Written by Adin Richards '23
Edited by Maximilian Bonnici MS '20
The fertile Gangetic plains span much of South Asia, encompassing parts of Pakistan, Nepal, Bangladesh, and India . The region is not merely productive, but also densely populated, hosting over six hundred million residents in India alone . Over half of the region’s annual rainfall comes from the Indian Summer Monsoons , a fact which, coupled with India’s heavily agrarian society, has given rise to the saying that the country’s economy is “a gamble in the hands of monsoons” . With temperatures rising globally, understanding the influence of our planet’s dynamic climate on the reliability and intensity of the monsoons is of paramount importance to South Asia’s policy-makers and citizens. A recent investigation provides new insight into monsoon behavior and impacts on society over the past millennia, as recorded by the composition of cave formations 
Current models forecasting future trends in Indian subcontinent precipitation predict increases in variability, but an overall trend toward greater amounts of rain. While many forces underlie this prospect, the association of warming temperatures and higher intensity monsoons stems from the impact of temperature differences between the land and sea, combined with higher evaporation rates . Water is relatively resistant to temperature changes, meaning that global warming impacts the land faster than the ocean . Hotter air above land rises, ushering in moist air from over the oceans, creating monsoons . Under conditions unaffected by global warming, the heating of the land is natural and simply the result of the summer months, and the moisture of above-ocean air is less due to lower evaporation rates that characterize warmer climates . While these dynamics play out in models, corroboration and further insight lie in the knowledge of past monsoons, provided by paleoclimate proxies. Records of the past millennia, however, are strikingly scarce, prompting researchers to examine the relative depletion of heavy oxygen isotopes in calcite cave formations, or speleothems .
Speleothems in northeast India’s Wah Shikah cave formed as a result of water percolating through overlying calcium-rich soil and rock. The water, acidified by dissolved carbon dioxide, collected calcium as it traveled to the cave. Eventually, it could no longer hold as much of the dissolved element once it lost the carbon dioxide from entering the open air of the cave . Consequently, calcite (CaCO3) precipitated onto the cave walls, making iconic stalactites and stalagmites, but also creating a record of the ratio of heavy to light oxygen isotopes present in the water at the time of these structures’ formation. If more of the water came from monsoons, then there would be a relative decrease in the amount of heavy oxygen (18O) since monsoonal rains are sourced from the evaporation of ocean water, a process which prefers lighter oxygen (16O) . The constant temperature and humidity of cave environments contribute to the utility of speleothems in preserving information about the time of their formation, with modern factors having little effect on their composition .
Using a method of radiometric dating which relies on the amount of thorium-230 (the result of decayed uranium-234 ) relative to a known baseline, researchers determined the dates at different intervals along a sampled stalagmite. This, combined with the record of monsoonal contribution to overall rain inferred from oxygen isotope ratios, provided an over nine hundred year record of monsoon variation over the Gangetic plains of Northern India . Stronger monsoons appeared to characterize a period known as the Medieval Climate Anomaly, a time of warmer temperatures across the globe . Conversely, an overall reduction in monsoon intensity corresponded with low temperatures during the Little Ice Age, corroborating the general forecasted association between higher temperatures and greater monsoonal rainfall. Beyond these broad correlations, narrow, specifically timed cooling and warming events respectively resulting from volcanic eruptions and heightened solar activity aligned closely with observed variation in monsoon strength. Prolonged cool periods not only corresponded with overall lower monsoon intensity but also a reduction in variability, potentially boding poorly for future precipitation stability in the region as our world heats up.
The historic interplay between environment and society in India is highlighted by a correspondence between the dates when canals were constructed across the region and when the monsoons were weak, since irrigation was an adaptation to insufficient water supply. This is further seen in the relationship between decreased monsoon strength and the decline of several dynasties, including the Sena of Bengal, the Solanki of modern-day Patan, the Paramara of west-central India, and the Deccan plateau-ruling Yadav. This relationship may stem from the reduced military capacity that follows agricultural losses associated with lower rainfall, as this decreases food stores’ requisites for standing armies and an empire’s ability to trade . More recently, the weakening of the Mughal Empire that facilitated British conquest times with a drop in monsoon activity and consequently agricultural output .
The impacts of a changing climate are multifaceted and only beginning to be fully understood . In other sciences, experiments can be run and rerun, with effects of small modulations on a broader system recorded with no impact on the world outside the lab. When the subject of study is our planet, however, climate science can’t wait for the effects of our current manipulation of the atmosphere, biomes, and oceans to play out before reporting back about how our earth responded. Researchers must therefore examine clues of the past, which are recorded on and below our planet’s surface. In doing so, the workings of our world can be pieced together to inform our actions in the face of our inadvertent experiment on our pale blue dot.
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