Frequently asked questions

  1. Why were these maps developed?
  2. Which regions do the maps cover?
  3. Why were these regions selected?
  4. What do the maps show?
  5. How accurate are these maps?
  6. Why don't the maps consider existing coastal protection works?
  7. Why are there maps for 3 sea level rise scenarios?
  8. Why did the scenarios go beyond the IPCC values?
  9. Who does this affect?
  10. What does this information mean for property exposed to higher sea levels?
  11. Why are the scenarios in the maps different to the planning benchmarks adopted in my area?
  12. Do the maps show what the coast will be like in 2100?
  13. What will be the impact of climate change on events such as 1-in-100 year events?
  14. How can we adapt to the impacts of climate change in time?
  15. What is the Australian Government doing about the risks of climate change?
  16. How were these maps developed?
  17. What is HAT?
  18. Why is it used?
  19. What HAT value was used for my region?
  20. Why is the HAT value so low for my region?
  21. What is AHD?

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1. Why were these maps developed?

The maps have been developed to give governments, business and communities access to good information, to help them understand what is potentially at risk.

We need to prepare Australia for the impacts of climate change now; for the impacts that are already 'locked in' due to the carbon pollution already released into the atmosphere, and for the impacts of future emissions.

Publicly available information can help to communicate risk and engage coastal communities and the private sector, so the range of views and social preferences can be considered as we develop new ways to prepare our coasts for a changing climate.

Every day, decisions are being made about where to build new houses and other infrastructure on the coast. We need to plan this new development better to cope with sea level rise and reduce future our exposure. We also have to address the challenge of managing impacts to our existing development – for example where past decision-making did not address climate change risks, but where areas may be vulnerable to flooding as sea levels rise. Government roles in planning and setting benchmarks will therefore be central to risk management and in managing the large number of public good assets in the coastal zone.

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2. Which regions do the maps cover?

The regions covered by the maps are:

  • Sydney, NSW
  • Hunter and Central Coast, NSW
  • Adelaide, SA
  • Melbourne, VIC
  • South East Queensland (including Brisbane and the Gold Coast)
  • Perth to south of Mandurah, WA.

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3. Why were these regions selected?

Australia is a coastal society. Around 85% of the population live in the coastal region and it is of immense economic, social and environmental importance to the nation.

The regions selected represent key urban areas, many of them our state capital cities. Understanding the risks to infrastructure and private property is particularly important for highly populated urban areas such as these regions.

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4. What do the sea level rise maps show?

The maps highlight the low-lying areas that are potentially vulnerable to flooding from an event that could be expected to occur at least once a year by 2100.

Maps have been developed for three sea level rise scenarios: a low (0.5m), medium (0.8m) and high (1.1m) scenario, relevant for the 2100 period. The maps have been developed to provide additional information for coastal communities to help them understand what is potentially at risk.

The flooding portrayed in the maps is based on a simple 'bucket fill' model that assumes a calm ocean surface. The maps should be considered as approximate only and do not replace the need for detailed local studies.

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5. How accurate are these maps?

The maps are based on high-resolution elevation data; however they are not a substitute for more detailed studies, as they do not:

  • consider the vulnerability of the coast to erosion
  • include any change to tidal flows in coastal waterways that may result from rising sea level and different coastal configurations
  • take account of existing sea walls and other protective structures, including some flood mitigation structures, as nationally consistent data is not available and some structures may not be feasible or sustainable in the longer term.
  • provide guidance about flood risk from an extreme event, such as storm surge and the influence of wind and waves etc, or
  • take account of the effects of coincident catchment flooding from extreme rainfall events.

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6. Why don't the maps consider existing coastal protection works?

The maps do not take existing sea walls and other protective structures into account as there is not a comprehensive national database containing this information. Protection works (such as sea-walls) may reduce the immediate risks from climate events but may not be feasible or sustainable in the long term. In the future, risks will increase with climate change and decisions will need to be made about if, when and how to upgrade or maintain protective structures.

In addition, there are a range of other factors that make the inclusion of existing coastal protection works difficult:

  • past experience with building protective structures (such as groynes) has shown that the problem is often displaced upstream or downstream;
  • the cost of protective structures such as sea walls can increase significantly with sea level rise.
  • a number of existing protective structures along the shoreline don’t provide protection to residential buildings along estuaries and lagoons; and
  • detailed local modelling is required to accurately predict the impact due to other channels for inundation, such as from stormwater or sewer drainage systems.

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7. Why are there maps for 3 sea level rise scenarios?

