6. Factors influencing the Region’s values

The Region is particularly vulnerable to the pervasive influence of a rapidly changing climate⁵³⁶ that is amplifying exposure to both pre-existing (for example, cyclones) and emerging (for example, marine heatwaves and ocean acidification) disturbances (Figure 6.7). The primary concern is the vulnerability of the Region’s key habitats and habitat-forming species, such as corals, seagrasses, mangroves and wetlands, based on their roles in supporting ecosystem resilience and maintaining biodiversity.⁵³⁵ 

The vulnerability of ecosystems is determined by the level of exposure and sensitivity of species and habitats to disturbances and the ability of ecosystems to tolerate and recover from impacts. The Region’s key habitats have a natural resilience against acute physical disturbances such as cyclones, intense rainfall, freshwater flood plumes and heatwaves. However, climate change is exacerbating both acute and chronic disturbances in the Region, shrinking recovery windows and affecting ecological processes that underpin ecosystem resilience.^{541,632,644,1604}  

The magnitude and pace of global warming (Section 6.3.1) will be pivotal drivers of ecosystem vulnerability by determining exposure to climate impacts. The combined impacts of near-term warming, ocean acidification, sea-level rise, increased ultraviolet radiation,¹⁶⁰⁵ and increased frequency, severity, and duration of extreme events, will negatively affect other species and habitats across the Region.^109,1572 For some habitats, direct climate impacts are likely to be the dominant drivers of change over the coming century. For example, in Halimeda bioherms and meadows (Section 2.3.8) these drivers are likely to be rising sea temperatures and ocean acidification, as well as changing ocean circulation patterns. Deoxygenation in the water column due to rising sea temperatures could have implications for a range of species and habitats (Box 6.2). Increasing fire frequency is a threat to coastal and island ecosystems ⁹⁶⁸ and may affect water quality through altered catchment hydrology (such as erosion rates and vegetation dynamics affecting runoff patterns)^1606,1607 and the atmospheric transport of limiting nutrients.⁷¹³ The sensitivity of other habitats, such as coral reefs, seagrass meadows, and mangrove forests, to climate impacts will be affected by concurrent near-term trends in other pressures, such as those associated with coastal development (Section 6.4), land-based runoff (Section 6.5) and direct use (Section 6.6). Studies have highlighted the potential for reducing near-term climate impacts through the targeted management of these local stressors.^535,1608

The repercussions of ecological vulnerability to climate change are potentially far reaching due to the diversity of the Reef’s ecosystem services.¹⁶¹² For example, reductions in the structural complexity of coral reefs reduces their effectiveness as natural barriers against ocean waves.¹⁵³ This would not only reduce their effectiveness as buffer zones for coastal communities ¹⁶¹³ (Section 6.3.3) but also for nearshore and coastal ecosystems, such as mangroves and seagrass meadows, which also play important coastal buffering roles.¹⁶¹⁴ On regional to global scales, changes in the condition of coral reefs and other ecosystems may indirectly affect the climate. For example, climate change may affect the release of sulfur compounds, which play a role in cloud formation (Section 3.3.1), from coral reefs into the atmosphere and surrounding waters.⁷²²

Coral reef habitats and coral-dependent species are among the most vulnerable to sea temperature increases (Figure 6.7).¹⁶¹⁵ Proximity to their thermal limits brought about by ocean warming signifies a precarious condition for the Reef's corals, as evidenced by the four mass coral bleaching events since 2016 (Sections 2.3.5, 2.4.4 and 8.3.1). These marine heatwave events were not equal in their severity or extent, and while the back-to-back 2016 and 2017 events resulted in cascading effects on coral-associated fish and invertebrates,^1616,1617 rapid recovery of coral cover was recorded up to 2022, with little change reported in 2023.¹⁶³ The rapid increase in coral cover across the Region reflects a lower disturbance regime since 2017 compared with the preceding decade. Despite their widespread impacts, the 2020 and 2022 mass coral bleaching events did not cause as much coral mortality as the 2016 and 2017 events. While coral mortality from crown-of-thorns starfish has occurred across the Region, outbreaks have been suppressed effectively and coral has been protected on reefs that have received targeted, timely and sufficient cull effort.^163,187,1015  While the recovery trend of coral cover in the mid-shelf and offshore reefs over the past 5 years is encouraging, the recent pause in recovery of coral cover highlights that exposure to even moderate disturbances (for example, the 2022 mass coral bleaching event and crown-of-thorns starfish losses) can have a widespread effect.¹⁶³ Potential increases in the strength of El Niño–Southern Oscillation events may amplify the impacts of future marine heatwaves.¹⁶¹⁸

