Sea level rise

Global sea level rise

There is a significant body of evidence that suggests the increase of ‘greenhouse' (heat-absorbing) gases in the atmosphere has resulted in a warming of the global climate during the previous century. Predictive work indicates that this warming will accelerate in the future due to continued anthropogenic greenhouse gas emissions. In the 20th century global average sea level has risen by 10-20 cm, primarily due to global warming . This sea level rise will continue, and possibly accelerate, over the next century and beyond, through a combination of mechanisms including:

thermal expansion of the oceans;
melting of glaciers and ice caps;
melting of the Greenland and Antarctic ice sheets; and
changes in terrestrial storage.

The Intergovernmental Panel on Climate Change's (IPCC's) Third Assessment Report - "Climate Change 2001: The Scientific Basis" - was prepared over the past three years by more than one hundred scientific authors assessing published scientific literature. In the context of sea level rise, the IPCC (2001) [1] used numerous greenhouse gas emission scenarios in various numerical climate models to predict the amount of sea level rise over the next century. A sea level rise of 0.09 to 0.88 m is projected from 1990 to 2100 (Table 1).

	Low	Mid	High
2040	3	12	30
2100	9	48	88

Table 1. Total predicted global mean sea level rise for 2040 and 2100 (in cm) (IPCC, 2001). Many projections of global climate change, including sea level rise forecasting, are affected by uncertainties, including those caused by incomplete scientific knowledge, and uncertainty surrounding future greenhouse gas emission trends.

Regional variation in sea level rise

Factors that can cause regional variations in sea level include [2]:

geological effects caused by the slow rebound of land that was covered by ice during the last Ice Age (‘isostatic rebound');
flooding of continental shelves since the end of the last Ice Age, which pushes down the shelves and causes the continent to push upwards in response (‘hydroisostasy');
changes in land height in tectonically or volcanically active regions;
changes in atmospheric wind patterns and ocean currents; and
local subsidence due to sediment compaction or groundwater extraction.

The estimated relative sea level trends for tide gauge locations around Australia which have at least 25 years of hourly data on the National Tidal Facility archive are shown in Table 2. The overall Australian average sea level rise of 0.30 mm per year is substantially lower that the global estimates of IPCC (2001) of 1-2 mm per year over the last 100 years. Table 2 also shows a considerable variation between sites, driven by combinations of the factors outlined above. A good example of this regional variation is the sea level fall of 0.19 mm per year at Port Pirie compared to the >2 mm per year sea level rise at nearby Adelaide.

Location	Years Of Data	Estimated Trend (mm per year)
Darwin	34.9	-0.02
Wyndham	26.4	-0.59
Port Headland	27.7	-1.32
Carnarvon	23.9	+0.24
Geraldton	31.5	-0.95
Fremantle	90.6	+1.38
Bunbury	30.2	+0.04
Albany	31.2	-0.86
Esperence	31.2	-0.45
Thevenard	31.0	+0.02
Port Lincoln	32.3	+0.63
Port Pirie	63.2	-0.19
Port Adelaide - inner	41.0	+2.06
Port Adelaide - outer	55.1	+2.08
Victor Harbour	30.8	+0.47
Hobart	29.3	+0.58
Georgetown	28.8	+0.30
Williamstown	31.8	+0.26
Geelong	25.0	+0.97
Point Lonsdale	34.4	-0.63
Fort Denison	81.8	+0.86
Newcastle	31.6	+1.18
Brisbane	23.7	-0.22
Bundaberg	30.2	-0.03
Mackay	24.3	+1.24
Townsville	38.3	+1.12
Cairns	23.6	-0.02

Table 2. The estimated relative sea level trends for tide gauge locations around Australia which have at least 25 years of hourly data on the National Tidal Facility archive. The overall average relative sea level trend of the above list is +0.30 mm per year [3].

Issues arising from sea level rise

Changes in sea level will be felt through [4]:

ncreases in intensity and frequency of storm surges and coastal flooding;
increased salinity of rivers, bays and coastal aquifers resulting from saline intrusion;
increased coastal erosion;
loss of important mangroves and other wetlands (the exact response will depend on the balance between sedimentation and sea level change [5]); and
impact on marine ecosystems i.e. coral reefs.

Any or all of these changes may have a severe impact on urban communities if unmitigated.

More information on sea level rise can be found at:

The CSIRO marine research website;
The National Tidal Facility of Australia website;
The IPCC website.

References

IPCC (2001) Climate Change 2001: The Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, UK.W
Walsh, K.J.E., Betts, H., Church, J., Pittock, A.B., McInnes, K.L., Jackett, D.R. & McDougall, T.J. (In press) Using sea level rise projections for urban planning in Australia. Journal of Coastal Research.
Mitchell, W., Chittleborough, J., Ronai, B. & Lennon, G.W. (2000) Sea Level Rise in Australia and the Pacific. Pacific Islands Conference on Climate Change, Climate Variability and Sea Level Change. Rarotonga, Cook Islands, 3-7 April 2000.
Scavia, D., Field, J.C., Boesch, D.F., Buddemeier, R.W., Burkett, V., Cayan, D.R., Fogarty, M., Harwell, M.A., Howarth, R.W., Mason, C., Reed, D.J., Royer, T.C., Sallenger, A.H., and J.G. Titus. 2002. Climate change impacts on U.S. coastal and marine ecosystems. Estuaries 25(2), 149-164.
Woodroffe, C.D. 1995. Response of tide-dominated mangrove shorelines in northern Australia to anticipated sea-level rise. Earth Surface Processes and Landforms 20, 65-85.

Author

Andrew Jones, Geoscience Australia