7-fold surge in Indians at risk due to sea level

By 2050, 36 million Indians may be at affected by flooding and inundation due to sea level rise by 2050, according to a new study published this week.

That projection is a seven-fold rise from a previous estimate of 5 million, and the impact is concentrated in Mumbai, Kolkata, Odisha, coastal Gujarat, as well as Chennai and Kochi, the study shows.

According to the new study, 300 million people globally live on land that will flood at least once a year by 2050 due to sea level rise—three times more than previous projections. Roughly 75% of those affected live in six Asian countries: China, India, Vietnam, Thailand, Indonesia, and Bangladesh.


Another 200 million live on land that may be completely submerged by 2100, said the study by scientists at Climate Central, a US-based institute.

The vulnerability of Asian coastal cities to global warming, including Mumbai, Kolkata, Chennai, and Surat, has been highlighted before, including in the most recent IPCC report which warned of a one metre rise in sea level by 2100 and storm surges occurring once a year rather than once a century.

But the new study provides an interactive map that allows one to see the potential impact under different scenarios.While the above numbers are based on a moderate sea-level rise scenario, if carbon emissions rise sharply or ice sheets melt faster, then 640 million people will be at risk by 2100, the study suggested.

Sharp reductions in carbon emissions and investment in coastal defense measures are vital to reduce risk , the study suggests.

The upward revision in flood risk estimates is due to the use of a finer-grained method of assessing land elevation. Flood risk is shaped not only by sea level rise—which in turn depends on factors such as how fast the ice sheets melt—but also the elevation level of the land. Climate Central’s model suggests that many of the world’s coastlines are lower than previously thought—raising risk of flooding or submergence due to sea level rise.

Scott Kulp, the senior computational scientist at Climate Central and the lead author of the study, told ToI he was surprised at the huge difference to the risk assessment made by this improved elevation data. "Now that we better understand the height of the ground beneath our feet, we can see that coastal communities in India and around the world are far more vulnerable to rising seas than we could have known.”

“Earlier studies on sea level rise used the elevation dataset SRTM, which overestimates elevation,” explained Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune. “This is because SRTM measures the top of trees and buildings. Hence the elevation of low lying coastal lands with dense built-up and skyscrapers (e.g. Mumbai) are generally overestimated in this data. As a result, the area and population which is under the threat of future sea level rise maybe underestimated.”

Recent estimates show that the increase in sea level in the Indian Ocean is at the rate of 5-6 cm per decade, says Koll. “At this rate, the new dataset shows that the densely populated low-lying cities like Mumbai and Kolkata are at greater risk.”

However, experts also point out that sea level rise can depend on local factors which may not be captured in global models. For instance, says Koll, sea level rise in the Indian Ocean is largely due to thermal expansion caused by ocean warming (water expands as it warms). Accurate understanding of sea level rise—and thus its impact on coastal India—requires better monitoring systems in the Indian Ocean, he said.

Pushpendra Johari, the head of Risk & Insurance at RMSI, a global natural hazard consultancy, said the study's numbers seemed high for India. “I don’t think the situation is as alarming as this study suggests; we need to take another look at it,” he said. Johari said that while it was true that the older datasets overestimate land elevation, this was a known fact and taken into account by researchers while making their projections.

The Climate Central study acknowledges its limitations: coastal defences such as sea walls that would reduce exposure were not taken into account, for example.