As the climate continues to change, the capacity of reef-building corals to survive, grow and reproduce will be increasingly compromised,^{1549,1619,1620} with resulting consequences for other species dependent on coral reefs (Section 2.3.5). Warming already locked into the climate system will lead to an increasing frequency of thermal stress events in the Region, and intensifying emissions could lead to severe bleaching conditions occurring annually by 2080.¹⁵⁹

The extent to which reef-building corals and other key species can adapt to rising sea temperatures is a major knowledge gap and a critical area of current research interest.^535,1621 For corals, their limited ¹⁶²² ability to adapt to changing conditions could be a critical factor influencing the future health and resilience of coral reefs (Figure 6.8).^109,1623 Improved understanding of the various genetic, physiological and ecological components of adaptation and acclimatisation to thermal extremes is vital to manage the response of coral reefs to climate change.^1621,1624 Some species might be able to shift their occurrence range southward as temperatures warm ¹⁶¹⁹ but their ability to move will depend on the availability of habitat outside the Region^1572,1620 and is fundamentally constrained by limited light availability at higher latitudes.¹⁶²⁵ Coral reef connectivity is projected to decline as the climate warms, primarily due to increased larval mortality and reduced competency duration in warmer waters.⁹³²

Locating potential climate refugia, including deeper banks and reefs where favourable hydrodynamic conditions confer a degree of protection from thermal extremes, is another area of active research interest.^158,180,664 One study identified 13 per cent of the Reef as potential refugia that avoid significant warming more than expected by chance.¹⁵⁸ These areas are potentially capable of delivering larvae to 58 per cent of the Reef.¹⁵⁸ Shallow reefs along the path of the North Queensland Current have been identified as potential refugia from bleaching because of upwelling of cooler waters.²⁴⁶ Deeper coral communities (Box 2.4) are also more likely to escape the effects of surface marine heatwaves.^181,246 Tidal and wind mixing of warm water away from the sea surface provides relief from warming for some local reef communities.⁶⁶⁷ These potential coral refugia are predicted to persist until global warming exceeds around 3 degrees Celsius.⁶⁶⁷

Studies show that extremes of warm temperature cause physiological stress to numerous other marine organisms including some species of fish, sponges and seagrasses.⁵³⁵ Sponges have been posited as potentially benefiting from reduced competition with corals under climate change ¹⁶²⁶ but this hinges on yet uncertain effects on ecosystem primary production.⁷⁹⁵ Additionally, some sponges can lose symbiotic microorganisms (bleaching)^1627,1628 and exhibit reduced reproductive output,¹⁶²⁹ decreased feeding behaviour ¹⁶³⁰ and mortality ^1627,1630 when exposed to prolonged thermal stress. Increasing water temperatures affect the energy balance of photosynthetic organisms, such as seagrasses and Halimeda.¹⁶³¹ Seagrass meadows are less vulnerable to temperature increases than coral reefs,^1632,1633 but a growing body of evidence links marine heatwaves to substantial losses of seagrass habitat,^{109,1632,1634,1635} which may reduce the effectiveness of these ecosystems for trapping and storing atmospheric carbon.^133,1636

