3.2. Rates of sea-level rise in history

As previously mentioned, sea-level rose and fell by as much as 130 m between glacial and interglacial periods. Some of the rises, particularly those just before and during the Holocene, were extremely rapid, and known as Catastrophic Rise Events (CREs). A 13.5 m CRE occurred about 14,000 years ago, another of 7.5 m at the end of the Younger Dryas, 11,500 years ago, and a third of 6.5 m as recently as 7,600 years before present. On average sea-level rose by around 10 m per millennium during the period of rapid post-glacial ice sheet melting, but far faster during CREs; up to 13.5 m in around 300 years. These rates – of up to 46 mm a year – are overprinted on an annual general post-glacial rise of between 4 and 13 mm, and reflect major and rapid changes in global climate and geography. Some, at least, may reflect the breaching of continental glacial meltwater lakes that then catastrophically emptied into the ocean. Discharge from Lake Agassiz in North America around 8,000 years ago is estimated, for example, to have caused a sea-level rise of between 20 and 42 cm. Such dramatic rises were succeeded in the late Holocene by a more steadily consistent rise in sea-level, the rate of which also began to slow, first to 5.45 mm per year between 7,000 and 4,800 years ago, and then to a time-averaged annual rate of 1.25 mm. Nearer our own time, a number of estimates for recent sea-level rise have been determined for the last 150 years or so, ranging from 1.8 ± 0.1 mm/y for the period from 1880 to 1980 to 2.4 ± 0.9 mm/y for the twentieth century up to that date. These rates are significantly higher than those prior to the middle of the 19^th century, for which some estimates suggest that the rise was either negligible or indistinguishable from zero. 20th century sea-level seems to have been rising more rapidly than at any time during the past 1000 years or more (Figure 5). Furthermore, since 1993, satellite observations have revealed that the rate of rise has increased to 3 mm/y.

Figure 5: Change in annually averaged sea-level at 23 geologically stable tide gauge sites with long-term records. The thick dark line is a three-year moving average of the instrumental records. This data indicates a sea-level rise of ~18.5 cm from 1900-2000. Because of the limited geographic coverage of these records, it is not obvious whether the apparent decadal fluctuations represent true variations in global sea-level or merely variations across regions that are not resolved. For comparison, the recent annually averaged satellite altimetry data from the TOPEX/Poseidon satellite are shown in red. Courtesy: Dragons flight.

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