Sea level

Global sea level has been rising since the beginning of the 20th century, and the rate of sea level rise is accelerating. This will have significant impacts on our coastal regions (see the Coasts chapter).

Since near-global satellite altimetry records began in 1993, global mean sea level has been rising at a rate of 3.3 millimetres per year (mm/yr), amounting to a total increase of about 9 centimetres (cm) from 1993 to 2020. Tide gauge records, whose global coverage is more limited, indicate a total increase since 1900 of approximately 20 cm.

Rates of sea level rise are relatively consistent on timescales of several years or longer, although there can be short-term periods (around 6–12 months) when global mean sea level is stable or slightly declining. This occurred in 2010−11, when extensive, slow-moving floodwaters on the Australian continent contained water that would otherwise have been in the ocean.

Sea level rise in Australia

Rates of sea level rise since 1993 have been above the global average over much of the western Pacific, including most of the Australian coastline, and below the global average in the eastern Pacific.

In some regions, particularly in northern Australia, the rate of post-1993 sea level rise has been up to 5 mm/yr. A major driver for these regional differences has been natural modes of climate variability, such as the El Niño–Southern Oscillation.

During El Niño events, sea level tends to be lower in the western Pacific and higher in the eastern Pacific, due to differential changes in ocean temperatures (and hence thermal expansion of sea water), and shifts in ocean currents and tropical winds. The reverse is true during La Niña events. Because El Niño events predominated early in the 1993–2020 period, with La Niña dominant in the late 2000s and early 2010s, this has amplified global sea level trends in the western Pacific. There is no clear indication that this amplification will continue in the future.

Tide gauge records at a few Australian locations extend back to the late 19th or early 20th centuries. Data from any individual location have a high level of uncertainty because they can be influenced by factors unrelated to global mean sea level, such as local subsidence, changes in port configuration (such as dredging) or gauge relocations. The first consistent national set of tide gauge sea level data is currently under development and, once completed, will allow assessment of regional sea level changes on the Australian coastline back to the 1960s.

Impacts of sea level rise

Sea level rise is accompanied by increased levels of coastal inundation and erosion. Coastal regions have many sensitive environmental features. There is also substantial infrastructure and other development in coastal regions, which is potentially at risk from further sea level rise (see the Coasts chapter).

Torres Strait Islander and coastal Indigenous communities are feeling the impacts of sea level rise. Along with posing threats to beaches and other coastal structures, sea level rise is a major threat to mangroves and coastal ecosystems (TSRA 2021). Valuable research is being undertaken by the Torres Strait Regional Authority into the impacts of sea level rise and climate change on Torres Strait Islander communities (see case study: Sea level rise and the Torres Strait islands) (see the Indigenous chapter) (TSRA 2021).

Case Study Sea level rise and the Torres Strait islands

Parts of the Torres Strait islands are highly vulnerable to sea level rise. A number of the islands are very low lying, and coastal inundation and erosion are significant issues even in the current climate. The most acute issues cover 2 groups of islands: the 2 alluvial islands in the top western region (Boigu and Saibai) and a number of small coral cays in the central part of the Torres Strait islands.

In the top western islands, there have been several inundation events since 2005. In 2011, during a strong La Niña event, some inhabited areas of Saibai were inundated to a depth of up to 0.5 metres (Systems Engineering Australia 2011). Some of the central coral cays have experienced significant coastal erosion. In addition to direct impacts on inhabited areas of the islands, these events threaten impacts on graves and other significant cultural sites, as well as saltwater intrusion into landfills, wastewater treatment sites and groundwater. Although tropical cyclones are relatively rare in the islands compared with areas further south in northern Queensland, storm surge associated with tropical cyclones is another potential risk.

Observed rates of sea level rise in the region over the period 1993–2010 were about 6 millimetres per year, somewhat greater than the global average of 3–3.5 millimetres per year and consistent with a broader pattern of increased sea level rise in the western tropical Pacific over this period. It is as yet unclear what contribution, if any, variability in the behaviour of the El Niño–Southern Oscillation (particularly the predominance of La Niña in the late 2000s and early 2010s) has made to this locally increased sea level rise, and whether it is likely to be sustained. Planning in the islands is widely based on projected sea level rise of 0.8 metres by 2100 (Green et al. 2010, Suppiah et al. 2011, TSRA 2014, Rainbird 2016).

Significant works to adapt to sea level rise are already taking place. New seawalls have been completed on Saibai and Boigu islands, replacing earlier community-built seawalls that had previously failed, as well as on Poruma Island in the central coral cay region. Further seawalls are in the planning stage. It is expected that, with such infrastructure works, existing communities will remain viable for at least several decades. Relocation of communities to other islands is regarded as a highly culturally disruptive option and would be considered only as a last resort.

Assessment Sea level rise in the Australian region