Mobile organisms such as fish have more capacity than benthic species to escape temperature stress, but the limited number of studies on reef fish have shown them to be vulnerable to both direct and indirect effects of temperature increase.⁵³⁵ Most marine organisms are ectotherms so are unable to regulate their body temperature. Increases in temperature affect their performance and fitness.^1637,1638 Studies have shown that increased temperatures have been associated with reduced pelagic larval duration,¹⁶³⁹ pre-settlement growth rates,¹⁶⁴⁰ and growth of juvenile and adult reef fishes.¹⁶⁴¹ Varied thermal tolerance between species ¹⁶⁴² has led to shifts in community assemblages as temperatures continue to rise.^1643,1644 Temperature experiments involving coral trout, one of the larger commercial species, found declines in survivorship, metabolism and swimming activity with relatively moderate increases in temperature.^1645,1646 Increasing energetic limitations with body size may have ramifications for fisheries productivity and ecosystem functions.¹⁶⁴⁷ Direct physiological effects of ocean warming may exceed the indirect effects of habitat loss on coral reef fish productivity.¹⁶⁴⁸

Herbivore populations can benefit in the short term from increased algal resources following coral bleaching and associated mortality.¹⁶⁴⁹ However, the subsequent loss of structural complexity has indirect impacts on coral-associated fishes, such as damselfish, including reduced recruitment ²⁵⁴ and decreased fish abundance.¹⁶⁵⁰ Similarly, bleaching of sea anemones has been linked to impaired threat responses in anemonefish.¹⁶⁵¹ Thermal stress reduces reproductive rates and alters behaviour in crustaceans (Section 2.4.5).^322,323 Although the sublethal effects of temperature are often inconspicuous, they can potentially de-stabilise how the ecosystem operates.^322,323,1652 Nevertheless, how various effects on individual species will manifest at the ecosystem level remains a major knowledge gap.¹⁶⁵

Rising temperatures are altering the sex ratios of marine turtles (Section 2.4.10), as the sex of hatchlings is determined by sand temperatures during incubation.⁵³⁵ Feminisation of green turtle hatchlings in the northern Reef has been an ongoing trend for more than 2 decades.¹⁶⁵⁴ This shift in the sex ratio of hatchlings may confer some adaptive capacity to these species ¹⁶⁵⁵ — since the sex that increases future fecundity (females) is produced in greater proportion — but the near complete feminisation of hatchlings under higher rates of warming compromises the future viability of sea turtle populations.^535,1656 Marine turtles are also subjected to the effects of rising sea levels on nesting habitat, and can experience changes in reproductive periodicity, shifts in latitudinal ranges, and changes in foraging success.¹⁶⁵⁷

Latitudinal shifts in species distributions and changes in the timing of biological events in response to temperature increases have already been shown for several plankton, bony fish and invertebrate species.^1539,1658 Little quantitative evidence exists about current shifts in the geographic ranges of tropical benthic species.⁵³⁵ Effects of increased sea surface temperatures, such as changes in timing of breeding and body size of breeding adults and loss of adequate food supplies, have been observed in many bird populations within the Region.^{1659,1660,1661} Under rates of projected warming, the persistence of many other species will depend on them moving well beyond the biogeographic realm where they are endemic, at rates of redistribution not previously seen.¹⁶⁶² For example, projections of the future distribution of seagrasses suggest a poleward shift due to increasing seawater temperatures.¹⁶⁶³ However, a species’ ability to migrate will depend on habitat availability.^1539,1620 

Ocean acidification is affecting many calcifying organisms, with corals and calcifying algae the most vulnerable^{535,540,1664,1665} (Section 3.3.2), but with considerable variability between and within species groups.¹⁶⁶⁶ Groups such as bivalves and sea urchins are recognised as generally sensitive to projected levels of acidification.¹⁶⁶⁶ Other marine calcifying organisms, such as benthic foraminifera, many echinoderms, crustaceans, bryozoans, polychaete worms and cephalopods, appear to be tolerant to the levels of ocean acidification expected this century.^{1666,1667,1668} There is some evidence that calcifying zooplankton in the Region may be sensitive to falling aragonite saturation at the range of values currently observed.³⁵⁹ Acidification has also been linked to an increased dominance of environmental microbes at the expense of those that contribute to a healthy coral holobiont.⁷⁸¹

