Human activities add more carbon dioxide to the air, which traps more heat and melts ice. More heat also means more water vapor in the atmosphere and more precipitation – mainly in the sea, depriving the continents of their share of rain.
Sediment cores taken from below the seabed in the Bay of Bengal, off the Indian coast, revealed that atmospheric carbon dioxide levels and associated warming were “major players” in shaping the intensity of the South Asian monsoon over the past million years, scientists said. in a new study. The results support numerical models that predict stronger monsoons with increasing concentrations of carbon dioxide in the atmosphere in the future.
Along with fluctuations in atmospheric carbon dioxide (CO2), the Pleistocene monsoons were also sensitive to the volume of continental ice and the import of moisture from the southern hemisphere of the Indian Ocean, according to research that verified at soil numerical models used to predict the future. climate change. Scientists have reconstructed monsoon activity over the past 900,000 years using clues safely hidden in sediments on the ocean floor.
âThis particular paper identified three different mechanisms that are responsible for the evolution of the monsoon over time. We have shown that the monsoon is also sensitive to the combined effects of ice volume and CO2 as well as the amount of moisture that flows from the southern hemisphere to the northern hemisphere during summer monsoon winds. ; these are the two factors that determine the changes in the monsoon at the timescale that we studied, âSteven Clemens, professor of geological sciences (research) at Brown University and senior author of the study, told Mongabay-India. ‘study.
Clemens shed light on the coupling between ice volume and atmospheric CO2 levels which together impact monsoons. âThe volume of ice in the northern hemisphere and the levels of CO2 in the atmosphere are intimately linked in the past. At about the same time that CO2 peaks, the global volume of ice is at a minimum, so when CO2 increases, the ice caps melt, sea ice retreats towards the poles, âhe said. .
The results of the study validate the model’s predictions suggesting that monsoons are indeed sensitive to CO2-related warming and that in a warming world, there is going to be more water vapor in the atmosphere, added a university press release. Research suggests heavier rains in the future as CO2 levels rise.
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Clemens, however, clarified that stronger monsoons do not necessarily mean strengthening winds associated with the monsoon per se “but the spread of strong winds over much larger portions of the Arabian Sea and the Bay of Bengal. “. âThis then helps to increase the rate of evaporation and the evaporated moisture is carried away from the continents. This is the local effect.
When it comes to the global impacts of CO2-associated warming, places that are now rainy are very likely to receive more precipitation. âEvery time you put more CO2 into the atmosphere, it gets warmer and a warmer atmosphere evaporates more moisture from the global ocean; thus the global atmosphere will carry more humidity. Therefore, India is not the only place that will receive more rainfall, âhe said.
Clemens expressed concern that “many governments have yet to fully understand” the extent to which humans impact Earth’s climate and are not yet taking enough action to mitigate the known impacts that occur now and in the future due to the burning of fossil fuels. âWe, as individuals, and our governments must take action to reduce emissions if we are to leave a healthy and functioning environment for our children and future generations. “
The study is one of many from an expedition, International Ocean Discovery Program Expedition 353, an international effort to drill sediment cores in the Bay of Bengal from November 2014 to January 2015. Clemens, working with a team international research vessel, sailed aboard the research vessel JOIDES Resolution, a converted oil drilling vessel, to the Bay of Bengal, to retrieve samples of sediment cores from the seabed.
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Departing from Singapore in November 2014, the vessel and her crew covered approximately 4,000 nautical miles in two months, collecting core at six locations in and around the Bay of Bengal, recovering 4,280m of sediment during 33 days of drilling on the spot. The expedition was funded in part by the US National Science Foundation, a 2015 press release says.
The Indian summer monsoon provides the majority of water for agriculture and industry in South Asia and is vital for the well-being of 1.4 billion people. The periods of activity and pause of the monsoon have a major influence on the success of agriculture, while the interannual variations in precipitation have economic consequences on an international scale.
Understanding the impacts of climate change on the monsoon is essential. Clemens adds that numerical models that seek to understand how the monsoon will change in the future are calibrated on the modern climate, then “simulate the future by changing conditions, for example the concentration of CO2 in the atmosphere”.
“For example, with the Indian monsoon, as we continue to increase the concentration of CO2 in the atmosphere, there will be more humidity in the atmosphere and the monsoon will become stronger to counter the influence of aerosols on India, which tend to moderate the monsoon, âClemens said, adding that these competing factors, pulling the monsoon in opposite directions, have been the subject of significant research.
However, precipitation is not that easy to model. âPart of the problem with digital models is that precipitation occurs at much smaller spatial scales than the model’s grid scales. It is therefore difficult to model precipitation; we can reconstruct precipitation changes in the past and this can be used as a target to confirm which of these models is working well or not, âClemens explained.
The results of the present study are based on cores extracted from the Mahanadi offshore basin at 1430 m below sea level on the Indian margin (Site U1446). The Bay of Bengal, surrounded on three sides by land, is the least salty ocean basin in the world, receiving about 2,950 cubic kilometers of runoff per year. Changes in precipitation and ocean surface salinity are captured and preserved in a number of chemical, physical, isotopic and biological components of the sediments deposited there.
Once the sediment cores returned to the lab, the researchers isotopically reconstructed from the past monsoon using indicators such as oxygen isotopes in planktonic fossils, carbon isotopes in leaf wax, and the rubidium from site U1446 on the Indian mainland margin.
The research is important because such a multi-proxy analysis helps us to better track the monsoon in the past, says Supriyo Chakraborty of the Indian Institute of Tropical Meteorology in Pune. âEach proxy is sensitive to a different factor that we want to look at; when we apply multiple proxies and if we get a convergent “result” and “interpretation”, it lends credence to our analysis. These proxies help us reconstruct the monsoon going back beyond 150 years of instrumental rainfall recording, âChakraborty added. It was not associated with the study.
For example, in an article published in 2020, scientists warned of an increased risk of flooding in the Brahmaputra River based on a 700-year reconstruction of the river’s water flow during the monsoon, from of data on tree rings, suggesting that the true flood of the transboundary river the risk is probably underestimated because the instrumental recordings were taken during a dry period.
India’s Nationally Determined Contributions (NDCs) under the Paris Agreement primarily target a reduction in gross domestic product (GDP) emissions intensity from 33 to 35 percent, by 2030; achieve around 40 percent of installed electricity capacity from non-fossil energy resources; energetic efficiency; and the creation of an additional carbon sink of 2.5 to 3 billion tonnes of carbon dioxide equivalent through additional forest and tree cover.
While India, the world’s third-largest carbon emitter, has yet to commit to a net-zero emissions schedule amid intense international pressure, the road to net zero is strewn with concerns about the availability of climate finance, the role of developed nations in increasing finance and India’s development priorities.
(The story first appeared in Mongabay)