Regions surrounding northern Bay of Bengal (BoB) received higher precipitation than the other parts of India for the last 10,200 years, says a new study that traced the dynamics of Indian Summer Monsoon Rainfall (ISMR) over 10,000 years – a period which witnessed the development and fall of numerous ancient civilisations around the world, many of which were associated with climate instability. “The study can help us understand long-term trends of climate change impacts on the ecosystems and may help mitigate future climate extremities,” says a press release from the Department of Science and Technology, Government of India.
Agriculture in India is heavily dependent on the Indian Summer Monsoon rains (ISMR). The Bengal Basin located at the trajectory of the Bay of Bengal (BoB) branch of the ISM is very sensitive to changes in the ISM strength. Even a minor change in ISM strength may have adverse effects on the agrarian-based socio-economic conditions of the region. However, no systematic long-term record (beyond the range of instrumental period) for the past ISM variability in the region was available.
Birbal Sahni Institute of Paleosciences, Lucknow, reconstructed the history of the ISM variability from this region by using both biotic and abiotic proxies that predates instrumental records (records taken before 19th century). The researchers show that a heavy ISMR was witnessed between 10,200 years and 5,600 years ago by this region, but the ISMR decreased 4,300 years ago. The ISM got strengthened again between 3,700 and 2,100 years following which it switched to a drier mode for a while. The ISM regained its strength some 200 to 100 years ago. Of the weakened phases, the one that occured around 4,300 years back was the most severe one, and had adverse impact of the ecosystem, the study reveals.
Scientists have collected sediment samples from the bed of a dried lake in the northern part of the Bengal Basin. Standard techniques were followed for building the age-depth model of sedimentary sequence and measuring different palaeo-climatological parameters. They also compared the proxy-based results with a few outputs from the palaeo modelling experiments for different time spans to validate the results of this study. The numerical models provided insights into the spatial-temporal dimensions of climate change and helped analyse the dynamic relations between different climatic components under specific boundary conditions. Combining these datasets, they investigated the timing, regional coherence and causes of Holocene ISM variability in the Bengal region.
They explored the drivers influencing the variability of the monsoon in the Indian part of the Bengal Basin and found that while the millennial-scale variations in the ISM rainfall may largely be attributed to changes in solar insolation and dynamics of inter-tropical convergence zone (ITCZ — an area where the northeast and the southeast trade winds converge), the centennial scale variations may be collectively triggered by phenomena like North Atlantic Oscillation, El Nino Southern Oscillation and Indian Ocean Dipole.
Focusing on monsoonal variability in the Indian part of the Bengal Basin, the scientists combined both biotic (phytoliths, NPPs and stable carbon isotopes) and abiotic (environmental magnetic parameters, and grain size data) proxy data to understand the ecosystem response to past hydroclimatic changes. They inferred that changes in lake ecosystem were strongly influenced by the ISM rainfall.