11 Coasts| 4 Risks and resilience of coastal communities and environments | 4.3 Sea level rise

State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.

11 Coasts

4.3 Sea level rise

The impact of sea level rise will be felt most strongly in our coastal regions. Sea level rise is a result of expansion of the oceans as they warm; addition of water to the ocean from melting glaciers, ice caps, and the ice sheets of Greenland and Antarctica; and changes in the relative level of seabeds and the land (see Chapter 6: Marine environment and Chapter 7: Antarctic environment for further discussion). Sea level is now rising globally, and the rate of rise increased from the 19th to the 20th century and during the 20th century (Figure 11.7). Since the early 1990s, the rate of rise has been almost double the average for the 20th century.

Sea level is forecast to continue to rise during the 21st century and beyond, in response to increasing concentrations of greenhouse gases in the atmosphere. Including an allowance for the melting of ice sheets, IPCC projections are for a rise of 18–79 centimetres by 2095 compared with 1990. However, our current understanding of the response of ice sheets to global warming is inadequate, and a larger rise is possible. The rate of sea level rise is currently near the upper end of current projections—observations indicate that global sea level is currently rising (since 1993) at around 3 millimetres per year.45

Figure 11.7

Source: Adapted from Church & White46 with kind permission from Springer Science+Business Media BV

Figure 11.7 Global mean sea level (GMSL) changes between 1860 and 2009, compared with the 1990 average sea level

The blue solid line is sea level estimated from coastal and island sea level data. The one standard deviation uncertainty estimates are indicated by the shading. The Church & White47 estimates for 1870–2001 are shown by the red solid line and dashed magenta lines indicate the one standard deviation errors. The series are set to have the same average value over 1960–90 and the new reconstruction is set to zero in 1990. The average rate of rise from 1900 to 2000 was around 1.7 millimetres per year. The rate of rise measured by satellite altimeters since 1993 has been around 3.2 millimetres per year, and from tide gauges around 3.0 millimetres per year.

Sea levels are rising around Australia. A sea level rise of a metre or more during this century is plausible.48 It could be less49 or much more.50 Between 160 000 and 250 000 individual homes are potentially at risk of inundation from a 1.1 metre rise in sea level (Figure 11.8).48

Figure 11.8

ACT = Australian Capital Territory; NSW = New South Wales; Qld = Queensland; NT = Northern Territory; SA = South Australia; Tas = Tasmania; Vic = Victoria; WA = Western Australia

Source: Australian Government Department of Climate Change and Energy Efficiency1

Figure 11.8 Estimated number of existing residential buildings at risk of inundation from a 1.1 metre sea level rise (including 1-in-100 storm tide for New South Wales, Victoria and Tasmania, and high-tide events for other states and territories)

Rising sea levels will result in inundation of low-lying coastal regions, and increased access of higher water levels and greater wave action to the shore. This will increase the rate of coastal erosion, particularly during extreme weather events, which are increasing in frequency.45 Expansion of mangroves into newly flooded coastal lands is also likely, where the topography is suitable. The capacity for coastal habitats (such as beaches, dunes and wetlands) to migrate inland to higher ground is limited in many parts of Australia by both the natural limits to the coastal plains (where steep hills approach the coast) and the human structures (such as seawalls, beach groynes and offshore reefs) built to defend against extreme weather events. The progressive effect is a reduction in natural habitats at the shoreline (particularly low-slope, soft-sediment habitats) and an increase in human structures that provide habitat of a different type (mainly hard structures, such as rock walls). The ecological impacts of these changes are currently gradual, but could become sudden and unexpected. They are likely to be profound.51

Direct impacts on certain types of cultural sites are possible. Aboriginal middens, for example, are invariably located close to foreshores and are particularly prone to erosion. Historical sites associated with defence, coastal trade or transport—such as gun emplacements, docks, wharves, shipyards, fishing ports and whaling stations—are also at direct risk from rising sea levels.

Potential impacts of sea level rise around Australia are summarised in Box 11.3.

