According to a recent study, the Bay of Bengal has warmed significantly over the past six decades, with experts warning that such changes could alter the underwater acoustic environment, resulting in increasing noise pollution & potentially disrupting feeding behaviour of marine animals. TOI takes a lookThe Bay of Bengal has warmed significantly over the past six decades, with temperatures in its upper 50 metres rising by up to 2.2°C and seawater becoming increasingly acidic, according to a new study that warns the changes could alter the region’s underwater acoustic environment and allow sound to travel longer distances beneath the sea.The study, published in ‘Ocean Dynamics’ on Sept 2 by researchers from Defence Research and Development Organisation (DRDO) and the Kerala University of Fisheries and Ocean Studies, found that surface pH levels in the Bay declined from around 8.3-8.4 in the 1960s and 1980s to 8.1-8.2 after 2000, while ocean heat content continued to rise, reflecting the growing impact of rising atmospheric carbon dioxide.The researchers compared historical observations made during the International Indian Ocean Expedition in 1963 with measurements collected during the Sagar Maitri expedition in 2019 and supplemented these with long-term temperature, salinity and pH datasets.Surface Getting WarmerOne of the most important findings of the study is the warming of the Bay of Bengal’s upper ocean. The comparison between 1963 and 2019 observations showed that temperatures in the upper 50 metres of water increased by about 0.2°C to 2.2°C at many locations. The warming was seen across all surveyed transects in the central Bay of Bengal.At the same time, researchers observed a contrasting trend below the surface. While the upper ocean warmed, waters between 100 metres and 200 metres in depth showed signs of cooling. Analysis of long-term ocean records from 1960 to 2020 confirmed these trends, indicating sustained warming near the surface and cooling in the upper thermocline layer.The study also found that the amount of heat stored in the upper 50 metres of the Bay of Bengal has been increasing steadily. Ocean heat content, an important indicator of climate change, showed a long-term rising trend, although year-to-year fluctuations were also evident. Researchers noted that heat storage increased by about 0.018 × 109 joules per square metre per decade during 1965-2020.Bay Waters Are Becoming More AcidicThe researchers also analysed pH data spanning around 65 years. Their findings indicate that the Bay of Bengal has become progressively more acidic since the 1990s.During the 1960s through the late 1980s, surface water pH in the Bay generally ranged between 8.3 and 8.4. Since then, pH levels have gradually fallen. After 2000, values commonly ranged between 8.1 and 8.2.According to the study, this decline coincided with a sharp rise in atmospheric carbon dioxide. Atmospheric CO2₂ concentrations exceeded 350 parts per million (ppm) during the period and increased to 414.72 ppm by 2020. As more carbon dioxide dissolves into seawater, it forms carbonic acid, lowering ocean pH and contributing to acidification.Researchers noted that previous studies have also reported a stronger decline in surface pH in the Bay of Bengal than in the Arabian Sea, partly because of freshwater inflow, mixed-layer processes and ocean circulation patterns.A Changing Ocean StructureThe Bay of Bengal is unique because of its large freshwater input from rivers and rainfall. This freshwater creates a low-salinity surface layer that affects how heat, salt and sound are distributed in the ocean.The study examined severalocean layers, including the isothermal layer, mixed layer, barrier layer and sonic layer. Researchers found substantial long-term variability in these layers. The barrier layer, which separates the surface from deeper waters, typically ranged between 25 metres and 45 metres in thickness, averaging around 32 metres.These layers are important because they influence underwater acoustics. Variations in temperature and salinity alter sound-speed gradients, affecting how sound energy travels through seawater. The study found that sound-speed gradients within the upper ocean have generally decreased over time, with the trend becoming more pronounced after 2000.What It Means for Underwater SoundA distinctive aspect of the study was its examination of how warming and acidification may affect underwater acoustics. Sound is essential in the marine environment and is used in underwater communication, navigation, monitoring and sonar systems.The researchers found that lower pH reduces the absorption of sound in seawater, particularly at frequencies above 1 kilohertz. Their calculations showed that a decrease of 0.1 pH units could reduce sound absorption by roughly 10% to 12% under representative Bay of Bengal conditions.More acidic Bay of Bengal waters can allow underwater sounds to travel longer distances, increasing noise pollution and potentially disrupting communication, navigation and feeding behaviour of marine animals.The study also showed that changes in salinity can significantly influence sound absorption, sometimes more strongly than temperature changes.
