Indicator description

This indicator reports on Tasmanian regional implications of data from the NTC in South Australia, which calculates annual sea-level trends at 35 longer-term sites (>25 years) under the ABSLMP and 16 high accuracy SEAFRAME sites around Australia. Other key references used for this indicator are the Indicative Mapping of Tasmanian Coastal Vulnerability to Climate Change and Sea Level Rise (Internal linkSharples 2006); Historical and Projected Sea-level Extremes for Hobart and Burnie, Tasmania (Internal linkHunter 2008); and the DPIW policy report titled: Sea-Level Extremes in Tasmania: Summary and Practical Guide for Planners and Managers (Internal linkDPIW 2008).

Why is it indicative?

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 due to global warming.

It is predicted that 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; melting of land ice in Antarctica and Greenland; and changes in terrestrial storage.

It is important to monitor baseline sea-level trends because any changes are influenced by natural climate variations such as El Niño and decadal oscillations in addition to longer-term climate change such as global warming due to the greenhouse effect. Over short periods of time the transient effects of climate variability are comparatively large and can conceal the slowly accumulating longer-term effects of climate change.

Tasmania is being affected by changes in climate. These changes are leading to a wide range of environmental impacts, including a rise in the level of the seas that has been occurring at a sustained rate, which has not been experienced for at least 5,000 years (see Internal linkHunter 2007). Rises in sea-level is currently being felt along the coastline of the State through an increased frequency of flooding events, coastal erosion and substantial changes to the bathymetry and topography of soft coastal margins (e.g. sandy shorelines).

According to the Australian Government Department of Climate Change, over 20% of the Tasmanian coastline will be a risk from sea-level rise, erosion and recession, and more severe storm surges associated with climate change in the future (Internal linkDCC 2008). No systematic increases in storm intensities have yet been detected in the Tasmanian region (Internal linkSharples 2006). However, inundation of low lying areas around Tasmanian estuaries could impact salt marshes, wetlands and intertidal sand flats (important wading bird habitat). For example, inundation in and around the Derwent Estuary could potentially impact 156 ha of salt marshes, 488 ha of wetlands and 1,000 ha of intertidal sand flats (Internal linkWhitehead 2009). Inundation of sewage infrastructure, landfills and other sites may have an impact on estuarine water quality.

Long-term sea-level changes provide a signal that impacts on estuarine, coastal and marine systems are occurring or are likely to occur in the future. It is predicated that changes in sea-level in association with increases in intensity and frequency of storm surges and coastal flooding accompanying climate change will continue to impact on the Tasmanian environment and coastal communities through:

• increased salinity of rivers, bays and coastal aquifers;
• increased shoreline recession and coastal erosion, particularly along exposed low gradient sandy coasts (i.e. a rough 'rule of thumb' of the Bruun Rule, is that for every 1 m of sea-level rise there will be 50–100 m of horizontal erosion of exposed sandy beaches, as outlined by Internal linkSharples 2006);
• loss of salt marshes, wetlands and intertidal sand flats;
• impacts on marine habitats (i.e. giant kelp and seagrass beds); and
• impacts on marine species that rely on inshore coastal and estuarine habitats (i.e. sharks, morwong and other finfish that use these areas as important nursery grounds).

Data availability and limitations

The climate of Australia is affected by complex interactions between the atmosphere and the ocean. The sea-level is determined by these interactions, as well as by tidal forcings, and tectonics. The challenges in isolating the components of change that can be attributed to thermal expansion due to the enhanced greenhouse effect are highlighted by Lambert (2002): 'Changes in sea level are usually expressed as a change in the level of the sea with respect to land. It is therefore a relative measure that is indicative of movement of the land, changes in ocean volume or, in most cases, of both. As such, sea-level change, both today and in the past, exhibits a complex spatial and temporal pattern that reflects tectonic, isostatic and climate contributions.' Future change, likewise, will be geographically variable (see Internal linkLambert 2002).

In addition, there are limitations in quantifying loss of water from ice sheets. The pattern of change of ice sheets is influenced by changing precipitation, atmospheric temperature and oceanographic conditions, and glacier dynamic effects. To quantify ice loss, four different methods have so far been used: (1) calculation of flux imbalance from separate measurements of snow accumulation and ice flow velocity; (2) detection of changing gravitational anomalies; (3) measurement of changing ice-sheet surface elevation; and (4) the use of high-resolution ICESat (Ice, Cloud and land Elevation Satellite) laser altimetry to map change along the entire grounded margins of ice sheets (Internal linkPritchard et al. 2009). The technique to estimate ice-sheet change using ICESat along-track data has been found to be an effective tool for mapping continental and local-scale changes.

Trends in relative sea-level have been derived for 35 Australian long-term sea-level records that have been measured for at least 25 years. The average length of these records is 42 years (Internal linkNTC 2007). The data are updated annually and archived by the NTC of BoM under the ABSLMP. The survey provides a synopsis of the annual mean sea-levels and trends in longer-term relative sea-level records. More information on the monthly data reports prepared by the NTC can be found at the External linkAustralian Baseline Sea Level Monitoring website.