The maps have been developed to provide additional information for coastal communities to help them understand what is potentially at risk. The simple scenarios for sea level rise (relative to 1990) have been developed to cover the range of plausible sea level rise projections and to help understand the potential impacts and risk to society.

The low (0.5m sea level rise), medium (0.8m sea level rise) and high (1.1m sea level rise) scenario shown in the maps are relevant for the 2100 period.

The sea level rise values were based on Intergovernmental Panel on Climate Change (IPCC) projections and more recent science and observations.

  • The low scenario (B1): considers sea-level rise in the context of a global agreement which brings about dramatic reductions in global emissions and represents the upper end of the range for sea-level rise by 2100 which is likely to be unavoidable.
  • The medium scenario (A1FI): represents the upper end of IPCC’s 4th Assessment Report (AR4) A1FI projections and is in line with recent global emissions and observations of sea-level rise.
  • The high-end scenario: considers the possible high end risk identified in the AR4 and more specifically in post IPCC AR4 research. This scenario factors in recent publications that explore the impacts of recent warming trends on ice sheet dynamics beyond those already included in the IPCC projections.

Although uncertainties are still present in some important aspects of climate science, the majority of these uncertainties operate in one direction – towards more rapid and severe climate change. There is an increasing recognition that a sea level rise of up to a metre or more this century is plausible, and possibly several metres within the next few centuries.

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8. Why did the scenarios go beyond the IPCC values?

There is a considerable body of evidence that points toward a sea level rise of 0.5 to 1.0 m by 2100 compared to 1990 values.

The main lines of argument include:

  1. recent observations have confirmed the conclusion that sea level has been rising near the upper bound of the IPCC projections since 1990 (Rahmstorf et al. 2007; Domingues et al. 2008; Church et al. 2008);
  2. the mid range of the statistical projection of Rahmstorf (2007), which used observational data to compare the statistical correlation of sea level rise and temperature, is 0.9 to 1.0 m;
  3. recent observations show increasing net mass loss from the Greenland ice sheet (Rignot and Kanagaratnam 2006) and the West Antarctic Ice Sheet (Cazenave 2006);
  4. physically based estimates of sea level rise due to dynamical loss of ice from the polar ice sheets suggest that a 0.8 m rise is plausible (Pfeffer et al. 2008).

Sea-level rise larger than the 0.5–1.0 m range – perhaps towards 1.5 m (i.e. at the upper range of the statistical projection of Rahmstorf 2007) – cannot be ruled out. There is still considerable uncertainty surrounding estimates of future sea level rise. Nearly all of these uncertainties, however, operate in one direction, towards higher rather than lower estimates.

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9. Who does this affect?

About 85 per cent of the population lives in the coastal region and it is of immense economic, social and environmental importance to the nation.

The maps have been developed to provide additional information for coastal communities to help them understand what is potentially at risk.

The risks from sea level rise are not confined to the coast itself. Some inland areas may experience inundation impacts (for example, buildings along estuaries, rivers, lakes or lagoons). Coastal infrastructure (for example, roads or rail) may also be impacted by sea level rise, leading to more widespread disruption eg to transport networks.

Coastal assets in some areas already face risks from the current climate. These risks are likely to increase and spread as sea levels rise.

Within 200 metres of the coastline are many facilities which support the delivery of community and emergency services. Nationally, they include 258 police, fire and ambulance stations, five power stations/sub stations, 75 hospitals and health services, 41 landfill sites, three water treatment plants, and 11 emergency services facilities.

The delivery of essential services such as electricity generation and wastewater management will increasingly be impacted by inundation, erosion, increased corrosion and the effects of sea water intrusion into coastal freshwater systems and drainage systems.

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10. What does this information mean for property exposed to higher sea levels?

The maps show the effect of both sea level rise and a very high tide, for the period around 2100. Houses in areas identified in blue are at a lower elevation than those in surrounding areas, and are more exposed to potential inundation, based on the sea level rise and high tide scenario considered. However, the maps do not show how deep the water may be, some areas may be only slightly wet, while other areas may be exposed to greater depths of inundation.

You don't have to live on the coast to be at risk from sea level rise. Some inland areas may see flooding impacts (for example, buildings along estuaries, rivers, lakes or lagoons). Coastal infrastructure (for example, roads or rail) may also be impacted by sea level rise, leading to more widespread transportation disruption.

Additionally, the maps do not take into account existing sea walls and other protective structures, which may reduce the impacts of sea level rise in the shorter term, but may or may not be suitable in the long term.

