The Indian monsoon season may soon become erratic and dangerous because of global warming and climate change, The New York Times has reported, citing new research.

The researchers found that over the past million years, “increases in carbon dioxide in the atmosphere have been followed by substantial increases in rainfall in the South Asian monsoon system”, Steven Clemens, the lead author of the study, told The New York Times.

The study, published in the Science Advances journal last week, analysed data from millions of years to understand if the annual monsoon season is impacted by human activities. Thirty scientists from India, the US, Australia, Japan and Europe travelled to the Bay of Bengal in 2014 on a two-month voyage and extracted core samples from the ocean. They reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal and northern Indian Ocean.

The core samples, several hundred metres long, comprised sediment runoffs from previous monsoons into the ocean. Seasons with more rainfall ensured more runoff into the ocean, thereby reducing the salinity (amount of salt) at the surface. The scientists studied the fossil shells of the plankton – microscopic organisms in water – found in these samples to get the salinity of the water that they had lived in. As plankton from the surface die, they sank to the sediment and were collected.

These durations of high rainfall and low salinity came after periods when the carbon dioxide was high in the atmosphere or there were lower levels of global ice volume and thereby, an increase in moisture-bearing winds.

“South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean,” the study said. “We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.”

Dr Pallavi Anand, one of the authors of the study, added that the findings supported existing climate change models, according to the Deccan Herald.

“We compared the monsoon rainfall pattern between two warm periods (thousands of years ago) and we found that there could be a potential shift in the loci of monsoon rainfall away from the continental landmass which would result in a drier monsoon in the Indian subcontinent.”

The monsoon in the Indian subcontinent is usually from June to September. It affects the lives of millions of people, who rely on the rain for agriculture. Either a dry monsoon or flooding would devastate crops and impact thousands of lives.