The ABSLMP also includes the maintenance of SEAFRAME stations, which measure sea-level very accurately, and also record meteorological parameters including atmospheric pressure, air and water temperatures, and wind speed and direction. The SEAFRAME stations consist of 14 standard stations at representative sites that are supported by the NTC as well as two customised stations supported by the private sector: Lorne by the Port of Melbourne Corporation and Stony Point by Toll Western Port (Internal linkNTC 2008). The installation of three of the standard stations (Spring Bay near Hobart, and Darwin and Cocos Islands) were supported by the National Oceanographic and Atmospheric Administration/National Ocean Service of the United States. The Division of Marine Research, CSIRO and the TOPEX/POSEIDON satellite altimetry experiment supported the installation of the gauge at Burnie.

According to the NTC, the TOPEX/POSEIDON and subsequent Jason-1 satellite altimeter missions have enabled sea-levels to be measured on a global basis every 10 days since late 1992, around the time the ABSLMP began. The SEAFRAME stations have provided important 'ground-truth' sea-level data for calibration and validation of the satellite altimeters. In shallow coastal waters satellite altimeter measurements are inaccurate and tide gauges are a necessity not only for monitoring long-term sea-levels but also tides and extreme events.

Some undocumented historical datum shifts have been identified at a few locations in the ABSLMP data. At Burnie, for example, the annual mean sea-levels prior to 1975 were contaminated. The damaged station at Burnie was replaced in June 2008 and upgraded with a more substantial supporting pile that offers improved protection from marine vessels (Internal linkNTC 2008). Adjustments have been made to the Burnie dataset to account for the damage at this monitoring site.

As identified in the ABSLMP data, there are two tide gauge stations in Tasmania with a sufficiently long record and the appropriate geographical separation for use in projecting the probabilities of extreme sea-levels in Tasmania for this century. These two tide gauge stations at located in Hobart and Burnie.

Data

Australia in a global context

Sea-level is the level of the sea surface at any one time. While the level of the seas have varied by more than 120 m during ancient ice age cycles that have occurred over the last 130 years, the overall rate of sea-level rise has increased. Since the launch of satellites to measure sea-level change in the early 1990s, the seas have risen over 3 mm/yr (Internal linkChurch et al. 2008). This is about 50% larger than the average rate of 1.7±0.3 mm/yr over the 20th century (Internal linkChurch et al. 2005). This rate has been unprecedented over the past century and is directly related to an increase of 'greenhouse' heat-absorbing gases in the atmosphere that has resulted in a warming of the global climate. For example, observations since 1961 also show that the oceans have warmed as the result of absorbing more than 80% of the heat added to the climate system.

Historical sea-level

internal SOE link to larger image

Source: Internal linkACE CRC 2008

• The best estimate of mean sea-level rise during the 20th century has been of 1.7±0.3 mm/yr (Internal linkChurch et al. 2005). For the period 1950–2000, the best estimate of mean sea-level rise is about 1.8 to 1.9±0.2 mm/yr (Internal linkAGO 2003). This rise accelerated to 3.1 mm/yr by 2003 (Internal linkChurch et al. 2008) and it is an important confirmation of climate change simulations that show an acceleration not previously observed. The 20th century global average sea-level has risen by a total of 10–20 cm.
   
• Disintegrating glacier ice constitutes a substantial and accelerating cause of global sea-level rise (Internal linkMeier et al. 2007). In a key scientific paper to the international journal Nature in 2009, scientists from the British Antarctic Survey and University of Bristol found that dynamic thinning of glaciers now reaches all latitudes in Greenland, has intensified on key Antarctic grounding lines, has endured for decades after ice-shelf collapse, penetrates far into the interior of each ice sheet and is spreading as ice shelves thin by ocean-driven melt (Internal linkPritchard et al. 2009). In Greenland, glaciers flowing faster than 100 m/yr^-1 have thinned at an average rate of 0.84 m/yr^-1, and in the Amundsen Sea embayment of Antarctica, thinning has exceeded 9.0 m/yr^-1 for some glaciers. The results from this study show that the most profound changes in the ice sheets currently result from glacier dynamics at ocean margins.
   
• Prominent in Greenland is the strong thinning of the southeast and northwest ice-sheet margins. Higher areas of the ice sheet in the south have thickened (Internal linkPritchard et al. 2009). The widespread dynamic thinning identified in the northwest of Greenland implies a sustained period of dynamic imbalance. In Antarctica, thinning has been found to be the strongest in the Amundsen Sea embayment area on the glacier scale, where it is confirmed as being localised on the fast-flowing glaciers and their tributaries. Profound dynamic thinning has also been identified of collapsed-ice-shelf tributary glaciers flowing from the Antarctic plateau to both the east and west coasts, and these glaciers have been found to be thinning at some of the highest rates recorded either in Antarctica or Greenland (up to tens of metres per year). Conversely, on the spine and western flank of the Antarctic Peninsula there has been an accumulation of ice and for the 400 km of coast feeding the Abbott Ice Shelf the ice thickness has been found to be unchanging.
   
• Sea-level rise for Australia is only slightly less than the global average (Internal linkNTC 2007; Internal linkChurch and White 2006) and coastal observations confirm that sea-levels have been rising since at least 1920 by approximately 1.2 mm/yr (Internal linkChurch et al. 2006; Internal linkIPCC 2001; see also Internal linkPittock and Wratt 2001; Internal linkLambeck 2002). A common feature in many model projections is a higher than the global average sea-level rise off the southeast coast of Australia and in a band stretching across the Indian and southern Pacific oceans at about 30°–45°S (Internal linkCSIRO 2009). Sea-level rise is greatest (about 3 mm/yr) in the eastern equatorial Pacific and western equatorial Indian Ocean. Observed rates of rise are smallest (about 1 mm/yr) in the western equatorial Pacific and eastern Indian Ocean, particularly the northwest coast of Australia. Two of the longest continuous Australian tide gauge records are from Fremantle in Western Australia (92 years) and Fort Denison in New South Wales (83 years) indicate that the observed rate of sea-level rise relative to the land has been 1.38 mm/yr and 0.86 mm/yr respectively (Internal linkDPIWE 2004). Regional variations in the rate of sea-level rise are weaker for much of the rest of the global oceans (Internal linkAGO 2003).
   

