India: the surprisingly difficult task of measuring sea-level rise

By Shreya Dasgupta

How do scientists determine if sea level is rising? Since 1992, satellite altimeters are the primary source of information. More locally, tide gauges can be helpful too.

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Scientists have predicted this because there is considerable evidence that the sea level is rising everywhere on Earth. In fact, measurements show that the mean sea level has been rising faster in recent decades than in the entire 20th century. Researchers are also quite sure, through meticulous measurements taken across the globe, that much of the sea-level rise is due to human-induced global warming that’s melting ice sheets and glaciers, and heating up the oceans.

In the 20th century, the global mean sea level rose at 1.4 mm per year, according to the 2019 IPCC Special Report on Ocean and Cryosphere in a Changing Climate (SROCC). Between 2006 and 2015, the rate increased to 3.6 mm per year. In fact, mean sea level is projected to rise beyond 2100 no matter how much we change our greenhouse gas emissions from this point on, according to the report.

Another report released by India’s Ministry of Earth Sciences (MoES) and the Indian Institute of Tropical Meteorology (IITM-MoES), Pune, in June this year suggests India is following similar trends. Between 1874 and 2004, the mean sea level in the northern Indian Ocean rose by 1.06-1.75 mm per year – but at about 3.3 mm per year from 1993 to 2017.

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So how do scientists determine if sea level is rising or not?

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Satellites

Since 1992, satellite altimeters have been the primary source of information on sea level. But these satellites measure a different kind of sea level compared to tide gauges. Data from tide gauges allows us to determine the relative sea level. Satellite altimeters on the other hand measure the absolute sea level – which are changes in the height of the ocean relative to a fixed centre of the planet. This way, it doesn’t matter if land nearby is rising or sinking.

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Data gaps

On the flip side, satellite altimeters can’t take a closer look at the land because of competing signals from shallow water and terrestrial sources. For many climate studies, getting closer to the coast isn’t very necessary, according to Nerem. But if people want to know what’s happening or going to happen on the coast where they live, say in Mumbai, then data from the coast is essential. “If you have to design a plan for coastal resilience, you have to depend on the local observations and assessments,” Hazra said.

This data comes mainly from tide gauges. Panickal said we need continuous monitoring of the Indian coast line with an extensive network of tide gauges with co-located GPS systems.

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