A recent IPCC report sounded the red alert and issued arguably its strongest warning on climate change. It says that the Indian Ocean is warming at a higher rate than other oceans. TOI breaks it down for you
How much has the Indian Ocean warmed?
The Indian Ocean has warmed faster than the global average. Surface temperature of the tropical Indian Ocean rose by 1°C on average during 1951–2015, compared to the global average of 0.7°C. The Indian Ocean is not the only ocean to warm faster than global average—the western equatorial Pacific Ocean has too.
Why has the Indian Ocean warmed faster than the global average?
A tropical ocean gets a lot of heat, according to Swapna Panickal, scientist at the Indian Institute of Tropical Meteorology. And in the case of the Indian Ocean, the body of water is landlocked to the north—by the South Asian land mass—-causing heat to accumulate in the northern parts. By comparison, the Atlantic Ocean is open to the northern polar regions allowing heat to dissipate into cooler waters. Another possible reason: the southwest monsoon circulation, which plays a role in directing heat of north Indian Ocean southwards, has weakened in recent decades. That may have allowed more heat to accumulate in north Indian Ocean.
Does that mean the region’s sea levels have risen faster too?
Not necessarily. Sea levels in the north Indian Ocean rose at the rate of 1.06–1.75 mm-1 a year from 1874 to 2004 and 3.3 mm year−1 between 1993–2015. That’s comparable to global mean sea level rise. However, relative sea levels are thought to have increased faster.
What are relative sea-levels and why have they increased faster in Asia?
Relative sea levels refer to the level of the sea in relation to land. For instance, the northern coasts of the Bay of Bengal–West Bengal and Bangladesh–have seen sea levels rising by 5mm and more a year (compared to the overall 3.3mm in the north Indian Ocean). This is because the Bengal delta is sinking, adding to the effect of sea level rise. Land subsidence is occurring in many parts of Asia, especially in deltas, due to both natural and development factors. Parts of Jakarta, for instance, are sinking as much as 10 cm a year.
What about future sea-level rise in the region?
If Indian Ocean continues to warm faster than global average, sea levels could start rising faster. Unlike some oceans, most sea level rise in the Indian Ocean is caused by warming—because water expands in volume when it warms—and not so much due to the melting of glaciers and ice. But there is uncertainty around the melting of ice sheets, such as the Greenland ice sheet, that contributes to global mean sea level rise.

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