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Has Sea-Level Stopped Rising In Hong Kong?

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Prof Wyss Yim, Hong Kong Engineer

Hong Kong’s tide gauges record shows a pause in the rise of sea level for almost¬†20 years

Sea-level record based on tide gauges in Victoria Harbour 1954-2015 (Source: Hong Kong Observatory website http://www.hko.gov.hk)

Hong Kong has extensive urbanised low-lying coastal land reclamations threatened by short-term typhoon-induced storm surges and long-term future sea level rise. It is therefore important to study past records in order to obtain information for the evaluation of future risks.

Chart datum, the lowest tide, is 0.146 m below the Hong Kong Principal Datum used in surveying. Local mean sea level is approximately 1.23 m above Principal Datum.

Based on tide gauge records in Victoria Harbour from 1954 to 2015, sea level rising at an average rate of 30 mm per decade was claimed by the Hong Kong Observatory. The projected rate of 25.5 cm by the year 2100 falls below the lower end of the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC) predictions for four carbon dioxide scenarios with mean sea level rise ranging from 28 cm to 98 cm.

Solar variability including Earth’s orbital changes and major volcanic eruptions are largely responsible for the Earth’s ‘long-term’ natural temperature variation. This controls ice balance and the thermal expansion and contraction of seawater. Additionally human activities including water usage, fossil fuel consumption, heat generation and land utilisation have altered the natural water cycle causing sea level changes.

In coastal regions like Hong Kong other possible factors include crustal stability; geoidal changes; meteorological conditions such as pressure changes, wind directions and the passage of typhoons; and ground movement both natural like deltaic sedimentation, and man-made like construction, groundwater extraction and mining. These reasons help to account for both short and long-term sea level variability.

For measuring sea levels, tide gauges became available in Hong Kong in 1954 and satellite remote sensing in 1993.

The oldest Hong Kong tide gauge was established on the North Point coastal reclamation. In 1987, it was relocated to the Quarry Bay coastal reclamation for fear of the constructional impact of the East Island Corridor. Neither tide gauge was founded on bedrock so uncertainties exist on the influence of post-constructional or residual ground settlement.

A 1993 study (Yim 1993) of the 1962-1986 record of the North Point tide gauge revealed an average rate of sea level rise of about 0.3 mm/year. Based on a positive correlation between sea level and rainfall, river discharge was identified as an important factor. The rate was a maximum rate because coastal reclamations are prone to ground settlement.

In spite of uncertainties, the Victoria Harbour 1954-2015 sea level record show:

– A pause in the average rate of sea level rise from 1959-1993.
– An abrupt 29 cm rise in sea level during 1987-1999.
– A post-1999 pause in the average rate of sea level rise.
– The average rate of sea level rise of 30 mm/decade from 1954-2013 in Victoria Harbour claimed by the Hong Kong Observatory is likely to be an overestimation because ground settlement of the North Point and Quarry Bay tide gauge stations has not been taken into account.
– The sea level trend observed over the South China Sea through satellite remote sensing since 1993 is too short to distinguish the trend from the noise. Nevertheless, an analysis of the South China Sea (Cheng and Qi 2007) showed sea level rose at an average rate of 11.3 mm/year during 1993-2000 and fell at an average rate of 11.8 mm/year during 2001-2005.
– River discharge is an important ‘noise’. For example, the low annual mean sea level in 1963 coincided with one of the most severe regional drought year on record triggered by the Agung volcanic eruption in Indonesia.
– Another ‘noise’ affecting tide gauges within Hong Kong is the Pacific Decadal Oscillation.
– Storm surge generated by the passage of typhoons is an immediate enemy, particularly when associated with heavy rainfall and high tide.
– Tide gauges used for the study of long-term sea level changes should be located on bedrock instead of reclaimed land in order to eliminate the possibility of ground settlement.
– Both tide gauge and satellite remote sensing records are too short for drawing reliable conclusions on projected sea level rise. Another 50 years may be needed.

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