SEA LEVELS

SEA LEVELS

Sea levels have been rising since the maximum of the last ice 20,000 years ago. The rate of sea level rise is regarded as an indicator of climate change. The change in sea levels is driven by two factors: the thermal expansion of the sea water as it warms and the melting of ice over land.

Long Term Sea Level Change

During an ice age, ice covers are large areas around both poles. The amount of water in the ice caps is such that sea levels are markedly reduced. Levels 20,000 years ago, at the maximum of the last ice age, were 140 m lower than they are today. Until about 7,000 years ago the rate of rise was about 100 mm/decade. Since then rate of rise has averaged 10 mm/decade.

Estimation of Sea Level Change

Global sea levels have traditionally been estimated from tide gauges. As can be imagined they show fluctuations of several meters due to tide and wave action. Identifying sea level changes of a few millimetres a year against this background “noise” is problematic. Since 1993, data are available from satellites. There are two other factors which add to the difficulty of estimate changes in sea level. The first is the way the earth has reacted to the melting of the ice caps. Where major ice melt has taken place, in northern Europe and North America for example, land levels have risen; the post glacial rebound (PGR). Conversely where sea levels have risen and encroached on previously dry areas, land levels have fallen under the increased weight of the oceans; glacial isostatic adjustment (GIA). (Some sources use the two terms interchangeably) These changes typically average around 4 mm/decade but can be higher in some locations. The second factor is the influence of atmospheric pressure. The changes in pressure can be seasonal and modify levels by 1 metre; often an allowance is made for these pressure difference by applying what is called “an inverted barometer.” As can be seen the adjustments to be made to sea level are of a similar order of magnitude to change in sea level itself. It is generally considered that the rate of change of sea level cannot be accurately estimated for periods of less than 10 years.

Sea Level Change

Figure 1 shows the sea level changes from 1807 to 2001 using two estimates based on tide gauges (Jevrejeva et all and Church at al). There is broad agreement between the two estimates. The Jevrejeva record show that sea levels fell for the first half of the 19th century. This suggests that the low temperatures recorded in Europe in this period may have been representative of global temperatures. It also follows the Dalton minimum of sunspot activity.

Figure 1



Figure 2 shows a composite record from the two tide gauge estimates and satellite based data from the TOPEX/JASON satellite system. To harmonise the two data sets the satellite data were adjusted to give the same average for the period of overlap. The graph also shows the rate of rise per decade. This was based on subtracting the difference in level for pairs of months 10 years apart. Over the last century or so the rate of rise has fluctuated from -20 mm/decade up to 40 mm/decade. The increase since 1880 has been around 250 mm.

Figure 2

Whilst at first sight the rise in sea level seems almost constant looking at the estimates of the rate of sea level rise shows that this does fluctuate. To clarify this the following graph, figure 3, shows the rate of rise in twent-year periods.

Figure 3

This appears to suggests that there is an underlying increase in the rate of sea level rise of about 0.015 mm/year/year and a fluctuation about this trend of ± 1 mm year.
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