Climate change due to continued anthropogenic greenhouse gas emissions is expected to lead to a continuing global increase in sea-level over the next century and beyond to thousands of years. This is anticipated to occur as the upper ocean expands, land ice (glaciers and ice sheets) melts due to global warming, and changes in terrestrial storage occur. In particular, there is increasing concern by the majority of scientists that the potential instability of both the West Antarctic Ice Sheet in Antarctica and the Greenland Ice Sheet in the northern hemisphere could lead to a more rapid rate of sea-level rise than the current model projections. For example, a study of Greenland ice melting published in 2005 found that the loss of ice has been occurring about five times faster from Greenland's southeastern region in the previous two years than in the previous year and a half. The Greenland study suggests that the amount of fresh water contributed from the melting of its ice sheet could add 0.56 mm annually to a global increase in sea-levels, higher than all previously published measurements (Internal linkEarth Observatory 2006). Researchers from the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) also predict that even if atmospheric greenhouse gas concentrations were stabilised at 550 ppm (CO₂ equivalent), a tipping point could result in a 50% (possibly irreversible) change of melting of the Greenland Ice Sheet. Other predications put this tipping point at between 350–450 ppm CO₂ (currently 380 ppm) if combined with an average global temperature rise of 2-3ºC (Internal linkHansen 2007). These two ice sheets alone hold enough grounded ice to raise sea-levels by 6 m and 7 m respectively if they melted entirely (Internal linkACE CRC 2008; Internal linkPyper 2007).

While a precise figure for sea-level rise remains uncertain, when taking these issues into account, sea-level projections for the 21st century from the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR) of 2001 and the Fourth Assessment Report (AR4) of 2007 identify that observed sea-level is currently tracking near the upper limit of the IPCC model projections from the start date of the projections in 1990 (see Internal linkRahmstorf et al. 2007; Internal linkIPCC 2001; Internal linkIPCC 2007). The IPCC TAR Report model projections estimated a sea-level rise of between 9–88 cm by 2100. The IPCC AR4 model projections (with a 90% confidence range) revised this estimated and calculated a sea-level rise of between 18–59 cm by 2095 plus an allowance of another 10–20 cm for a potential dynamic response related to the melting of land ice in Antarctica and Greenland (see also Internal linkChurch et al. 2008; Internal linkIPCC 2007; Internal linkChurch et al. 2006; Internal linkEarth Observatory 2006). However, more recent published work suggests that a rise of up to 2 m is possible (Internal linkACE CRC 2008) and it could be more than 5 m by 2100 (Internal linkHansen 2007).

TAR and AR4 projections

internal SOE link to larger image

Source: Internal linkChurch et al. 2008

• CSIRO has projected a sea-level rise of between 18–79 cm for Australia by 2100 (Internal linkCSIRO 2009).
   
• Even with a rise of mean sea-level of approximately 50 cm, it is predicated that flooding, storm and inundation events which now happen every few years would happen every few days in 2100 and that larger increases in the frequency of extremes could occur in Bass Strait, along the coastline of Western Australia and capital cities such as Sydney, Brisbane and Hobart (Internal linkHunter 2007).
   
• In Australia, even a moderate rise in sea-level of 20–50 cm will have serious implications for coastal ecosystems and coastal zone management (Internal linkACE CRC 2008).
   

Future sea-level change is not expected to be geographically uniform, so information about its distribution is needed to inform assessments of the impacts on coastal regions in Tasmania. The Statewide regional pattern depends on ocean surface fluxes, interior conditions and ocean circulation. Serious impacts are caused by changes in mean sea-level and extreme sea-level events that result from storm surges and exceptionally high waves forced by meteorological conditions. The southward extension of the warmer EAC waters is also predicated to result in a sea-level rise above that of global background sea-level rise. Therefore, climate-related changes in these phenomena need to be considered in any future assessment of changing sea-levels in the State. This implies that revised estimates of global sea-level rise should feed into planning and adaptation to sea-level rise at the local, regional and State scales in Tasmania.

Tasmania in a regional context

Sea-level measurements based on an early colonial tide gauge at Port Arthur suggest a rise of at least 13 cm, with an average annual rate of 0.8 mm/yr±0.2 mm/yr relative to the land in the southeast of Tasmania during the period 1841 to 2002 (Internal linkPugh et al. 2002; Internal linkHunter et al. 2003; see also Internal linkSharples 2006). When combined with estimates of land uplift, this yields an estimate of average sea-level rise due to an increase in the volume of the oceans of 1.0±0.3 mm/yr, over the same period. This represents a lower rate than that observed elsewhere in the Australia/New Zealand region. However, historical data from other sites suggests that this figure may be an under-estimate and the rate of sea-level rise in Tasmania may have been 1 to 2 mm/yr as evident elsewhere in the Australian region. More information on the early colonial tide gauge at Port Arthur can be found in the embedded document Internal linkMeasuring sea-level rise at Port Arthur.

