Sea Level Observations
Sea level is usually measured by “tide gauges” and “altimeter on board of satellite”. Tide gauge observations are mostly from coastal and island stations, and some of the global stations have provided more than hundred years of data. One such Indian station is Mumbai, which is located in the west coast of India. In addition to the above two conventional measurement strategies, temperature and salinity observations over a long period can also provide sea level rise estimates accounting the steric effect. Melting of ice sheets and mountain glaciers add freshwater to the oceans, which may also increase the water mass of the ocean. The continuous increase in water mass of the ocean can modify the Earth’s rotation rate as well as gravitational oblateness. Measurements of these possible changes due to water mass increase can in turn help to estimate sea level change due to melting of sea ice and glacier (Cazenave and Nerem 2004).
Tide gauge observations are the main source for understanding the long-term sea level change (Douglas 2001; Woodworth and Player 2003), and it measures the sea level in mm. One of the limitations of tide gauge observations is that it records sea level only at the coast (it cannot provide observations of open ocean sea level); secondly, they are mounted to land which may be experiencing vertical movements (Cazenave et al. 1999; Nerem and Mitchum 2002). The second limitation can be rectified by geodetic leveling to some extent. These limitations can introduce error in the long recorded sea level information, which indeed mislead the rate of sea level estimation (Cabanes et al. 2001b). Tide gauge in general measures the sea level at every hourly interval. To extract sea level trend information, monthly or yearly averaged tide gauge data are used. However, the factors such as data length and impact of short-term climate variability in a specific region add uncertainty in the estimate of sea level trend from these data. Permanent Service for Mean Sea Level (PSMSL), is the nodal agency for the global tide gauge data.