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11. Why are the scenarios in the maps different to the planning benchmarks adopted in my area?

The sea level rise scenarios broadly agree with the planning benchmarks used by state governments around Australia. However, the scenarios used for these maps do not take regional variations into account, for example in NSW the state government has adopted a 0.9m benchmark for 2100 to account for regional variance in sea levels and based on work undertaken by CSIRO. The maps and have been designed to help communicate risks of sea level rise and are an additional set of information to consider.

For a detailed comparison of planning benchmarks see the table below:

State/Territory20502100Policy/planning document 
NSW0.4m0.9mNSW Policy Statement on SLR and Coastal Planning Guidelines: Adapting to SLRValue set at 0.9m based on CSIRO studies that indicate sea level rise will be slightly higher along the NSW coast.
NTNilNilNT Climate Change Policy 2009 
QLD0.3m(draft)0.8m(draft)Draft State Coastal Management Plan (2009) 
SA0.3m1mCoast Protection Board Policy Document 2002  
TASNilNilState Coast Policy 1996  
VIC 0.8mVictorian Coastal Strategy (2008) 
WA 0.9m**(draft)State Coastal Planning Policy 2003; review underway – interim position statement released**for the period 2110, not 2100

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12. Do the maps show what the coast will be like in 2100?

The maps do not show what the actual sea level for 2100 might be. The maps illustrate a range of scenarios for an event that might be expected to occur at least once a year by 2100. They highlight the low-lying areas that may be vulnerable flooding.

The actual impacts of sea level rise will depend on many things:

  • erosion that will occur between now and the turn of the century,
  • changes in the shape and characteristics of lakes, rivers, and estuaries caused by rising sea levels, and
  • coastal planning decisions to protect, accommodate or retreat in response to sea level rise (including building new coastal protective structures).

The maps do not take into account local variations or existing coastal protective structures (which may or may not be feasible in the long term).

Scientists agree that sea levels will rise as a result of climate change, but uncertainty remains about the extent of the rise in sea level by the turn of the century. Recent findings, however, suggest the climate is changing faster than projected. The scenarios shown in the maps are based on projections developed by the Intergovernmental Panel on Climate Change (IPCC) and more recent science and observations.

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13. What will be the impact of climate change on events such as 1-in-100 year events?

The frequency of what we currently consider a high sea level event (with 1-in-100 being more at the extreme end) will increase. Importantly, this increasing frequency is non linear, that is, with 10 cm rise in sea levels, an event that may now occur once a year will occur at least twice a year and in some places four or six times a year. But with a 50cm rise in sea levels the frequency of the event will increase in places at least 100 times a year and in some places 1,000 or 10,000 times a year.

For more information, see Chapter 2 of Climate Change Risks to Australia's Coast.

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14. How can we adapt to the impacts of climate change?

The Australian Government recognises the need for national leadership and cooperation between all levels of government to develop an integrated, cohesive and effective national approach for the management of Australia’s coastal zone. To this end, the Australian Government will work with states, territories and, through the Australian Local Government Association, with local government to develop a national coastal adaptation agenda.

Managing this risk will be a significant planning issue for governments, but it will also require action from business and the community. Publicly available information can help to broadly communicate risk and engage coastal communities so that the range of views and social preferences can be considered as we develop new ways to prepare our coasts for a changing climate.

We have time to prepare our coasts, but we need to start now. Many government agencies recognise this will require new approaches. For example state governments have established sea level rise policies and local governments are increasingly adapting their decision making processes to take into account the changing climatic conditions.

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15. What is the Australian Government doing about the risks of climate change?

The Australian Government is committed to addressing climate change and reducing Australia's carbon pollution. However, even if global mitigation efforts are successful, the science shows some climate change impacts are now unavoidable and society will need to adapt to a certain level of climate change.

The Australian Government's position paper Adapting to Climate Change, sets out the Government's vision for adapting to the impacts of climate change and proposes practical steps to realise that vision. It outlines the Australian Government’s role in adaptation, which includes building community resilience and establishing the right conditions for people to adapt; taking climate change into account in the management of Commonwealth assets and programs; providing sound scientific information; and leading national reform. The position paper identifies six national priority areas for action: water, coasts, infrastructure, natural ecosystems, natural disaster management, and agriculture.

The Australian Government recognises the need for national leadership and cooperation between all levels of government to develop an integrated, cohesive and effective national approach for the management of Australia’s coastal zone. To this end, the Australian Government will work with states, territories and, through the Australian Local Government Association, with local government to develop a national coastal adaptation agenda.