There are many low lying coastal areas in the State. The following Google Earth images provide a range of examples of exposed low gradient sandy coasts and other areas such as salt marshes, wetlands and intertidal sand flats that could be subject to storm surges and sea-level rise.

Bakers Beach internal SOE link to larger image

Ansons Bay internal SOE link to larger image

Hazards Lagoon internal SOE link to larger image

Swan Bay internal SOE link to larger image

SwanLagoon internal SOE link to larger image

Bruny Island Neck internal SOE link to larger image

Eaglehawk Neck internal SOE link to larger image

Racecourse Flats internal SOE link to larger image

Southport Lagoon internal SOE link to larger image

Moulting Lagoon internal SOE link to larger image

Weymouth area internal SOE link to larger image

Maria Island internal SOE link to larger image

Great Musselroe Bay internal SOE link to larger image

Little Musselroe Bay internal SOE link to larger image

As outlined by DPIWE in 2004, an alternative estimate of sea-level rise for Tasmania may be derived from the local sea-level datum (Internal linkDPIWE 2004). The Tasmanian State Datum was adopted in the mid-1940s and was based on mean sea-level in Hobart during the period 1874/5 to 1905 (see also Internal linkHunter 2008). This datum had been used prior to the 1940s, and is still in use throughout the Hobart City Council region. The AHD for Tasmania was declared in 1983, based on mean sea-level in Hobart and Burnie for 1972. The AHD (Tasmania) is 16.5 cm above the Tasmanian State Datum, implying an average rate of sea-level rise of about 2.0±0.3 mm /yr (relative to the land) over the period 1874/5–1905 to 1972. The uncertainty of this estimate has been calculated to be several tenths of a millimetre per year.

Tasmania can also be affected by tsunami events that can result in changes to sea-levels. For example, magnitude Mw7.4 earthquake occurred in September 2007 near the Auckland Islands, New Zealand and it generated a tsunami that propagated across the Tasman Sea and was detected by the SEAFRAME station at Spring Bay approximately 2 hours later (Internal linkNTC 2008). The tsunami was also detected by the Australian tide gauges at Macquarie Island (30cm) and Hobart (10cm). The trough-to-peak size of the tsunami at these stations was 20cm and 30cm respectively. Tasmani events that reach the State can also occur further away. For example, another magnitude Mw8.0 earthquake occurred in August 2007 near the coast of central Peru. Evidence of an associated small tsunami was detected at Spring Bay approximately 17 hours later with trough-to-peak size of around 10–20 cm. More recently, a tsunami event in July 2009 that was generated from a Mw7.8 earthquake in the south island of New Zealand caused storm surge waves of approximately 20–30 cm along the east coast of Tasmania.

National Tidal Centre tide gauge stations

In the following tables, trends in relative sea-level have been derived for 35 Australian long-term sea-level records (>25 years) that are collected under ABSLMP and archived at the NTC. The following map shows the distribution of relative sea-level trend estimates for these 35 tide gauge stations based on data that is collected hourly.

Distribution of relative sea-level trend

internal SOE link to larger image

Source: Internal linkNTC 2007

The overall average relative sea-level trend for these tide gauge stations to December 2007 is estimated at 0.8 mm/yr with a standard deviation of 1.3 mm/yr (Internal linkNTC 2007). Although the length of record from the baseline stations is relatively short in climatic terms there are a number of clear results emerging. At the same time, some of the stations exhibit unrealistic trends due to undocumented datum shifts. A more realistic average trend obtained from 27 stations (whose trends are within 1 standard deviation of the mean) is 1.2 mm/yr with a standard deviation of 0.5 mm/yr.

Total sea-level trend estimates for tide gauges around Australia

Regional variation in sea level rise

Locations	No of stations	Years of data	Trend (mm/yr)
		Avg	Avg	Std Dev
All	35	42.1	0.8	1.3
Excluding*	27	44.7	1.2	0.5

* = Trend values that differ by more than +/- 1 standard deviation from the overall mean.

Source: Internal linkNTC 2007

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Relative sea-level trend estimates for tide gauges around Australia

Regional variation in sea-level rise

Location	Longitude	Latitude	Years of data	Trend (mm/yr)
Darwin	130.850	-12.467	44.9	1.6
Wyndham*	128.100	-15.450	36.1	2.2
Broome*	128.100	-18.000	33.5	2.3
Port Hedland	118.583	-20.300	37.9	1.2
Carnarvon	113.617	-24.883	33.8	1.5
Geraldton	114.583	-28.783	41.9	0.6
Fremantle	115.733	-32.050	100.6	1.5
Bunbury	115.633	-33.317	40.3	0.9
Albany	117.883	-35.033	41.4	0.6
Esperance	121.900	-33.867	41.2	0.6
Thevenard	133.650	-32.150	40.9	0.8
Port Lincoln	135.867	-34.717	42.1	1.5
Port Pirie	138.017	-33.167	64.3	0.4
Port Adelaide-inner	138.500	-34.850	49.7	2.1
Port Adelaide-outer	138.483	-34.783	65.1	2.1
Victor Harbor	138.633	-35.567	40.5	0.8
Portland	141.600	-38.333	25.2	1.8
Williamstown	144.917	-37.867	41.6	1.4
Geelong	144.433	-38.167	34.3	0.7
Point Lonsdale*	144.617	-38.300	42.9	-1.6
Stony Point*	145.217	-38.367	27.8	-2.3
Burnie	145.917	-41.050	35.2	-1.5
Hobart	147.333	-42.883	37.8	0.7
Port Kembla	150.917	-34.483	29.6	0.7
Fort Denison	151.233	-33.850	91.7	0.9
Newcastle	151.800	-32.917	41.8	0.9
Lord Howe Island*	159.067	-31.517	29.4	-2.2
Brisbane	153.167	-27.367	35.6	1.8
Bundaberg	152.383	-24.767	40.2	0.2
Gladstone	151.250	-23.833	28.4	2.0
Mackay	149.233	-21.117	34.5	1.4
Townsville	146.833	-19.250	48.3	1.2
Cairns	145.783	-16.917	34.0	1.5
Weipa*	141.883	-12.667	30.7	2.7
Melville Bay*	136.700	-12.217	29.4	-2.1

