Since 1880, sea level in the world’s oceans has risen about nine inches. But one pocket of the Indian Ocean has puzzled scientists for the last 40 years. The Arabian Sea and the Bay of Bengal rose four inches more than the world’s oceans — a total of 13 inches to date. The north Indian Ocean is creeping up its coastlines.

Why is the Indian Ocean rising so fast? A weakening of Indian summer monsoons has meant warmer ocean water, due in part to a process called “thermal expansion,” according to a recent study published in the journal Geophysical Research Letters.

The summer monsoon typically blows heat from north to south. But researchers from the Centre for Climate Change Research in Pune, India have found that these monsoons, for the last three decades, have been too weak to properly push heat through this system. As a result, the heat gets absorbed by the ocean. When water warms, it expands and takes up more volume. That’s thermal expansion.

The research: The team, led by climate scientist Swapna Panickal, reviewed climate and sea level data from the 1950s to 2016 and conducted ocean simulation experiments in the lab. The team’s analysis indicated that summer monsoon winds and rainfall had significantly decreased in frequency and intensity over this time.

After concluding that thermal expansion played a role in this particular region and knowing that global sea level rise is primarily due to melting ice caps and glaciers, they concluded that the two combined were responsible for the acceleration of sea level rise in the northern Indian Ocean.

Why it matters: Rising sea levels are already putting lives at risk. According to the study, 2.6 billion people — or about 40 percent of the global population — live in and around the Indian Ocean. Places in India like Chennai, with its seaside nuclear plant, and Mumbai, with thousands of homes, offices and slum settlements along the coast, are in danger of getting submerged.

From Panickal: “Aerosols, pollutants and ongoing warming is often viewed in a global perspective. And here we see all of these things acting together on a regional scale and causing drastic changes.”

Panickal cautions: “We will have to adapt to this situation and prepare for global temperature extremes and sea level rise. Actions and policies need to be taken by the government. Our institute provides information that the country can use to help those living near the Indian Ocean.”