Sea level changes and vertical land movements


Sea level changes due to vertical land movements

 Land movements relative to the sea could cause sea level to rise or fall locally. Other well-known causes are expansion of sea water due to warming and melting of ice sheets and glaciers.

Formerly geodesists or land surveyors used spirit level and telescope to measure land movement locally with an accuracy of tenth of mm. The method could not be applied on a continental scale Satellites has come to the rescue for precise, economic and continuous and Earth-scale measurements of land and sea vertical movements.

Using the Global Navigation Satellite Systems (GNSS) with stations at thousands of locations on Earth, the heights of the land at these locations can be measures. However, the stations are clustered in densely populated areas of developed countries, as are permanent tide gauges for sea level measurements. Co-location of land stations and tide gauges enable to separate rising sea level sinking of land.

By comparing ground radar data of a location at different times, a technique known as interferometry,  scientist determine changes in the surface.

Uplift or subsidence are due to both natural and human causes. Natural drivers include shifting of tectonic places and rebounding of Earth crust once the weighty ice melts. Human generated changes include oil and water extraction. Natural changes are of the order of mm/year compared to human-induced of cm/year.

River deltas are particularly subject to risks of subsidence. The annual rate of land subsidence is often faster than the global  mean of 3.3 mm/yr. In these places, human impacts add to natural causes.

A growing number of people live in coastal areas. Adaptation to sea level changes are essential for sustainability of these people and the coastal nations. Small Island Developing States relying on the beaches for tourism. Remote sensing monitoring systems like the GNSS are key tools for long-term adaptation and mitigation of disruptive hazards. (NASA)


                                                             (NASA: Image 10 November, 2020)