* = Trend values that differ by more than +/- 1 standard deviation from the overall mean.

Relative sea-level = the level of the sea surface, relative to the land, at any one time.

Source: Internal linkNTC 2007

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The data indicates that the overall pattern of relative sea-level trends around the Australian coastline is geographically uniform. The Australian average relative sea-level rise is consistent with the global average sea-level rise over the same period.

• Annual mean sea-levels around the Australian coastline are strongly correlated with the ENSO signal. Annual mean sea-levels generally fluctuate in accordance with the Southern Oscillation Index (SOI). The longest sea-level records show decadal sea-level oscillations with periods of around 20 years.
   
• Burnie (-1.5 mm/yr) is among five sites showing negative trends partly due to unstable tide gauge datum (particularly prior to 1975). The overall average relative sea-level rise around Australia is 1.2 mm/yr (Internal linkNTC 2007). This is consistent with a global average sea-level rise over the last 100 years of 1.7 ± 0.3 mm/yr (Internal linkChurch et al. 2005).
   

The following table details short-term relative sea-level trends at the 14 standard SEAFRAME stations and two customised stations supported by the private sector. SEAFRAME stations measure sea-level very accurately, and also record meteorological parameters including atmospheric pressure, air and water temperatures, and wind speed and direction.

Short-term relative sea-level trends based upon SEAFRAME data from June 2007 to June 2008

Location	Installation date	Trend (mm/yr)	Trend from 2007 (mm/yr)
Cocos Islands	Sep-92	7.9	-0.1
Groote Eylandt	Sep-93	6.3	0.9
Darwin	May-90	7.2	0.6
Broome	Nov-91	8.7	0.4
Hillarys	Nov-91	8.2	1.1
Esperance	Mar-92	5.4	0.8
Thevenard	Mar-92	3.7	0.6
Port Stanvac	Jun-92	4.9	0.5
Portland	Jul-91	2.6	0.4
Lorne	Jan-93	1.7	0.1
Stony Point	Jan-93	1.7	0.6
Burnie	Sep-92	1.8	-0.2
Spring Bay	May-91	3.2	0.3
Port Kembla	Jul-91	3.3	-0.2
Rosslyn Bay	Jun-92	1.6	0.3
Cape Ferguson	Sep-91	2.8	0.1

Relative sea-level = the level of the sea surface, relative to the land, at any one time.

Source: Internal linkNTC 2008

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Precise levelling support for the ABSLMP is provided by relevant state agencies and Geosciences Australia. The purpose of levelling sea-level monitoring gauges is to establish whether they are moving vertically with respect to the land. The following table details the surveyed heights of the SEAFRAME stations with respect to the local primary tide gauge benchmark and the rates of vertical movement. The SEAFRAME station at Cocos Island appears to have undergone subsidence at a rate of 0.8 mm/yr, and the SEAFRAME station at Esparence appears to have undergone subsidence at a rate of 0.4 mm/yr. Corrections for these movements will reduce the observed relative sea-level trend. Other stations around the Australian mainland, including those at Burnie and Spring Bay, appear more vertically stable. However, there is evidence of both subsidence and emergence at some stations.

Trends in the datum of the SEAFRAME sea-level sensor as determined from precise levelling between the sensor and the tide gauge benchmark

Location	Installation Date	Trend in the Datum of the Sea Level Sensor (mm/yr)
Cocos Islands	Sep-92	-0.8
Groote Eylandt	Sep-93	-0.1
Darwin	May-90	0.3
Broome	Nov-91	-0.1
Hillarys	Nov-91	0.1
Esperance	Mar-92	-0.4
Thevenard	Mar-92	0.2
Port Stanvac	Jun-92	-0.1
Portland	Jul-91	0.1
Lorne	Jan-93	0
Stony Point	Jan-93	0
Burnie	Sep-92	0
Spring Bay	May-91	0
Port Kembla	Jul-91	0
Rosslyn Bay	Jun-92	0
Cape Ferguson	Sep-91	0.2

Source: Internal linkNTC 2008

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The following table details the combined net rate of relative sea-level trends after the effects of the vertical movement of the land and the inverse barometer effect are removed. The data indicates that sea-level rise over the duration of SEAFRAME has not been geographically uniform, with the largest trends observed around the north and west Australian coastline adjacent to the Indian Ocean (Internal linkNTC 2008). With ongoing sea-level monitoring the expectation is that better estimates of the longer-term sea-level change signal will increasingly emerge from the 'noise' of decadal fluctuations. Over 2007–08, the overall sea-level trends are mostly larger than they were in the pervious 12 months to mid-2007, due in part to higher than normal sea-level around Australia during the recent La Niña.

