Tuesday, July 29, 2008

Cave records of the monsoon

At the laggardly rate I am going, it is going to take me forever and then some to eventually post all, or at least a decent number, of my articles here. Anyway, here’s a story that appeared in Deccan Herald earlier this year. I found the whole idea of using stalactites and stalagmites – those things we usually ooh and aah over in caves – to study monsoon history just fascinating. Read on…

The Indian monsoon is notoriously unpredictable, foiling most intelligent attempts at modeling it. Now, a stalagmite in a Chhattisgarh cave has provided researchers with a record of rainfall from 600 to 1500 AD which shows that the monsoon may be even more variable than we thought. The data showed severe monsoon failures of a magnitude not known during the 150 years that rainfall has been measured, deficits which tallied with historical accounts of famines and droughts.
India’s economy and the lives of millions in the subcontinent are closely tied to the vagaries of the monsoon. Recognizing the need to understand the phenomenon which sustained the region, the British began recording rainfall in 1813, when the first recording station was set up in Chennai. By 1871, 306 stations around the country recorded monthly rainfall, providing today’s meteorologists with ~150 years of instrumental records to study the monsoon and its variations. This instrumental record shows fairly strong variations from year to year and place to place. But though the record has been useful in helping us understand the phenomenon better, the summer monsoon still remains difficult to simulate and predict.
Enter the speleologists. Cave formations like stalactites and stalagmites (together called speleothems) that have us lay people marveling at their fantastical shapes, have been a boon to scientists studying past climate conditions. Speleothems are essentially calcium carbonate deposits and their formation depends on rainfall. In brief, rainfall, which contains some carbon dioxide dissolved in it, dissolves small amounts of limestone (essentially calcium carbonate) as it percolates through soil. When it enters a cave, the dissolved carbon dioxide is outgassed, leading the calcium carbonate to precipitate out. Over years, this leads to the formation of a speleothem. Because their rate of formation depends on the amount of water available, speleothems are petrified records of the climate of their period.
But the reason speleothems have become a focus of paleoclimate research is because they can be dated accurately. The water percolating into the cave contains trace amounts of radioactive uranium, which gradually decays into thorium. Since thorium itself is insoluble in water, any thorium in the speleothem can only have arisen from radioactive decay of uranium. Because we know their half lives (i.e., their rates of decay), we can use the uranium-thorium ratio in the speleothem to accurately date the structure back to several thousand years.
This is essentially how Dr Ashish Sinha of California State University, Dominguez Hills, Carson, USA and colleagues from the USA and India dated and measured rainfall using a speleothem in Dandak Cave in Kanger Valley National Park, Chhatisgarh. Co-author Prof R Ramesh, Physical Research Laboratory, Ahmedabad, who has been working on cave-related research since 1995, says they were directed to the Dandak cave by chance, which proved useful because the cave has not been used by modern man and is closed to tourists. The cave has two chambers with a small connecting passage between the two, which researchers had to crawl through to get to a 27-cm long stalagmite from the second cave, some 220 m away from the cave entrance.
Their paper, published recently in the journal Geophysical Research Letters, has some astounding, if disturbing, results. Analysis of the stalagmite revealed that substantially poor monsoons occurred during the 14th and 15th centuries with rainfall deficits in the range of 30%. Though shortfalls of this magnitude also occur in the modern instrumental record, the difference is that the failures lasted several decades. Strikingly, several famines or droughts in India for which we have historical reports correspond with a decreased rainfall record in the Dandak stalagmite. One of the earliest recorded famines in India occurred in 650 AD, at which period the Dandak stalagmite shows the monsoon had failed for about three decades. In the late 1330s and early 1340s, Ibn Batuta records how towns and districts were depopulated by famines and people were reduced to eating animal hides and human flesh – the Dandak record shows that the rains had failed since about the late 1320s. The infamous Durga Devi famine (1396 to 1407 AD) which devastated the country and led to vast tracts of land being left uncultivated for several years coincides with the most severe monsoon deficit in the 900-year record, spanning several decades.
The authors sound a note of caution based on their findings, pointing out that in a region where the population has grown exponentially since the 1500s, multi-decadal monsoon failure of the sort reflected in Dandak would have disastrous consequences, particularly since we have no preparedness for such events. Prof Ramesh, who is an IPCC member and authored a chapter on paleoclimate in the recent IPCC report, feels “an integrated societal response” is required to deal with severe monsoon deficits. He outlines some simple measures that may help sustain us through rainless periods. “Agriculture will be the first thing to be affected so we have to strengthen our storage of food grains,” he says, adding wryly that stories of rats eating stored grain will have to go. “Small changes in food habits could help; we could switch to crops that are less water demanding instead of depending on rice which is water intensive,” he says. Though Prof Ramesh adds the caveat that “there are always uncertainties regarding climate predictions,” common sense declares it would be wiser to be well prepared for the whims of our capricious monsoon.

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