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16. How were these maps developed?

The maps have been developed using Digital Elevation Models (DEMs) that are based on high resolution elevation data with a vertical accuracy of +/- 10-15cm.

DEMs map the ground surface topography, or elevation. The DEMs have been further processed to provide a better appreciation of how the water would flow over the land including streams and channels.

Sea level rise values were combined with a nominal highest astronomical tide (HAT) value to illustrate an event that can be expected to occur at least once a year, but possibly more frequently, by 2100. The HAT values used for each region were based on tidal planes for the nearest port(s) provided by the National Tidal Centre in the Bureau of Meteorology.

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17. What is HAT?

Contributions to coastal sea level from tides, storm surge and wave processes. Mean sea level approximates to Australian Height Datum (AHD). Source: ACE CRC
Contributions to coastal sea level from tides, storm surge and wave processes.
Mean sea level approximates to Australian Height Datum (AHD).
Source: ACE CRC

The highest astronomical tide (HAT) is the highest level of water which can be predicted to occur under any combination of astronomical conditions.

Tide predictions do not include any allowance for weather conditions, such as the influence of wind and waves. So, while a HAT technically only occurs once every 18 years, in practice water levels can often exceed HAT values because of the influence of wind and waves. Tide records indicate that sea levels often exceed HAT, particularly along the southern Australian coastline.

The HAT values used for each region were based on tidal planes for the nearest port(s) provided by the National Tidal Centre in the Bureau of Meteorology.

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18. Why was HAT used?

Sea level rise values were combined with a nominal highest astronomical tide value (HAT) to illustrate an event that could be expected to occur at least once a year, but possibly more frequently, around 2100. The maps therefore show low-lying areas that will experience regular flooding around the turn of the century.

Current flood mapping generally illustrates risks for a 1-in-100 year event; or a 'worst case' infrequent event. Managing risks from an event that will occur regularly will require different responses than managing for an event that occurs very infrequently.

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19. What highest astronomical tide (HAT) value was used for my region?

The highest astronomical tide (HAT) value used for your region can be calculated by reading the scenario text on the right hand side of the map. Tide values were converted to an Australian Height Datum (AHD) reference (see Why is the HAT value so low for my region?).

Sea level rise values were combined with a nominal HAT to provide an indication of an inundation event that could be expected to occur at least once a year, but possibly more frequently, around 2100. Usually, tide predictions do not include any allowance for weather conditions, such as the influence of wind and waves. So, while a HAT technically only occurs once every 18 years, in practice water levels often exceed HAT values because of the influence of wind and waves.

Flood risk maps commissioned by local governments commonly identify areas vulnerable to flooding from a 1-in-100 year event – which is a less frequent and more extreme event, i.e. an event that could be expected to occur once in a hundred year period.

The HAT values used in the maps were based on a nominal value for the region, although at a finer local scale there may be differences in HAT values up and down the coast and around estuaries and lakes.

The maps provide an indication of low-lying areas that may be vulnerable to inundation from rising sea levels. It is likely that the shape and position of the coastline due to erosion will be different in 2100, structures may be built to protect buildings and infrastructure, and these may all change the actual pattern of inundation that occurs at the turn of the century. The maps have been produced to provide information and to support engagement of coastal decision makers and communities on the implications of climate change and the need for early planning. The maps should not be relied on for decision making.

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20. Why is the HAT value so low for my region?

Tides are generally measured relative to a low water reference or hydrographic chart datum, traditionally ISLW (Indian Springs Low Water) but more recently LAT (lowest astronomical tide). This is not the same as AHD (Australian Height Datum; AHD71). Elevation data is referenced to the AHD standard and therefore tide values need to be converted from the low water reference datum to AHD.

To do this, the mean sea level (MSL - which is roughly equivalent to AHD) was subtracted from the highest astronomical tide (HAT) value to provide a new HAT value relevant to AHD.

For example in the figure below the highest recorded tide in Sydney was 2.4m (relative to hydrographic chart datum) or 1.4m (relative to AHD).

IPCC Sea Level Scenarios - Sydney Tides, Credit: NSW Department of Environment and Climate Change, 2009

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21. What is AHD?

Australian Height Datum (AHD71) is roughly equivalent to mean sea level, and is a measure of the average height of the ocean's surface. All heights in the sea level rise maps are relative to AHD71.

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If you have questions about coastal climate change please contact the Department of Climate Change and Energy Efficiency: enquiries@climatechange.gov.au

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