Net relative sea-level trend estimates from June 2007 to June 2008

Location	Installation date	Net relative trend (mm/yr)	Trend from 2007 (mm/yr)
Cocos Islands	Sep-92	7.2	-0.3
Groote Eylandt	Sep-93	6.6	0.5
Darwin	Sep-93	7.1	0.5
Broome	Nov-91	7.9	0.4
Hillarys	Nov-91	8	1.1
Esperance	Mar-92	4.5	0.6
Thevenard	Mar-92	3.5	0.6
Port Stanvac	Jun-92	4.6	0.4
Portland	Jul-91	2.8	0.5
Lorne	Jan-93	1.7	0.2
Stony Point	Jan-93	1.7	0.8
Burnie	Sep-92	1.7	-0.1
Spring Bay	May-91	3.5	0.5
Port Kembla	Jul-91	1.8	0.3
Rosslyn Bay	Jun-92	1.3	0.1
Cape Ferguson	Sep-91	2.5	0

Relative sea-level = the level of the sea surface, relative to the land, at any one time.

The net relative sea-level trend estimates from June 2007 to June 2008 after vertical movements in the observing platform and the inverted barometric pressure effect are taken into account. The net trend is defined to be the relative sea-level trend after subtracting the effects of the vertical movement of the platform and the inverse barometric pressure effect.

Source: Internal linkNTC 2008

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Indicative mapping of Tasmanian coastal vulnerability to climate change and sea-level rise

As part of the project titled: Indicative Mapping of Tasmanian Coastal Vulnerability to Climate Change and Sea Level Rise, Chris Sharples reviewed coastal flooding events and available tide-gauge data for Tasmania (Internal linkSharples 2006). In order to evaluate the vulnerability to sea-level extremes under conditions of rising sea-level, records were assessed from seven sites at which there was sufficient data to derive statistics for extreme events including: Hobart (29 years), Georgetown (29 years), Burnie (31 years), Spring Bay (18 years), Devonport (18 years), Stanley (3 years) and Granville Harbour (4 years). The following map shows the location of these key tide gauge stations.

Key tide gauge stations in Tasmania

internal SOE link to larger image

Source: Compiled by SoE Unit using data provided by Internal linkSharples 2006

Base topographic data supplied by the LIST (State of Tasmania)

Storm Surge Flooding Scenarios have been modelled and digital polygon maps prepared to provide an indicative identification of Tasmanian coastal areas potentially vulnerable to storm surge flooding under contemporary (2004) mean sea-level conditions, and under projected mean sea-level conditions in 2100 according to the range of global sea-level rise projections that were provided by the IPCC in 2001 (see Internal linkIPCC 2001; Internal linkChurch and Gregory 2001). These three scenarios are included in the following table and it presents a summary of Tasmanian Tide Gauge Data that has been used to map an indicative 0.01% exceedance storm surge flood vulnerability zone. The three scenarios are as follows:

• 2004 predictable storm surge flood levels: based on 0.01% exceedance storm surge water levels historically recorded by Tasmanian tide gauges;
• 2100 minimum predictable storm surge flood levels: based on 2004 predictable levels plus minimum sea-level rise projected for 2100 by the IPCC in 2001; and
• 2100 maximum predictable storm surge flood levels: based on 2004 predictable levels plus maximum sea-level rise projected for 2100 by the IPCC in 2001.

Summary of Tasmanian Tide Gauge Data used to map an indicative 0.01% exceedance storm surge flood vulnerability zone

As part of the project titled: Indicative Mapping of Tasmanian Coastal Vulnerability to Climate Change and Sea Level Rise

Station	Period	Hourly records	Calculated return*	Exceedance level 0.01%	Minimum 0.01% exceedance level	Maximum 0.01% exceedance level
Hobart	1960-2001	250,002	3.2	1.103	1.183	1.943
Georgetown	1965-1997	252,512	1.4	1.783	1.863	2.623
Burnie	1952-2003	273,484	1.2	1.812	1.892	2.652
Spring Bay	1985-2003	156,214	2	0.907	0.987	1.747
Devonport	1965-2002	156,093	1.5	1.827	1.907	2.667
Stanley	1965-1969	23,230	2.7	1.738	1.818	2.578
Granville Harbour	1974-1994	36,825	2.1	1.413	1.493	2.253

Total period includes gaps for some records, but all records include a minimum of 2.7 years of hourly data. Granville Harbour: 4.2 years of actual record only.

* = Calculated return period for 0.01% exceedance event (years). Return period is calculated by determining the number of 0.01% exceedance events over the period of actual data recording. Discrete exceedance events are considered to be events separated by a minimum of 1 day. The calculated return period is more statistically reliable for the longer data records.

Exceedance level 0.01% = 0.01% exceedance level for 2004 (m above AHD); Minimum 0.01% exceedance level = Minimum 0.01% exceedance level for 2100 (m above AHD) plus 8 cm (minimum IPCC projected sea-level rise, 2001); and Maximum 0.01% exceedance level = Maximum 0.01% exceedance level for 2100 (m above AHD) plus 8 cm (minimum IPCC projected sea-level rise, 2001).

Source: Internal linkSharples 2006

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Indicative areas of Tasmanian coastal regions that would predictably be flooded by 0.01% exceedence storm surge events under each of the three scenarios identified above were also estimated as part of this coastal vulnerability assessment. The following table details Indicative total Tasmanian coastal areas (including Bass Strait islands) that would be flooded by 0.01% exceedance storm surge events under each of the three scenarios.

