Impacts of climate change on the coast are both insidious and extreme. While sea level slowly creeps up and threatens to inundate coastal land, extreme weather events periodically impact the coast with increasing frequency and intensity. Coastal resilience adaptation plans that respond to the impacts of climate change, including storms, cyclones, sea level rise, heatwaves, bushfires and water security, are critical to reducing climate risks to coastal communities.
Indigenous peoples have acquired knowledge about the environment and its changes for over 60,000 years; this includes adaptation responses (Bird et al. 2013, Lyons et al. 2019). The survival of Indigenous people over this time means that cultural and traditional knowledge has been passed down during some of the severest climate years in human history (Charles & O'Brien 2020). Climate change, and in particular sea level rise, is of significant concern for many communities around Australia, especially those on the low-lying islands of the Torres Strait (O'Neill et al. 2012).
Most climate pressures are considered high impact, and, for pressures where trends through time are discernible, are deteriorating. Sea level rise currently has low impact, but is projected to have very high impact in the future. The Indigenous assessments for the state of climate change pressures found 1 pressure has a high impact and 2 have a very high impact, and the trend is deteriorating for 2 pressures and stable for 1. Local government assessments (see Approach) showed that extreme weather events are a concern Australia-wide, while concern over other climate-related impacts varies between local government areas. Related to United Nations Sustainable Development Goal targets 13.2, 14.3, 17.2
Extreme weather events apply significant pressure to the Australian region. Changes in trends on 5-year timescales (between assessment reports) may be due to low-frequency climate mode variability that can act to misrepresent the signatures of long-term climate change. The Indigenous assessment for some regional areas was low, with a stable trend.
There has already been a significant increase in the frequency of extreme high sea levels and this will increase as sea levels rise. The Indigenous assessment for some local areas was low, with a deteriorating trend.
Inundation associated with extreme sea levels is slowly increasing in line with sea level rise. Beach erosion can now be monitored nationwide with satellite imagery. Most beaches are dynamic but stable, apart from legacy erosion ‘hotspots’. The Indigenous assessment for some local areas was high, with a deteriorating trend.
Extreme weather events
At a national scale, extreme events are typically increasing in frequency and duration around Australia. However, information on historical trends needs to be at regional scales to help decision-makers develop adaptation plans. Armed with this information, Indigenous peoples, policy-makers, managers, and other marine and coastal stakeholders can aim to work together, including across organisational levels and potential barriers, to build resilience to extreme weather events. In the next 5 years, more information is expected to be available to support decision-making, through improvements in our knowledge of weather predictability, and better forecasting of extreme events in the coastal zone and ocean.
Extreme weather events cause pressures to the marine and coastal environments through:
- severe storm-induced winds (including tropical and extratropical cyclones), which can cause wind damage and storm surges
- heatwaves or shorter-term heat extremes, which can stress human infrastructure and species at their thermal limits (see the Marine chapter). However, terrestrial heatwaves can also impact intertidal species
- extreme rainfall events, which can cause flooding and erosion
- extremely dry conditions and drought, which can stress natural coastal systems such as mangroves.
Apart from cyclones, which predominantly occur in the tropics, all listed pressures can affect all coastal regions of Australia.
Extreme wind and cyclones can generate high to extreme waves, sea level and storm-surge conditions (McInnes et al. 2016), which can cause coastal erosion and damage to coastal structures within and around coastlines and embayments.
Extreme rainfall events can generate local coastal flooding, either directly or via run‑off from slopes or river transports. These floodwaters may carry pollution from agriculture such as soils, pesticides and fertilisers, and can cause turbid coastal plumes. This impacts marine and coastal ecosystems such as coral reefs, seagrass and kelp environments, and the species these habitats support (Fraser et al. 2014).
Extreme dry periods can raise coastal salinity levels through evaporation, affecting marine and coastal habitats and species (Babcock et al. 2019). Sea level extremes in shallow-water and coastal marine environments can also be caused by swings in modes of climate variability, with El Niño–Southern Oscillation (ENSO) playing an important role (Holbrook et al. 2020). Mangrove dieback around the Gulf of Carpentaria in 2015–16 provides recent evidence for this (Duke 2017, Harris et al. 2017).
