Estimates on the degree of change in sea level have been calculated by considering both sea-level variations and vertical activity of the land at specific tide measurements (Teferele et al. 2006). The global sea level has been estimated to have significantly increased in the last century (Woodworth, 1990; Douglas, 1992, 2001), after approximately 2,000 years of maintaining a constant sea level. The greenhouse effect has been identified to influence the thermal expansion of the ocean water in terms of mass and volume.
Analysis of measurements and models of the current sea level has shown that there are certain discrepancies in the differences in the sea level across the last 100 years, resulting in large-scale variations in the estimates for current rate of sea level increase (Munk, 2002). Historical measurements of sea levels use the tide-gauge readings, which employ corrected vertical land movements in relation to measurements of the sea level, as well as correlation with waves, tides, meteorological forcing, ocean currents, mass and heat changes in the ocean and geophysical and man-made processes.
In addition, changes in the wind current also affect changes in the sea level, especially along coastal regions. Estimating global sea levels using tide-gauges have been difficult because of a number of factors. Firstly, tide-gauges are only present for a limited time that variations in the measurements of sea level are not stable enough within a year or a decade. Such variations are mostly due to ocean processes and atmospheric forces. Secondly, tide-gauges are not well distributed around the Earth, but are actually located together or bunch up at only certain areas around the world.
This large spatial deficiency in tide-gauges around the Earth make it impossible to determine a global sea-level change, and unfortunately, can only estimate relative sea-level changes. Thirdly, the vertical movement of the land to which tide-gauges are attached make estimations more complicated. This may be observed when ice sheets and glaciers load and deform the surface of the Earth and in turn, affect the Earths gravity field. Such impact redistributes the ocean mass and results in a rise in the sea level.
More importantly, any exchange of mass between the ocean and other water reservoirs such as glaciers and ice sheets results in the generation of a specific spatial fingerprint. A good example may be observed in the two largest ice sheets, Antarctica and Greenland, which are constantly changing in mass as well as too large to measure for relative sea levels. There is currently no robust model or theory that will facilitate the estimation of increase in the global sea level as influenced by glaciers and ice sheets.
In addition, the movement of the earths crust or plate tectonics are continually occurring, aside from the vertical movement of the land, although certain investigators have suggested that plate tectonics may be disregarded during estimations. A comprehensive review of all factors influencing the measurement of global sea level may facilitate in the understanding of the effect of climate change on the rise in sea level.