Due to variations in solar radiation, it started weakening 4,200 to 2,000 years ago, say researchers
A study carried out by researchers from the Central University of Kerala has found that the monsoon was much stronger 7,000 years to 5,000 years ago and it underwent a cyclic change caused by variations in solar radiation.
A.V. Sijinkumar, Assistant Professor, Department of Geology, who led the research team, said the study was carried out in the Indian Ocean to understand changes in the monsoon in the past. This would help researchers forecast monsoon variability and the results could be used for building models that would offer insight into future variations in the monsoon, especially in the context of global warming.
“The monsoon is critical to Indian economy. A weak monsoon has led to severe droughts, famine, affecting the economy and food security, whereas a strong monsoon may lead to floods like those in Kerala in 2018 and 2019,” he said.
Mr. Sijinkumar said the main finding of the report was that the monsoon started weakening 4,200 to 2,000 years ago, before arriving at the present condition. The study found a strong summer monsoon in approximately every 10,000 years in the last 55,000 years.
Long-term records
He said the researchers looked for monsoon details in geological records because instrumental records (rain gauge data) were available only for the past 150 years. To improve forecasting, it was necessary to have long-term rainfall records, he said. “We have reconstructed monsoon variability by using the marine sediment core collected from the Andaman Sea. The Andaman Sea was selected because of its excellent preservation of fossils and the significant influence of river water run-off,” he said.
Mr. Sijinkumar said the sample fossil shells were dated using the radiocarbon dating technique. The study covered the time span of the past 55,000 years with a time resolution of 200 to 400 years. The microfossil shells, which look like sand grains to the naked eye, but were shaped in spirals, disks, spheres, tubes, and cones under microscope, were picked, identified, and their abundance recorded in each sample.
The isotopic and chemical composition of each samples was measured. The analyses gave the temperature and salinity of seawater at the time when foraminifera, a single cell organism, was alive. They tell about past freshwater runoff levels in the ocean from rivers. Less saline conditions indicate times of increased rainfall.
The results of the study were published in the Journal of Quaternary Science.
