Reduced rainfall was among the factors that created multicentennial-scale drier conditions that may have contributed to the Indus Valley deurbanization. Lack of rainfall is believed to have been among the reasons for the social upheaval and the eventual fall of the Tang, Yuan and Ming dynasties of China. Drought interspersed with violent monsoon rains sounded the death knell of the Khmer empire of south-east Asia that flourished between 802 and 1431 CE.
There can be no doubting the profound impact of the abrupt shifts of rainfall on human history – a fact we need to constantly remind ourselves in this day and age of irretrievable climate change. Abrupt shifts in the Indian summer monsoon (ISM) precipitation has similarly impacted history in India. For example, bad precipitation is known to have worsened the agrarian crisis that tore asunder the mansabdari system of the Mughals, thereby hastening their fall.
But linking of ISM’s variability to human history in the subcontinent has been limited by the scarcity of high-resolution paleoclimatic data for the period encompassing the last 1000 years or earlier. Researchers at IIT Kharagpur and Wadia Institute of Himalayan Geology, Dehradun have been able to present high resolution oxygen isotope proxy record from the Wah Shikar cave in Meghalaya that identifies abrupt shifts in the ISM that has had profound impact on human society in the region for the past 900 years.
Given that India has seen extreme precipitation in the recent years that has caused devastation such as the Kedarnath floods of 2013 or the recent floods in Kerala and Tamil Nadu, researchers also believe that we need to investigate the long-term spatio-temporal variability of the ISM in order to understand the complex mechanisms that drive this variability, and therefore help improve predictive capabilities of climate models that help future planning. The speleothem proxy from Meghalaya, believe the researchers, holds crucial answers.
A 233 mm long stalagmite sample (WSS-3) was collected from the Wah Shikar cave. The sample was split into two halves and as many as 465 subsamples were extracted from them at every 0.5 mm interval parallel to the central growth axis. Palaeoclimatic records using oxygen isotope proxy in speleothems from northeastern India have been widely used to understanding ISM variability on millennial time scales. But data have not been derived from such finely divided samples.
“This is what is most definitive in this study,” says Prof. Anil Gupta, who has led the research and pioneered speleothem study in the country. “We took samples from every half millimetre or sometimes even one-third of a mm, and we dated using uranium-thorium time series. Such fine sampling of less time interval means we are covering data at 2-3 years’ interval while most research collect data at 20-30 years’ interval. In fact, we have even captured the drought events of last few centuries,” he added.
The analysis of the data derived from the samples show marked changes in ISM strength over northeastern India during the past 900 years coinciding with intervals of severe droughts and floods in the Indian subcontinent. The data pertains to the three main time slices – Medieval Climate Anomaly (MCA), Little Ice Age (LIA) and Current Warm Period (CWP). The findings have been corroborated with other proxy time series.
The data shows strong ISM conditions during MCA which is attributed to the warming of climate due to high solar insolation, which pulled the Inter Tropical Convergence Zone, or ITCZ, northward and strengthened the tropic monsoon circulation (ITCZ is a belt of low pressure which circles the Earth, generally near the equator, where the trade winds of the Northern and Southern Hemispheres come together). Data collected from caves in central India, northwestern India and western Himalaya corroborate the research team’s conclusion about ISM conditions during MCA.
During LIA, there were three different phases of precipitation in the ISM – all of them related to the cooling of the atmosphere owing to a host of factors – reduction in the number of sun spots, leading to the decreased solar insolation, volcanic forcings, and resultant atmospheric cooling. Each atmospheric cooling interval coincides with dry ISM episodes and long droughts as well as famine in India. Contrary to the earlier record from the same cave, the data collected by the IIT Kharagpur research team suggests reduced rainfall conditions during LIA.
The abrupt changes in the ISM during the last millennium had large socio-economic impact on Indian society. Several dynasties, such as the Sena in Bengal, Solanki in Gujarat in the mid-13th century and Paramara and Yadav in the early to mid-14th century – all of which flourished during abundant rainfall – declined during the drying phases of the ISM, suggesting role of the climate in the socio-political crisis. Weak monsoon meant decline in agriculture and trade, and thereby societal discontent that often resulted in upheaval and rebellions. The researchers suggest that the increase in rain-water harvesting structures in northern India during the period attests to the decline in precipitation.
From the beginning of the 19th century, the changes in ISM became more abrupt with a rise in atmospheric temperature that coincides with the dawn of Industrial Revolution and enhanced societal developments along with long term astronomical changes. The study says, “An increase in the frequency of abrupt shifts in the ISM during the last three centuries, coincidental with a rise in atmospheric temperature, suggests occurrence of more climatic surprises in future consequent to future rise in the global temperature and subsequently more precipitation in the form of rain at higher altitudes.”
The data from the Wah Shikar cave also indicates increased frequency of extreme rainfall events in India during the Current Warm Period that portends further increase in near future with the rise in global atmospheric temperature. This has already been predicted by the 5th Assessment Report of Inter-Governmental Panel on Climate Change, 2013. The study says that higher rainfall in high altitude regions in the Himalaya will increase the risk of flash floods and landslides as well as reduction in glacier extent that will affect Asia’s water resources and agricultural wealth, consequently harming south Asia’s economy in the decades to come.
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