The recent Intergovernmental Panel on Climate Change (IPCC) Special report on the ocean and cryosphere in a changing climate (Collins et al. 2019) says with high confidence that anthropogenic (human-caused) climate change has increased winds (globally), extreme sea levels, and precipitation events associated with tropical and extratropical cyclones. Further, it points out with medium confidence that extreme wave heights have increased globally by 5% over the past 3 decades, and the strongest El Niño and La Niña events since pre-industrial times have occurred in the past 50 years.
However, despite these global trends, the frequency of tropical cyclones appears to be decreasing in Australia. A recent review and assessment of Australian tropical cyclone season data found statistically significant decreasing trends in the frequency of tropical cyclones (around −0.115/year) and frequency of severe tropical cyclones (around −0.112/year) from 1981–82 to 2017–18, with at least 95% confidence(Chand et al. 2019). Frequency of landfalling tropical cyclones also shows a decreasing trend through time (Chand et al. 2019).
There are some key uncertainties and knowledge gaps associated with providing an assessment of the current state and recent trends of extreme weather events:
- Tropical cyclone trends in the Australian region have only been assessed since 1981–82 (Chand et al. 2019), which is no longer than one cycle of the Interdecadal Pacific Oscillation (IPO) (Henley et al. 2015). During this time, the IPO has shifted from an IPO-positive phase (El Niño-like) to an IPO-negative phase (La Niña-like). This runs contrary to the observed tropical cyclone trends, which might be otherwise expected to increase over this period.
- There are significant uncertainties in extreme weather event trends at regional scales because of limitations in observational data quality and quantity, as well as model constraints (limits to configurations and resolution), in the coastal zone.
- The diversity of the ENSO system makes it difficult to manage the risk of future impacts from the system (Santoso et al. 2019).
- There is significant uncertainty around the role of compound events (Zscheischler et al. 2018).
Sea level rise
Sea levels are rising as a result of climate change, and the rise is projected to accelerate over the coming century and continue for centuries into the future.
For the 2 millennia before the industrial revolution, sea level was almost constant, with oscillations no greater than 15–20 centimetres (cm) (Lambeck et al. 2014). This changed with the industrial revolution in the 19th century, when sea level rise began to accelerate (Church & White 2006, Church & White 2011). The largest acceleration has occurred since 1970 (Dangendorf et al. 2019), and 70% of the rise since 1970 is a result of a warming climate from anthropogenic climate change (Slangen et al. 2016). Most of the 20th century global mean sea level rise was caused by ocean thermal expansion (i.e. as warmer waters expand due to increasing ocean temperatures, the ocean water takes up more volume) and the addition of mass to the oceans from glaciers (Church et al. 2011, Gregory et al. 2013), with a recent growth in addition of mass from the ice sheets of Greenland and Antarctica (Shepherd et al. 2012, Shepherd et al. 2018).
Sea level rise will have profound impacts on the coastal environment and communities.
Global and Australian sea level rise
From 1900 to 2018, global sea level rose by approximately 20 cm (Church et al. 2013, Fox-Kemper et al. 2021, Gulev et al. 2021), and the rate of rise is increasing. Measurements since 1993, when global satellite altimeter data became available (Watson et al. 2015, Nerem et al. 2018), show that the rate of global mean sea level rise since 1993 is 3.1 millimetres per year (mm/yr), and over 4 mm/yr over the past decade (Cazenave et al. 2018, Wang et al. 2021).
Sea level does not change uniformly around the globe, or even around Australia, due to differences in ocean density and circulation, changes in Earth’s gravitational field from the redistribution of water on Earth, and vertical land motion. The latest comprehensive assessment of sea level around Australia (White et al. 2014) reported average sea level rise rates of 0.7 mm/yr at Fort Denison, Sydney (based on tide gauge records from 1886 to 2010), and 1.6 mm/yr at Fremantle, Western Australia (based on tide gauge records from 1897 to 2010), with higher rates over recent decades. The average rate around Australia, after removing the variations correlated with the El Niño–Southern Oscillation, increased from 1.6 mm/yr (1976–2010) to 2.7 mm/yr (1993–2010). This was approximately equal to the corresponding global average rate of rise after correcting for estimates of vertical land motion from glacial isostatic adjustment (the ongoing movement of the land following the loss of ice on land since the last glacial maximum) and the impact of changes in regional atmospheric pressure.
