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Monday, 2nd November 2009

Future sea level in Hong Kong

Further to a previous blog on rainfall, I would like to share with you the latest about the local sea-level rise because of global climate change. The material used here had been presented in the International Conference on Climate Change 2009 in early October.

A number of tide gauges were installed in Hong Kong, the earliest in the 1950s. This gives us over 50 years of sea level data, and serves as an important basis for studying sea level changes in Hong Kong. The mean sea-level in Hong Kong's Victoria Harbour has risen by about 13 cm over the past 50 years or so.

The United Nations IPCC (Intergovernmental Panel on Climate Change) report predicted in 2007 that globally the overall sea-level will rise between 18 and 59 cm by the end of the century. Since 2007, however, scientists have learnt more about what is behind the rise and have pinpointed the leading causes of sea-level rise in the past century.

The first contributor is the melting of mountain ice and glaciers in such places as Alaska and the Himalayas. This is expected to add another 10 to 20 cm to sea level by 2100. The second contributor is the thermal expansion of ocean water, which depending on the future scenarios in respect of emission of greenhouse gases is expected to raise the sea level by 20 to 50 cm by 2100.


Figure 1:Muir Glacier, Alaska's Glacier Bay on 13 August 1941 (upper) and 31 August 2004 (lower).  Between 1941 and 2004 the glacier retreated more than twelve kilometers and thinned by more than 800 meters
Figure 1:Muir Glacier, Alaska's Glacier Bay on 13 August 1941 (upper) and 31 August 2004 (lower).  Between 1941 and 2004 the glacier retreated more than twelve kilometers and thinned by more than 800 meters
Figure 1    Muir Glacier, Alaska's Glacier Bay on 13 August 1941 (upper) and 31 August 2004 (lower). Between 1941 and 2004 the glacier retreated more than twelve kilometers and thinned by more than 800 meters (Image source : National Snow and Ice Data Center, W. O. Field, B. F. Molnia).


The third contributor is the melting of the great land ice on Greenland and Antarctic, which hold enough water to raise sea level by 70 metres. IPCC predicted that Greenland would contribute up to 12 cm to sea-level rise by 2100, while Antarctica would eventually gain ice. However, recent studies show that this may be too conservative. Monitoring of both these ice caps over the past couple of years shows that they are melting at an accelerating rate.

Taken together, these three factors are expected to contribute 0.4 to 0.8 m to sea-level rise by 2100. This is way higher than IPCC's estimate.

Another simpler, statistical approach taken recently relates the sea level to temperature and has identified good correlation between the two for the past 120 years. The projection based on this approach is 0.5 to 1.4 m. Some scientists however argue that the linear relation would not hold for the future because there is multiple positive feedback, such as lubrication of glaciers by meltwater. Their estimates come to an alarming 0.8 to 2 metres.

So what does this hold for Hong Kong? Whatever the future projection, it now looks more than likely that IPCC's estimate is too optimistic, as the lowest and most conservative estimates are now already comparable to its highest estimate.

Like many coastal cities, Hong Kong will be affected by more severe flooding and a higher storm surge, which is elevated sea level brought about an approaching tropical cyclone. There have been incidences of flooding caused by rain runoff backed up by high tides and/or wind waves. There have also been cases of freshwater salination as a result of seawater intrusion into the Pearl River estuary. A raised sea level will make things worse.

In respect of storm surge, we do not have to look too far back. In September 2008, Typhoon Hagupit brought a storm surge exceeding 1.4 m, resulting in severe flooding in number of low-lying places. The worst hit was Tai O, on Lantau Island west of the international airport. You can imagine what it is going to be like when, with a raised sea level brought by global warming, Hong Kong is hit by a similar storm in future. We certainly need to ask ourselves what we can do to combat or reduce the effects of climate change.


Figure 2:Flooding in Tai O after the passage of Typhoon Hagupit in September 2008 (Photograph courtesy of TVB)
Figure 2    Flooding in Tai O after the passage of Typhoon Hagupit in September 2008
(Photograph courtesy of TVB)


B.Y. Lee


Sources:

New Scientist, 4 July 2009 issue.

Pfeffer, W.T., J.T. Harper and S.O. Neel, 2008. Kinematic Constraints on Glacier Contributions to 21st-Century Sea-Level Rise. Science, Vol. 321, 5 September 2009 issue.

Rahmstorf, S., 2007. A Semi-Empirical Approach to Projecting Future Sea-Level Rise. Science, Vol. 315, 19 January 2007 issue.

Rignot, E., J. Bamber, M. van den Broeke, C. Davis, Y. Li, W. van de Berg, E. van Meijgaard, 2008. Recent mass loss of the Antarctic Ice Sheet from dynamic thinning, Nature Geoscience, Nature Geoscience, Vol 1, 106 - 110.



Last revision date: <17 Jan 2013>