Sea level varies as a result of processes operating on a great range of time-scales, from seconds to millions of years, so that current sea level change is also related to past climate change.
The local change in sea level at any coastal location as measured by a tide gauge depends on the sum of global, regional and local factors and is termed relative sea-level change.
It is so called because it can come about either by movement of the land on which the tide gauge is situated or by the change in the height of the adjacent sea surface.
Relative sea levels are also measured by dating buried coastal vegetation (salt marshes, mangroves, etc.).
Most of the tide gauges are located in mid-latitude northern hemisphere, few in middle of oceans, and contaminated by earth movements.
The main source for the uncertainties in using tide gauge records still remain: poor historical distribution of tide gauges, lack of data from Africa and Antarctica, the GIA corrections used, and localized tectonic activity.
Sea-level rise due to global warming occurs primarily because water expands as it warms up. The melting ice caps and mountain glaciers also add water to the oceans, thus rising the sea level.
The contribution from large ice masses in Greenland and Antarctica is expected to be small over the coming decades. But it may become larger in future centuries.
Sea-level rise can be offset up by irrigation, the storage of water in reservoirs, and other land management practices that reduce run-off of water into the oceans. Changes in land-levels due to coastal subsidence or geological movements can also affect local sea-levels.
Average Rate of Sea Level Rise and the Estimated Contributions from Different Processes: 1910 - 1990
CLIMATE CHANGE AND SEA LEVEL RISE
About 20,000 years ago during the LGM, large ice sheets melted causing a rise in sea level of about 100m, most of the melting occurred about 6,000 years ago.
Over the past 1,000 years and prior to the 20th century, the average global sea level rise was of the order of 0.2 mm/yr.
The rate of sea level rise climbed to about 1-2 mm/yr during the 20th century, with a central value of 1.5 mm/yr (IPCC TAR). The most recent estimate during the 20th century is 1.4 -2.0 mm/yr, with a central value of 1.7 ± 0.3 mm/yr (Church & White, 2006).
This significant rate of rise in sea level is attributed to global warming caused by industrialization during the second half of the 19th century.