Estimated total Tasmanian coastal areas (including Bass Strait islands) that would be flooded by 0.01% exceedance storm surge events

Scenario	Indicative flooding area*
2004 0.01% exceedance storm surge flood levels	240 km2
Minimum 2100 0.01% exceedance storm surge flood levels	247 km2
Maximum 2100 0.01% exceedance storm surge flood levels	247 km2

* = Indicative area of flooded land above the mean high water mark as mapped by current 1:25,000 LIST topographic mapping at the time of the assessment. Calculated from map polygons using ESRI Arcview GIS software.

Flooded by 0.01% exceedance storm surge events under each of the three scenarios considered in this assessment

Source: Internal linkSharples 2006

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The project concluded that in some areas where coastal flats grade up more gradually and any break of slope is less distinct, there may be a more significant increase in the areas flooded in these places under the maximum 2100 scenario as compared to the 2004 scenario (Internal linkSharples 2006). However, in many other flood prone coastal locations the land area that would be flooded under the maximum 2100 scenario is only a little greater than that which is potentially prone to flooding under the 2004 scenario. Although the 2100 maximum flooding scenario involves a significant rise in water levels, this extra rise is expected to create only a minor additional area of flooding since the increased flooding depth is accommodated by a minor horizontal extension over the more steeply-rising ground.

This modelling research also identified that shoreline recession in Tasmania does not occur gradually as the sea-level rises, but rather occurs episodically during major storms when energetic waves can reach the back of the beach to cause erosion of beach and dunes (Internal linkSharples 2006). Therefore, there is a lag between the sea-level rising and a corresponding degree of erosion taking place. The lag will depend on the frequency and intensity of storms affecting particular coasts, and erosion can be expected to progress most rapidly on coasts receiving the most frequent large storms. In Tasmania, the west and southwest coasts currently receive the highest annual wave energies and probably the most frequent intense storms. It is on the west and southwest coasts of the State that sandy shoreline erosion is currently most prominently in evidence. More information on the vulnerability of various shoreline geodiversity types to erosion and recession caused by wave action and other hazards such as sea-level rise and storm surge flooding can be found in the Internal linkVulnerability of Coastal Geodiversity Indicator. Vulnerable shores are those that are:

• predominantly composed of unconsolidated sand-grade sediment in the intertidal zone (i.e. sandy beaches and dune fronts);
• 'open' coasts, exposed to oceanic swells and particularly storm waves (as opposed to estuarine or coastal re-entrant shorelines protected from oceanic waves but affected by other processes such as tidal currents which are not generally characteristic of open coasts); and
• coastal areas that are backed by low-lying (low-profile) plains underlain by unconsolidated sandy sediments in the immediate backshore, which may include coastal dune systems or may be low plains without significant dunes. Where a sandy beach is immediately backed by hard bedrock rising above sea-level, the beach may still erode, but further recession will be retarded by the hard bedrock (Internal linkSharples 2006).

Where coastlines remain in a natural state and are more or less undeveloped, dynamic and highly mobile coastal landforms and ecosystems such as beaches, dune systems, wetlands, saltmarshes and intertidal sand flats have the potential to adjust to rising sea-levels (Internal linkSharples et al. 2008). They are also able to adapt to changing groundwater levels and increased frequencies of inundation by migrating landwards, particularly when they are backed by low-lying soft-sediment environments that allow such migration. However, coastlines that are subject to development are less resilient to changes in sea-levels. In addition, if human responses to rising sea-levels are to defend the coast with artificial structures such as sea walls, existing potential for natural shoreline adjustment to the changing conditions will be further reduced. This may result in less natural shoreline and fewer areas for the retreat of vulnerable plants, animals and landforms.

Exceedance probabilities for Tasmania

There are two tide gauge stations in Tasmania with a sufficiently long record and the appropriate geographical separation for use in projecting the probabilities of extreme sea-levels in the State for this century (Internal linkDPIW 2008). Sea-level height data from these tide gauges has been reviewed by John Hunter from the ACE CRC with the aim of assessing historical extreme sea-level events, and determine sea-level rise probabilities for the 21st century. The George Town tide gauge has a similar record to Burnie, however, data from this tide gauge was not used in the analysis because it is considered to be in effect the same as data from the Burnie tide gauge. The associated report titled: Historical and projected sea-level extremes for Hobart and Burnie, Tasmania provides a set of 150 tables and 300 figures to demonstrate the probabilities that an asset would suffer flooding at various intervals for reference by planning and management authorities responsible for setting standards for assets in the coastal zone (Internal linkHunter 2008).

