• Mohor Sengupta

Did you know, sea levels vary at different coastlines?

Updated: Aug 8, 2019

'Sea level' is entwined to my life in many ways. I was born in a metropolis at the sea level. I spent several years of my adolescent years in another sea-level megacity and now I live near the capital of U.S., again a few meters above the sea level. I love being near the sea and a big gain of altitude on land is not generally something I'd enjoy.



The Atlantic coast of North America in Maryland.


Most people (including me) have a black and white notion about sea level. We know that tides in the seas and oceans are created by gravitational pull of the moon. But, we think that the general sea level, not accounting for tides, is the same everywhere on Earth. Because water will distribute itself and attain similar levels everywhere, right?


Wrong!


Sea levels are very different at different coastlines. For example, the sea level at Colombo on the Indian Ocean is different (and lower) from the sea level at the Big Island of Hawaii on the Pacific Ocean. The water level on the Atlantic and Pacific sides of the Panama Canal are, likewise, different.


This is because the Earth's gravitational pull is not the same everywhere.


One reason for non-uniform gravitational field of Earth is the centrifugal force created by its spinning. The second and more relevant reason in terms of this post, is the non uniformity of the Earth's mass. The Earth is not uniformly spherical. Neither is the Earth's mass uniformly distributed across its combined land masses. Gravity measurements on the land and also the sea (measured from ships) have given different results at different areas. With the advent of satellites buzzing around the planet, it was observed that these don't follow a uniformly spherical or elliptical orbit either.



Coastline of the Big Island of Hawaii.


The huge mass of ice and rock of Greenland has its own powerful gravitational pull on the surrounding Arctic Ocean waters. This is true for the Big Island of Hawaii, with the nearest gravity-competing land mass of North America at least 4000 km away to east and Asia at more than 6500 km to the west. In the north and south are vast stretches of the gigantic Pacific ocean with Australia at more than 9000 km far to the south. The Big Island, therefore, exerts a considerably greater gravitational pull on the Pacific waters surrounding it, than the island of Sri Lanka does on the Indian Ocean waters near it. The gravity of Sri Lankan land mass is dissipated by the huge Indian subcontinent just above it.



Southern coastline of Sri Lanka.


If the ice of Greenland suddenly melts (it will eventually), there will be an expected sea level rise of 23 feet. But, the sea level at Greenland coast itself will fall from its present height. This is because the decrease in the mass of ice (which will melt) will lower the gravitational pull it currently exerts on the waters surrounding it. The Arctic waters around molten Greenland will relax in the absence of the strong gravitational pull. The excess water from ice-melt will travel far and cause the sea level to rise several meters in places like the Big Island coast.


By studying fossil records at coastlines, scientists have discovered past changes in the sea levels and therefore, the Earth's gravitational field. These are gravitational fingerprints left on Earth's oceans by melting glaciers of the past.


It's not just the land that can create gravitational field though. Rapidly shifting masses of water create their own gravitational pull as well. That is much harder to detect!