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A 1500-Year History of Northern Hemisphere Monsoon Activity
Asmerom, Y., Polyak, V.J., Rasmussen, J.B.T., Burns, S.J. and Lachniet, M. 2013. Multidecadal to multicentury scale collapses of Northern Hemisphere monsoons over the past millennium. Proceedings of the National Academy of Sciences USA 110: 9651-9656.

The authors note that knowledge of regional responses to past climate forcings and their associated atmospheric teleconnections "is likely to be the key to building a unified global understanding of future climate change." And in this regard they say that the past 1500 years, including the Medieval Climate Anomaly and Little Ice Age, may be "of particular interest," since "this period contains observed climate variability before a significant anthropogenic forcing," and thus provides "important validation points for models of future climate change."

What was done
In harmony with this reasoning, and working with stalagmite BC-11, which was collected while it was still actively growing in 2004 within the Bat Cave of Carlsbad Caverns, New Mexico, USA, Asmerom et al. measured δ13C and δ18O values at intervals of approximately 10 years, as well as the thickness of annual growth bands, which - like the isotopic data - had been shown by some of their earlier work (Polyak and Asmeron, 2001; Rasmussen et al., 2006) to be positively correlated to moisture amount.

What was learned
The five U.S. researchers say "the speleothem band thickness and δ13C and δ18O isotopic data show the Medieval Climate Anomaly-Little Ice Age interval to have been a time of extreme climate variability, punctuated by multidecadal dry periods (here referred to as megadroughts), some of which were previously shown to have been spatially extensive throughout the western United States (Cook et al., 2004)." In addition, they identified an extended multicentury dry period that they dubbed the Super Drought, which occurred during the early Little Ice Age ca. AD 1350-1650, and which their data showed to have been the most severe of the entire record.

Another interesting finding was the synchronicity they observed among southwestern North American, Chinese and West African monsoon precipitation, which seemed to suggest that the megadroughts were hemispheric in scale. They also found them to be associated with cooler than average sea surface temperature (SSTs) and Northern Hemisphere temperatures (NHTs). And they observed that "the megadroughts, including the Super Drought, coincide with solar insolation minima, suggesting that solar forcing of sea surface and atmospheric temperatures may generate variations in the strength of Northern Hemisphere monsoons.

What it means
Asmerom et al. say their analysis suggests that "Northern Hemisphere monsoon megadroughts over the past 1.5 millennia are associated with cold NHTs and SSTs and intervals of low solar irradiance, whereas pluvials are associated with the opposite patterns," which would seem to suggest that if solar activity were to begin to decline sometime in the near future, humanity might very well - and quickly - find itself in a world of hurt due, ironically, to global cooling.

Cook, E.R., Woodhouse, C.A., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Polyak, V.J. and Asmerom, Y. 2001. Late Holocene climate and cultural changes in the southwestern United States. Science 294: 148-151.

Rasmussen, J.B.T., Polyak, V.J. and Asmerom, Y. 2006. Evidence for Pacific modulated precipitation variability during the late Holocene from the southwestern USA. Geophysical Research Letters 33: 10.1029/2006GL025714.

Reviewed 16 October 2013