Mean sea level
Mean sea level is the average height of the sea over longer periods of time (usually a month or year), with the shorter-term variations of tides and storm surges averaged out. Eustatic (or absolute) mean sea level reflects only the change in sea height, whereas relative mean sea level represents the change in sea height and changes in the level of the land at a local or regional scale. Changes in sea height are caused by changes in water volume or mass, and by variations in the shape of the oceanic basins over geological time scales. The main factors that increase the volume or mass of the ocean are: (1) the melting of land-based sources of ice (glaciers and ice caps, and the ice sheets of Greenland and Antarctica); (2) thermal expansion of seawater as it heats up; and (3) changes in water storage on land. Vertical land movements arise from both natural geological and anthropogenic processes. Natural processes include tectonics (i.e. earth quakes) and glacial isostatic adjustment (i.e. the ongoing adjustment of the earth following the reduction of ice coverage from the last glacial maximum 25,000 years ago). Anthropogenic processes usually result in subsidence, caused for example, by withdrawal of ground water.
Over many hundreds of thousands of years, mean sea level has varied by as much as 130 m as major ice sheets grew and shrank during glacial-interglacial cycles. Palaeo data from corals indicate that during the last interglacial period (approximately 125,000 years ago) sea level was 4 to 6 m above that of present day. Sea level fell to more than 120 m below the present day level as water became trapped in ice sheets during the last ice age. Levels then rose rapidly until about 6,000 years ago as ice melted. Between 6,000 and 2,000 years ago sea level rose more slowly and there has been little change in sea level from 2,000 years ago until more recently.
There has been considerable interest in mean sea level in recent decades because of concern that sea levels are rising as result of climate change, and the negative impact this will have on rapidly growing coastal communities. Palaeo sea level data from coastal sediments, the few long (pre-1900) tide gauge records and reconstructions of mean sea level (made by combining tide gauge records with satellite altimetry measurements), all indicate that there was an increase in the rate of global mean sea level rise during the late 19th and early 20th century. Over the last 3,000 years, changes in mean sea level were at the most a few tenths of millimeters per year; whereas the global average rate of rise estimated for the 20th century was 1.7 mm yr-1. Measurements from altimetry suggest that the rate of mean sea level rise has almost doubled over the last two decades. The Intergovernmental Panel on Climate Change’s Fifth Assessment Report predicts that global mean sea level is likely to rise a further 28 to 97 cm by 2100.
Figure: Time series of annual mean sea level at: Stockholm, Sweden showing a fall in sea level due to post-glacial rebound – i.e. land is rising); Fremantle, Australia showing a steady increase in sea level; Bangkok, Thailand showing a rapid rise in sea level post-1960 associated with ground water extraction; Nezugaseki, Japan showing a 18 cm change in sea level in 1964 caused by ground depression associated with the Niigata earthquake.