Box 11.3 Potential impacts of sea level rise around Australia

Global sea levels increased by 1.7 millimetres per year during the 20th century. Over the past 15 years, this trend has increased to approximately 3.2 millimetres per year. The rate varies significantly around Australia.

New South Wales

A sea level rise of 1.1 metres will put at risk between 43 900 and 65 300 residential buildings, with a current value of between $14 billion and $20 billion, and up to 4800 kilometres of roads, 320 kilometres of railways and 1200 commercial buildings, with an estimated value of up to $10.4 billion, $1.3 billion and $9 billion, respectively.

Local government areas of Lake Macquarie, Wyong, Gosford, Wollongong, Shoalhaven and Rockdale contain more than 50% of the residential buildings at risk in New South Wales.

Since the early 1990s, New South Wales has experienced a sea level rise of approximately 2.1 millimetres per year.

Northern Territory

A sea level rise of 1.1 metres will put at risk between 260 and 370 residential buildings, with a current value of between $100 million and $134 million, and up to 2045 kilometres of roads, 32 kilometres of railways and 24 commercial buildings, with an estimated value of up to $1.8 billion, $100 million and $500 million, respectively.

Darwin is particularly vulnerable to riverine flooding and more intense cyclonic activity. Impacts on infrastructure are expected to be extreme under a ‘business as usual’ climate scenario, including major threats to vital port infrastructure on the Northern Territory coast.

Since the early 1990s, northern Australia has experienced sea level rises of up to 7.1 millimetres per year.

Queensland

Queensland’s highly developed and populated coastal communities are likely to be significantly affected by the impacts of climate change. A sea level rise of 1.1 metres will put at risk between 48 300 and 67 700 residential buildings, with a current value of between $15.4 billion and $20 billion, and up to 4700 kilometres of roads, 570 kilometres of railways and 1440 commercial buildings, with an estimated value of up to $12.9 billion, $2.3 billion and $15 billion, respectively.

Since the early 1990s, northern Australia has experienced sea level rises of around 7.1 millimetres per year, while eastern Australia has experienced increases of around 2.0–3.3 millimetres per year.

South Australia

A sea level rise of 1.1 metres will put at risk between 25 200 and 43 000 residential buildings, with a current value of between $4.4 billion and $7.4 billion.

Nearly half (47%) of the South Australian coastline is sandy beaches, and more than half of these sandy beaches are backed by soft-sediment plains. Rising sea levels will make these coastlines significantly more vulnerable to shoreline recession and foredune destabilisation.

Since the early 1990s, southern Australia has experienced sea level rises of 2–7 millimetres per year.

Tasmania

A sea level rise of 1.1 metres will put at risk between 12 000 and 15 000 residential buildings, with a current value of $4 billion, and up to 2000 kilometres of roads, 160 kilometres of railways and 300 commercial buildings, with an estimated value of up to $4.5 billion, $700 million and $1 billion, respectively.

Since the early 1990s, Tasmania has experienced sea level rises of 2.6–3.4 millimetres per year.

Victoria

A sea level rise of 1.1 metres will put at risk between 31 000 and 48 000 residential buildings, with a current value of between $8 billion and $11 billion, and up to 3500 kilometres of roads, 125 kilometres of railways and 2000 commercial buildings, with an estimated value of up to $9.8 billion, $500 million and $12 billion, respectively.

Since the early 1990s, south-eastern Australia has experienced sea level rises of 1.3–2.8 millimetres per year.

Western Australia

Western Australia has the longest coastline of any Australian state or territory. A sea level rise of 1.1 metres will put at risk between 20 000 and 30 000 residential buildings, with a current value of between $5 billion and $8 billion, and up to 9000 kilometres of roads, 114 kilometres of railways and 2100 commercial buildings, with an estimated value of up to $11.3 billion, $500 million and $17 billion, respectively.

Since the early 1990s, the southern coast of Western Australia has experienced sea level rises of up to 4.6 millimetres per year, while the western coast has experienced increases of up to 7.4 millimetres per year.

Source: Australian Government Department of Climate Change and Energy Efficiency52