Background
Writing of "the potential consequences that would be associated with a drought lasting years to decades, or even a century (megadrought)," the authors say that such a phenomenon "constitutes one of the greatest threats to human welfare," noting that it would be "a particular serious threat for the predominantly agrarian-based societies of monsoon Asia, where the lives of billions of people are tightly intertwined with the annual monsoon cycle."

What was done
In exploring this ominous subject in great detail, Sinha et al. review what is known about it as a result of numerous pertinent studies, relying heavily on the work of Sinha et al. (2007) and Berkelhammer et al. (2010), based on the δ¹⁸O record of a speleothem from Dandak Cave in central-eastern India, which documents Indian monsoon rainfall variations between AD 600 and 1500.

What was learned
The eight researchers, hailing from China, Germany and the United States, report that "proxy reconstructions of precipitation from central India, north-central China [Zhang et al., 2008], and southern Vietnam [Buckley et al., 2010] reveal a series of monsoon droughts during the mid 14th-15th centuries that each lasted for several years to decades," and they say that "these monsoon megadroughts have no analog during the instrumental period." They also note that "emerging tree ring-based reconstructions of monsoon variability from SE Asia (Buckley et al., 2007; Sano et al., 2009) and India (Borgaonkar et al., 2010) suggest that the mid 14th-15th century megadroughts were the first in a series of spatially widespread megadroughts that occurred during the Little Ice Age," and that they "appear to have played a major role in shaping significant regional societal changes at that time." Among these upheavals, they make special mention of "famines and significant political reorganization within India (Dando, 1980; Pant et al., 1993; Maharatna, 1996), the collapse of the Yuan dynasty in China (Zhang et al., 2008); Rajarata civilization in Sri Lanka (Indrapala, 1971), and the Khmer civilization of Angkor Wat fame in Cambodia (Buckley et al., 2010)," noting that the evidence suggests that "monsoon megadroughts may have played a major contributing role in shaping these societal changes."

What it means
In light of the fact that there were, in the words of Sinha et al., "at least five episodes of monsoon megadroughts during the Little Ice Age (nominally, AD 1350-1850)," the peoples of the world should be extremely thankful that the earth has finally emerged from this unique period of global coolness, which is universally recognized as being the coldest interval of the current interglacial. Especially should this be so, for they add in their closing paragraph that "the present-day water-resource infrastructure and planning are barely sufficient to meet the welfare of billions of people during a single season of anomalous weak monsoon, let alone a protracted failure," such as what occurred repeatedly during the global chill of the Little Ice Age.

References
Berkelhammer, M., Sinha, A., Mudelsee, M., and Cannariato, K.G. 2010. Persistent multidecadal power in the Indian summer monsoon. Earth and Planetary Science Letters 290: 166-172.

Borgaonkar, H.P., Sikdera, A.B., Rama, S. and Panta, G.B. 2010. El Niņo and related monsoon drought signals in 523-year-long ring width records of teak (Tectona grandis L.F.) trees from south India. Palaeogeography, Palaeoclimatology, Palaeoecology 285: 74-84.

Buckley, B.M., Anchukaitis, K.J., Penny, D., Fletcher, R., Cook, E.R., Sano, M., Nam, L.C., Wichienkeeo, A., Minh, T.T. and Hong, T.M. 2010. Climate as a contributing factor in the demise of Angkor, Cambodia. Proceedings of the National Academy of Sciences USA 107: 6748-6752.

Buckley, B.M., Palakit, K., Duangsathaporn, K., Sanguantham, P. and Prasomsin, P. 2007. Decadal scale droughts over northwestern Thailand over the past 448 years: links to the tropical Pacific and Indian Ocean sectors. Climate Dynamics 29: 63-71.

Dando, W.A. 1980. The Geography of Famine. John Wiley, New York, New York, USA, p. 209.

Indrapala, K. 1971. The Collapse of the Rajarata Civilization and the Drift to the Southwest. University of Ceylon Press.

Maharatna, A. 1996. The Demography of Famines: An Indian Historical Perspective. Oxford University Press, Delhi, India, p. 317.

Pant, G.B., Rupa-Kumar, K.N., Sontakke, A. and Borgaonkar, H.P. 1993. Climate variability over India on century and longer time scales. In: Keshavamurty, R.N. and Joshi, P.C. (Eds.). Tropical Meteorology. Tata McGraw-Hill, New Delhi, India, pp. 149-158.

Sano, M., Buckley, B.M. and Sweda, T. 2009. Tree-ring based hydroclimate reconstruction over northern Vietnam from Fokienia hodginsii: eighteenth century mega-drought and tropical Pacific influence. Climate Dynamics 33: 331-340.

Sinha, A., Cannariato, K.G., Stott, L.D., Cheng, H., Edwards, R.L., Yadava, M.G., Ramesh, R. and Singh, I.B. 2007. A 900-year (600 to 1500 A.D.) record of the Indian summer monsoon precipitation from the core monsoon zone of India. Geophysical Research Letters 34: 10.1029/2007GL030431.

Zhang, P.Z., Cheng, H., Edwards, R.L., Chen, F.H., Wang, Y.J., Yang, X.L., Liu, J., Tan, M., Wang, X.F., Liu, J.H., An, C.L., Dai, Z.B., Zhou, J., Zhang, D.Z., Jia, J.H., Jin, L.Y. and Johnson, K.R. 2008. A test of climate, sun, and culture relationships from an 1810-year Chinese cave record. Science 322: 940-942.