Satellite altimeter data and tide gauge observations show slightly different trends around Australia (Figure 24). Satellite altimeter data (1993 to July 2020) indicate that to the north of Australia and off the south-eastern coast, the rate of sea level rise was higher than the global average. Tide gauges (reported by the Australian Baseline Sea Level Monitoring Project) indicate the rate of rise in northern Australia since the early 1990s is above the global average at approximately 4–6 mm/yr (similar to the altimeter record and higher than the global average), but along the south-eastern coast, the rate is similar to the global average at approximately 2–4 mm/yr (Figure 24).
At least part of the spatial variability reported by both sources is a result of climate variability (Zhang & Church 2012, White et al. 2014), while other differences can be explained by local factors. For example, the local rate of rise at Marmion (north of Perth, Western Australia) is higher than surrounding sites by several millimetres per year, as a result of sediment compaction after ground water was withdrawn (White et al. 2014).
Projections of sea level rise to 2100
Projections of sea level rise (relative to the land) for Australia to 2100 (Figure 25), based on the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) (Church et al. 2013), have been produced by McInnes et al. (2015) and Zhang et al. (2017), and are available for every coastal council in Australia on the CoastAdapt website.
The likely range (central 66% of probabilities) of the 2021 IPCC Sixth Assessment Report (Fox-Kemper et al. 2021) projections are similar to the AR5 but use a slightly larger Antarctic ice sheet projection. For low-emission shared socioeconomic pathway (SSP)1-1.9 (consistent with the Paris Agreement Target of 1.5°C), the projected global mean sea level rise by 2100 (relative to 1995–2014) is 0.38 metres (m) (likely range: 5–95% confidence interval (CI) 0.28–0.55 m) and for high-emission SSP5-8.5 the rise is essentially double this at 0.77 m (CI: 0.63–1.02 m). Including ice sheet instabilities, in which there is only low confidence, increases the projected SSP5-8.5 rise to 0.88 m (CI: 0.63 to 1.61 m) in 2100. A global mean sea level rise approaching 2 m by 2100 cannot be ruled out due to large uncertainty in ice sheet processes.
High-resolution projections (Figure 25d) indicate that, under the high-emissions scenario (RCP8.5), rises on the eastern Australian coast will be slightly larger (by up to around 0.1 m by 2100) than elsewhere around Australia. However, the highest rates of rise in the Tasman Sea are projected to be seaward of the continental shelf, consistent with the difference between terrestrial and satellite observations of recent sea level change (Figure 24). Rising sea levels have already increased the frequency of extreme sea levels (Church et al. 2006) and this will continue through the 21st century (McInnes et al. 2015).
It is important to note that under all scenarios, sea level will continue to rise for centuries because of the long timescales of ocean warming and ice sheet responses but the amount of rise is strongly dependent of future emissions. Projections of rise by 2300 in the IPCC AR6 under low-emission RCP2.6 are for a sea level rise of 0.5 to 3 m, and under high-emission RCP8.5 a rise of 2 to 7 m. They also noted rises higher than 15 m cannot be ruled out (IPCC 2021). These higher levels are consistent with paleoclimatic data indicating sea levels were metres higher than they are now in past periods of warmer climate (Dutton et al. 2015). However, there remains significant uncertainty about the rate of contributions from the Greenland and Antarctica ice sheets.
Erosion and inundation
One of the most significant impacts of sea level rise on Australia’s coast is erosion and movement of beaches, and permanent inundation of low-lying areas. To define the terms used in this section:
- Erosion is defined as beach retreat during extreme storm events followed by a period of sand recovery, with no net shoreline change.
- Recession involves net landward shoreline movement, usually associated with a deficiency in the nearshore sediment budget (when less sediment is being added than is removed).
