditions after 1850 AD," which has been slightly muted -- but only "in some records" -- over the past 20 years by an increase in humidity.

What it means
Chen et al. "propose that the humid LIA in the ACA, possibly extending to the Mediterranean Sea and Western Europe, may have resulted from increased precipitation due to more frequent mid-latitude cyclone activities as a result of the strengthening and equator-ward shift of the westerly jet stream ... coupled with a decrease in evapotranspiration caused by the cooling at that time," which cooling was brought about by the gradual demise of the Medieval Warm Period, which in turn speaks volumes about the great significance of that centuries-long period of much-lower-than-present atmospheric CO2 concentration but of equivalent or even greater warmth than that of the Current Warm Period, which ultimately suggests that the 20th-century increase in the air's CO2 content may have had little, or maybe even nothing, to do with 20th-century global warming.

References
Esper, J., Frank, D.C., Wilson, R.J.S., Buntgen, U. and Treydte, K. 2007. Uniform growth trends among central Asian low- and high-elevation juniper tree sites. Trees -- Structure and Function 21: 141-150.

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., Braeuning, A., Johnson, K.R. and Shi, Y.F. 2002. General characteristics of temperature variation in China during the last two millennia. Geophysical Research Letters 29: 10.1029/2001gl014485.

Reviewed 28 July 2010