Rising CO2 in air, melting ice make monsoon ‘stronger’

Sediment cores harvested from beneath the seafloor in the Bay of Bengal, off the Indian coast, have uncovered that atmospheric carbon dioxide levels and related warming were “major players” in shaping the intensity of the South Asian monsoon over the past million years, said scientists in a new study. The findings support numerical models that predict stronger monsoons with increasing carbon dioxide concentrations in the atmosphere in the future.

Along with fluctuations in atmospheric carbon dioxide (CO2), the monsoons in the Pleistocene were also sensitive to continental ice volume and moisture import from the southern hemisphere of the Indian Ocean, according to the research that ground-truthed the numerical models used to predict future climate change. The scientists reconstructed the monsoon activity over the past 900,000 years using clues tucked away safely in the sediment resting on the ocean floor.

“This particular paper identified three different mechanisms that are responsible for changing the monsoon over time. We demonstrated that monsoon is equally sensitive to the combined effects of ice volume and CO2 as well as how much moisture is flowing out of the southern hemisphere into the northern hemisphere in the summer monsoon winds; those are the two factors that drive the changes in monsoon at the timescale we studied,” Steven Clemens, a professor of geological sciences (research) at Brown University and lead author of the study, told Mongabay-India.

Clemens threw light on the coupling between ice volume and CO2 levels in the atmosphere that together impact the monsoons. “Northern hemisphere ice volume and CO2 levels in the atmosphere are intimately coupled in the past. At approximately the same time CO2 reaches a peak, global ice volume reaches a minimum, so when CO2 rises, the ice caps melt, sea ice retreats Polewards,” he said.

The study findings validate model predictions suggesting monsoons are indeed sensitive to CO2-related warming and that in a warming world, there’s going to be more water vapour in the atmosphere, added a media release from the university. The research suggests stronger rains in the future as CO2 levels rise.

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Clemens, however, clarified that stronger monsoons do not necessarily mean the strengthening of the winds associated with the monsoon per se “but the spreading out of the strong winds across much larger portions of the Arabian Sea and the Bay of Bengal.” “That then helps increase the evaporation rate and the evaporated moisture is rained out of the continents. That’s the local effect.”

As for the global impacts of CO2-associated warming, places that are rainy now are very likely to receive more rainfall. “Anytime you put more CO2 in the atmosphere it becomes warmer and a warmer atmosphere evaporates more moisture out of the global ocean; so the global atmosphere will carry more moisture. Hence, India is not the only place that is going to receive more rainfall,” he said.

Clemens expressed concern that “many governments have yet to fully appreciate” the extent to which humans are impacting Earth’s climate and are not yet taking sufficient action to mitigate the known impacts happening today and into the future due to the burning of fossil fuels. “We, as individuals, and our governments need to take actions to reduce emissions if we are to leave a functioning and healthy environment to our children and future generations.”

The study is one among many that have come out from an expedition, International Ocean Discovery Program Expedition 353, an international effort to drill for sediment cores in the Bay of Bengal from November, 2014 to January, 2015. Clemens, working with an international team of researchers, sailed aboard the research vessel JOIDES Resolution, a converted oil-drilling vessel, to the Bay of Bengal, to recover sediment core samples from beneath the seafloor.

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Setting sail from Singapore in November, 2014, the ship and crew travelled about 4,000 nautical miles over two months, collecting cores from six locations in and around the Bay of Bengal, recovering 4,280 m of sediments during 33 days of on-site drilling. The expedition was funded in part by the U.S. National Science Foundation, states a 2015 media release.

The Indian summer monsoon supplies the majority of water for agriculture and industry in South Asia and is vital to the wellbeing of 1.4 billion people. Active and break periods in the monsoon have a major influence on the success of farming, while year-to-year variations in the rainfall have economic consequences on an international scale.

Understanding the impacts of climate change on monsoon is critical. Clemens adds that the numerical models that seek to understand how the monsoon is going to change in the future are calibrated to the modern climate and then “simulate the future by changing the conditions, for example, the CO2 concentration in the atmosphere.”

“For example with the Indian monsoon, as we continue increasing the CO2 concentration of the atmosphere there is going to be more moisture in the atmosphere and the monsoon is going to become stronger to counteract the influence of aerosols over India, which tend to dampen the monsoon,” Clemens said adding that these competing factors, driving the monsoon in opposing directions, have been the subject of significant research.

However, rainfall is not that easy to model. “Part of the problem with numerical models is that rainfall happens at spatial scales much smaller than the grid scales of the model. So it hard to model rainfall; we can reconstruct rainfall changes in the past and that can be used as a target to confirm which of these models are operating properly or not,” explained Clemens.

The present study’s findings are based on the 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 world’s least saline ocean basin, receiving around 2950 cubic kilometre of runoff annually. Changes in rainfall and surface ocean salinity are captured and preserved in a number of chemical, physical, isotopic, and biological components of sediments deposited there.

Once the sediment cores were back in the lab, researchers isotopically reconstructed the past monsoon using indicators such as oxygen isotopes in planktonic fossils, leaf wax carbon isotopes and rubidium from Site U1446 on the Indian continental margin.

The research is significant because such multi-proxy analysis helps us track monsoon better in the past, says Supriyo Chakraborty at the Indian Institute of Tropical Meteorology, Pune. “Each proxy is sensitive to a different factor that we want to examine; when we apply multiple proxies and if we get a converging ‘result’ and the ‘interpretation’ then it gives credence to our analysis. These proxies help us reconstruct the monsoon going back beyond the 150 years of the instrumental record of rainfall,” Chakraborty added. He was not associated with the study.

For example, in a paper published in 2020, scientists warned of heightened flood risk in the Brahmaputra river based on a reconstruction of 700 years of the river’s water flow in the monsoon season, from tree-ring data, suggesting that the transboundary river’s real flood risk is likely an underestimate as instrumental records were taken during a dry period.

India’s Nationally Determined Contributions (NDCs) under the Paris Agreement primarily target, a reduction in the emissions intensity of Gross Domestic Product (GDP) by 33 to 35 percent, by 2030; achieving about 40 percent installed power capacity from non-fossil fuel-based energy resources; energy efficiency; and creating an additional carbon sink of 2.5-3 billion tonnes of carbon dioxide equivalent through additional forest and tree cover.

While India, the third-biggest carbon emitter in the world, is yet to commit to a net-zero emission timeline amid intense international pressure, the path to net-zero is riddled with concerns around the availability of climate finance, the role of developed nations in augmenting the finance and India’s developmental priorities.

(The story first appeared in Mongabay)