Summary of available tide-gauge observations for Hobart

Period	Description	Source	Digitised	Datum info
1889	Data collected by Mault (1889)	Various references	Yes	Yes
1889 to 1946	Records of high and low water only	Archives Office of Tasmania (MB2/57)	Yes	No
1923 to 1944	Miscellaneous charts	Archives Office of Tasmania (MB2/58)	No	No
1950 to 1960	Miscellaneous charts	Tasmanian Ports Corporation	No	Some
1960 to 2004	Digitised hourly data	National Tidal Centre	Yes	Some
2004 to 2009	Data collected by the National Tidal Centre	National Tidal Centre	?	Yes

Source: Internal linkHunter 2008; Internal linkNTC 2009

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Summary of available tide-gauge observations for Burnie

Period	Description	Source	Digitised	Datum info
1952 to 1992	Data collected by Burnie Port Authority	National Tidal Centre	Yes	No
1992 to 2004	Data collected by National Tidal Centre	National Tidal Centre	Yes	Yes
2004 to 2009	Data collected by National Tidal Centre	National Tidal Centre	?	Yes

Source: Internal linkHunter 2008; Internal linkNTC 2009

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The 2008 Technical Report completed by John Hunter provides a companion document to the DPIW guide titled: Sea-Level Extremes In Tasmania: Summary and Practical Guide for Planners and Managers (Internal linkDPIW 2008). As part of this guide, exceedance probabilities for an asset life of 100 years are outlined for the current regime of sea-levels (year 2000). These exceedance probabilities are detailed in the following table. Exceedance probabilities are also provided for the years 2040 and 2100 to provide information on the increasing level of risk in Tasmania due to sea-level rise. For example, based on the Hobart tide gauge data, over a 100 year time period, there is a 70% chance of the height 1.49 m above AHD being exceeded at year 2000 sea-levels. By the year 2040, the same level of chance of exceedance over the same 100 years rises to 1.59 m above AHD. By the end of the century, the height has risen to 1.92 m above AHD. The exceedance probability threshold levels of 90%, 80%, 70% and 60% are given for an asset life of 100 years for each of the current, 2040 and 2100 year timeframes.

Exceedance probabilities for Hobart and Burnie

Tide gauge	Hobart			Burnie
Statistic	Min	Mean	Max	Min	Mean	Max
Annual exceedance probability (AEP)
1% AEP – yr 2000	1.41	1.5	1.65	1.905	1.92	1.94
1% AEP – yr 2040	1.54	1.6	1.74	2.06	20.7	20.8
1% AEP – yr 2100	20.9	2.15	2.2	2.7	2.75	2.8
Exceedance Probability (asset life 100 yrs)
90% EP (100yrs) – yr 2000	1.36	1.4	1.46	1.9	1.91	1.92
90% EP (100yrs) – yr 2040	1.45	1.5	1.55	1.95	1.97	2
90% EP (100yrs) – yr 2100	1.63	1.72	1.81	2.11	2.15	2.18
80% EP (100yrs) – yr 2000	1.39	1.45	1.854	1.91	1.92	1.94
80% EP (100yrs) – yr 2040	1.48	1.55	1.63	1.97	2	20.025
80% EP (100yrs) – yr 2100	1.71	1.83	1.96	2.19	2.23	2.27
70% EP (100yrs) – yr 2000	1.41	1.49	1.61	1.91	1.925	1.951
70% EP (100yrs) – yr 2040	1.51	1.59	1.7	1.985	2.02	20.5
70% EP (100yrs) – yr 2100	1.79	1.92	20.6	2.28	2.3	2.34
60% EP (100yrs) – yr 2000	1.43	1.53	1.69	not plotted	not plotted	1.97
60% EP (100yrs) – yr 2040	1.53	1.63	1.77	20.1	20.3	2.41
60% EP (100yrs) – yr 2100	1.86	2	2.15	2.33	2.37	2.41

Numbers refer to meters above Australian Height Datum (AHD). The numbers do not include the impact of waves. For the purposes of determining probabilities for extreme high sea-level events around Tasmania:

• The Hobart figures can be used for the south of the State from approximately Macquarie Harbour to Freycinet Peninsula.
• A transitional zone can be considered for between approximately Freycinet and Cape Naturaliste, and approximately Macquarie Harbour to Cape Grim – here, it is possible to use the mean of the tidal data from the Hobart and Burnie tide gauges.
• The Burnie figures are generally representative of the zone through Bass Strait.
• For the Furneaux Group, Burnie figures can be used for the western shorelines, while thetransitional zone figures can be used for the eastern shorelines. For King Island, Hobart figures can be used for the western shorelines, Burnie figures for the eastern shorelines, and thetransitional figures for the northern and southern end of the island.

Source: Internal linkDPIW 2008

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Related Indicators

Biological Changes and ClimateInternal link

Vulnerability of Coastal GeodiversityInternal link

Potential Area Affected by ErosionInternal link

Acid Sulfate SoilsInternal link

Population DistributionInternal link

Coastal Inundation HazardInternal link

Historic Heritage EnvironmentsInternal link

New Dwelling Completions and Subdivision ActivityInternal link

Artificially Modified CoastlinesInternal link

Related Issues

Within the structure of State of the Environment Tasmania, an indicator can be related or associated with any number of issue reports (or vice versa). The data within an indicator is used to inform an issue report and any related recommendations. For example, the Climate Change Issue Report is linked to indicators on sea-level change, atmospheric concentrations of greenhouse gases, and ecological responses to climate change. The following table lists those issue reports that have been related with the indicator you are currently reading. The link below takes you to the introduction to each Issue Report, so you can navigate to the indicator section within each issue report to find out more about why and how an indicator is informing an Issue Report.

Aboriginal HeritageInternal link

Climate Variability and ChangeInternal link

Estuarine, Coastal and MarineInternal link

GeodiversityInternal link

Historic HeritageInternal link

Population and Settlement PatternsInternal link

Threatened Species and CommunitiesInternal link

Acknowledgment

Data for this indicator is provided courtesy of the National Tidal Facility, Bureau of Meteorology (Internal linkNTC 2003). The indicator is based on the Key Atmosphere Indicator 1.3 (Internal linkManton and Jasper 1998).