- Inundation refers to marine inundation generated by extreme sea levels that accompany storms, and can include storm surge, wave set-up, wave run-up and spring tides. The impact of these forces may be exacerbated by the local coastal configuration. Inundation may also include the creeping effect of sea level rise around shores of estuaries, evidenced through increasing frequency of inundation, such as when king tides flood settlements and intertidal vegetation migrates landward. This effect is seen already in low-lying areas at Newcastle (New South Wales), and Gulf St Vincent and Spencer Gulf (South Australia).
Current levels of erosion, recession and inundation of Australian coasts are dynamic but stable. However, sea level rise is expected to significantly increase these threats for all regions of Australia, and especially for coastal wetlands. Infrastructure Australia (2020) has identified a coastal inundation strategy as a ‘priority initiative’ stating that ‘the initiative is for a proactive infrastructure strategy in advance of the inundation risks materialising’.
Shores around Australia are monitored to measure shoreline trends, erosion and hazard lines, beach fluctuations, and storm demand (the removal of sand from beaches during storms). These measures are used in coastal management programs to map hazard lines at various timeframes, which in turn are used to assess future risk to coastal development. Recent developments in satellite-based monitoring tools include CoastSat (Vos et al. 2019), Digital Earth Australia Coastlines (Geoscience Australia 2020b), and the citizen science tool CoastSnap (Harley et al. 2019).
Most sandy shorelines, including sites monitored since the 1970s, appear dynamic but stable, showing no clear impact of sea level rise. Beaches that are receding have been doing so for decades or centuries, because of a deficient sediment budget, while a few beaches are building seaward where there is abundant supply of sediment from the adjoining nearshore zones.
However, while most beaches remain resilient at present, sea level rise is slowly intruding on estuaries and low-gradient shores, and is expected to begin affecting beaches in the coming decades. By 2100, the IPCC project a likely range (17–83% exceedance) for global sea level rise of 0.29–0.59 m under a low-emissions scenario and 0.61–1.10 m under a high-emissions scenario, relative to 1986–2005. They also caution coastal managers with low risk tolerance that they may need to consider scenarios above this range. There is high confidence that sea levels will continue to rise for centuries beyond 2100, due to continuing deep ocean heat uptake and mass loss of glaciers and ice sheets, and remain elevated for thousands of years. This ongoing rise is likely to have a significant and unavoidable impact on coastal communities within Australia over the coming decades, presenting a serious challenge for coastal managers and planners.
The National Coastal Risk Assessment was conducted in 2011 to estimate damage to Australian coastal infrastructure that would result from a sea level rise of 1.1 m (a high-end scenario for 2100), considering impacts of both inundation and shoreline recession (DCCEE 2011). There is a strong need for a coherent response. A typical site is Kingscliff Beach; here, erosion is caused by sandwaves moving northwards along the beach and generating excessive erosion, followed by some recovery. This occurs every few years and is a recurrent and predictable event, and could be dealt with systematically, rather than through the uncoordinated approaches currently in use (Figure 26).
The combined value of damage to major asset classes across Australia is predicted to exceed $226 billion, not including damage to large public spaces such as beaches and foreshores (Table 3).
In New South Wales, a risk assessment of coastal recession for the entire coast to 2100 was undertaken by the government’s Office of Environment and Heritage (Kinsela et al. 2017). Risk assessments of erosion hazard and recession have also been undertaken in Queensland, South Australia, Tasmania and parts of Western Australia. These studies may also highlight areas that are more likely to be affected by storm-surge inundation during cyclonic events.
Marine inundation poses the greatest threat to coastal lowlands, particularly estuarine shorelines. The online viewer Coastal Risk Australia (Geoscience Australia 2020a) contains images of the entire Australian coast and maps of marine inundation levels in 2020 and 2100, the latter with a 0.74 m rise in sea level (Figure 27). A 2020 CSIRO–Bureau of Meteorology report states: ‘Sea levels are rising around Australia, including more frequent extremes, that are increasing the risk of inundation and damage to coastal infrastructure and communities’ (CSIRO & BOM 2020).
In New South Wales, all estuaries and their potential inundation levels have been mapped at current sea level and with scenarios of 0.5 m, 1 m and 1.5 m sea level rises. The mapping also looks at the number of properties exposed at each of these levels (Hanslow et al. 2018). Likewise, South Australia is mapping potential inundation associated with extreme storms and rising sea level (DEWSA 2021).