Extreme changes in India’s summer monsoon threatens food sources in Bay of Bengal

Analysis spanning the last 22,000 years shows that extreme changes in India’s summer monsoon due to climate change could see dramatic declines in Bay of Bengal marine life – a crucial regional food source.

The Bay of Bengal covers less than 1% of the global ocean but supplies nearly 8% of fishery production.

Monsoons are essential for providing freshwater to the region. But increasingly extreme differences in intensity between strong and weak monsoons over centuries is causing significant disruptions, according to the new study published in Nature Geoscience.

According to the research, previous extreme changes in monsoons have caused a 50% reduction in surface food availability for marine life.

Climate change from the release of greenhouse gases from fossil fuel burning is expected to make monsoons more intense and variable, destabilising the marine food web again.

“Millions of people living along the Bay of Bengal rely on the sea for protein, particularly from fisheries,” says author Yair Rosenthal, from Rutgers University in the US. “The productivity of these waters – the ability of the ocean to support plankton growth – is the foundation of the marine food web. If ocean productivity declines, it will powerfully affect the ecosystem, ultimately reducing fish stocks and threatening food security for coastal communities.”

The geoscientists measured ocean productivity over the millennia by studying the shells of foraminifera –single-celled plankton that live in the ocean and build calcium carbonate shells. The shells also preserve information about ancient ocean and climate conditions.

“By analysing their chemistry and tracking the abundance of certain types that thrive in productive waters, we reconstructed long-term changes in rainfall, ocean temperatures and marine life in the Bay of Bengal,” says lead author Kaustubh Thirumalai, from the University of Arizona, US.

Bay of Bengal productivity collapsed during periods of very weak and very strong monsoons.

The period known as Heinrich Stadial 1, a significantly cold period, occurred between 17,500 and 15,500 years ago and had very weak monsoons. Rapid warming and sea level rise because of melting glaciers at the end of the last ice age, occurred between about 10,500 and 9,500 years ago, leading to very strong monsoons.

“Both extremes threaten marine resource availability,” Thirumalai says.

Modelling based on current climate trajectories suggest future warming of surface waters and stronger freshwater runoff. These conditions match past periods of sharp marine productivity drop.

“The relationship between monsoons and ocean biology we have uncovered in the Bay of Bengal gives us real-world evidence of how marine ecosystems have reacted to warming and monsoon shifts and may do so in the future,” explains Rosenthal. “These insights can help refine projections and inform sustainable management of fisheries and coastal resources as the impacts of climate change accelerate.”