The Indian Ocean is experiencing unprecedented and accelerated warming, which may continue throughout the century unless greenhouse gases (GHGs) are reduced immediately. A new study published in journal Science Direct has predicted that between 2020 and 2100, the Indian Ocean could warm at a rate of 1.7-3.8 degrees Celsius per century.
This could lead to widespread socio-economic impacts, along with significant effects on biodiversity, especially on coral reefs.
“The future increase in heat content is equivalent to adding the energy of one Hiroshima atomic bomb detonation every second, all day, every day, for a decade,” said Roxy Mathew Koll, a climate scientist at Indian Institute of Tropical Meteorology and lead author of the study, in an interview with Down To Earth.
This warming of the ocean’s surface, along with the heat accumulated in the first 2,000 metres below the surface, has significant repercussions for the southwest monsoon season, which provides about 70 per cent of India’s annual rainfall and also affects rainfall patterns in other South Asian countries.
Currently, the heat content of the first 2,000 metres below the ocean surface is increasing at a rate of 4.5 zettajoules per decade, with projections indicating a dramatic increase to 16-22 zettajoules per decade in the future, according to the study.
This warming could also lead to more frequent and intense extreme weather events, such as tropical cyclones and floods, as well as a rise in sea levels due to thermal expansion.
The study also examined the Indian Ocean’s warming over the past century, finding that the ocean basin heated at a rate of 1.2°C per century between 1950 and 2020.
The northwestern parts of the Indian Ocean, including the Arabian Sea, experienced the most significant warming, while the southeastern parts of the ocean, off the coasts of Sumatra and Java, experienced the least warming.
The study also discovered that the seasonal cycle of surface temperatures has shifted and might shift further due to warming. Between 1980 and 2020, the maximum sea surface temperatures (SST) across the Indian Ocean generally stayed below 28°C (ranging from 26-28°C).
If GHGs are emitted at current levels, the minimum SSTs in the Indian Ocean could exceed 28°C (28.5-30.7°C) all year round by the end of the 21st century, according to the study. Generally, SSTs above 28°C are conducive to the formation of tropical cyclones and extremely heavy rainfall.
Heavy rainfall events and extremely severe cyclones have already become more frequent since the 1950s and are projected to increase further with rising ocean temperatures, according to Koll.
The increased ocean heat would also cause thermal expansion in the Indian Ocean, resulting in rising sea levels in the 40 countries that border it. Thermal expansion of water accounts for more than half of the sea level rise in the ocean, exceeding the contribution from glacier and sea-ice melting, Koll said.
Other natural climatic phenomena, such as the Indian Ocean Dipole (IOD), are also expected to change due to the warming of the Indian Ocean. The positive and negative phases of the IOD influence rainfall during the southwest monsoon and affect the formation of tropical cyclones.
In the positive phase of the IOD, when the western parts of the Indian Ocean are warmer than the eastern parts, monsoon rainfall generally increases across many regions in India and the rest of South Asia.
In the negative phase, when the western parts of the ocean are cooler than the eastern parts, less than normal rainfall is observed during the post-monsoon period in northwestern India.
The study predicted that the frequency of extreme IOD events may increase by 66 per cent, while the frequency of moderate events is projected to decrease by 52 per cent by the end of the century.
The authors of the study also describe a concerning outlook for the Indian Ocean in terms of marine heatwaves, which are events when large areas of the ocean become much warmer than usual and remain so for extended periods.
The number of days with marine heatwaves is expected to increase from 20 to 220-250 days a year, according to the study. This would mean that most of the Indian Ocean could be in a near-permanent state of marine heatwave conditions.
Marine heatwaves lead to habitat destruction through coral bleaching, seagrass loss and the degradation of kelp forests, adversely affecting the fisheries sector. They also result in the rapid intensification of cyclones, where a cyclone can intensify from a depression to a severe category within a few hours, according to the study.
“The Indian Ocean, a climate change hotspot, faces rapid and strong increases in marine heatwave frequency and intensity unless global CO2 emissions are substantially cut,” said Thomas Frölicher, a climate scientist at the University of Bern, Switzerland, and one of the authors of the study.
The bio-geochemical characteristics of the Indian Ocean are also expected to change due to the warming. For instance, the pH levels of the ocean’s waters are projected to decrease from about 8.1 currently to 7.7 by the end of the century.
“The projected changes in pH may be detrimental to the marine ecosystem since many marine organisms — particularly corals and organisms that depend on calcification to build and maintain their shells — are sensitive to the change in ocean acidity,” said Koll.
“The change may be easier to fathom when we realise that a 0.1 fall in human blood pH can result in rather profound health consequences and multiple-organ failure,” he added.
The productivity and surface levels of chlorophyll are also expected to decline by 2100, with the greatest reduction in the western Arabian Sea, where levels could fall by 8-10 per cent from their current state.
“It is crucial to recognise that the impacts of these changes are not distant concerns for our grandchildren and future generations alone. As the current generation, we are already witnessing the repercussions firsthand,” said Koll.
The study recommended a multifaceted approach to address the imminent challenges in the Indian Ocean. Reducing GHG emissions and building climate-resilient infrastructure are the most effective strategies to mitigate the current and future impacts of warming.
Concurrently, conserving marine ecosystems through sustainable practices and improving forecasting capabilities can strengthen the region’s resilience to extreme weather events. Additionally, promoting adaptive agriculture and encouraging international collaboration are essential for ensuring food security and preserving biodiversity, according to the study.