Even relatively small decreases in ocean pH are associated with changes to carbonate chemistry that reduce the capacity of corals to build skeletons, which in turn reduces their capacity to create habitat ^{743 ,1669} (Section 3.3.2). Reef-building processes are also affected, with increasing ocean acidity leading to faster dissolution of calcium carbonate sediments in lagoons and interreefal areas ^757,758 and affecting calcification rates of organisms involved in the cementation of the coral reef matrix, such as crustose coralline algae ^743,744 and calcifying microbes⁷⁷⁶ (Section 3.4.8). The palaeo-record shows that the microbial calcification process is highly sensitive to ocean acidification.^889,1670 Individual organisms and reef structures weakened by ocean acidification are less able to resist and recover from physical damage caused by cyclones.¹⁶⁷¹

Declines in calcification rates of Halimeda (Section 2.3.8) are anticipated under high emissions scenarios,^1672,1673 although the ability to regulate carbonate chemistry within the intercellular spaces where calcium carbonate is precipitated is likely to insulate these algae to some degree from the effects of ocean acidification.¹⁶⁷⁴ Some species, such as copepods that are a significant component of the reef lagoon infauna and an important food source for higher trophic levels, have been shown to increase under low pH conditions.⁷⁵⁷ Increases in carbon dioxide also provide photosynthetic organisms, such as seagrass and algae, with improved access to an essential resource for photosynthesis.¹⁶⁷⁵ The ecosystem consequences of relative winners and losers under ocean acidification represent a major knowledge gap.^1666,757

The relative saturation of aragonite is a key factor in reef development.⁵³⁵ Observed declines in aragonite saturation state ⁷³⁹ are well on the way to exceeding thresholds associated with steep changes in reef biota and may exceed thresholds for positive reef accretion within decades²⁸¹ (Section 6.3.1). Numerous studies have documented decreases in community calcification in the Region and globally, although at widely varying rates.^749,898 More than 55 million years ago, under high atmospheric concentrations of carbon dioxide, a reduction in calcification occurred.^1676,1677 It took tens of thousands of years for those changes to reverse,¹⁶⁷⁸ suggesting emerging risks associated with ocean acidification will be essentially irreversible on human timescales.⁶⁶⁵ Many of the effects of ocean acidification on reefs, such as proliferation of algae, greater bioerosion, negative effects on coral recruitment and crustose coralline algae, are similar in their direction to the effects of poor water quality, suggesting water quality improvement may mitigate some of the effects of ocean acidification on inshore reefs.⁵³⁵

Towards the end of this century, sea-level rise is projected to place significant pressure on coral reefs,¹⁶⁷⁹ mangrove forests ^95,1680 and seagrasses, as well as on saltmarshes and islands.¹⁶⁸¹ For these habitats to maintain their extent, growth and migration must keep pace with sea-level rise.¹⁶⁷⁹ The compounding pressures of current climate change threaten the ability of reef-builders to keep pace with future sea-level rise.⁸⁸⁴ Losses of biodiversity and functional integrity are likely to occur over decades on the Reef’s ecosystems in which upward growth is insufficient to keep pace with rising waters.^1679,1682 The Reef’s low-lying coral islands are likely to exhibit considerable variability in their vulnerability to sea-level rise and other climate-related stressors. Factors affecting reef-building process affect island geomorphology but indirectly and over longer timescales.¹⁰⁰ The capacity of any individual island to continue to grow vertically with rising sea levels depends on a complex set of factors,^884,1683 including the physical characteristics of the cay itself, the local carbonate budget and surrounding oceanographic and weather patterns.^100,1684

Overall, future warming is expected to drive range shifts in mangrove forests, with significant losses projected under all future emissions scenarios by mid-century and substantially greater losses by 2100.^109,95 Recent research suggests that vertical growth of mangroves and saltmarshes will be likely to fall behind a relative sea-level rise (Section 3.2.5) of 4 millimetres per year and highly likely to do so at 7 millimetres per year.⁹⁵ Losses will be greatest where space for retreat is constrained or where other non-climate drivers exacerbate risk from climate-induced drivers.^591,109 The effects of sea-level rise will take time to manifest in ecosystems and regional communities, but the effects will continue for centuries and are highly dependent on emissions trajectories (Section 6.3.1). Under a very high emissions scenario leading to 3 degrees Celsius of global warming, nearly all the world’s mangrove forests and coral reef islands and almost 40 per cent of mapped tidal marshes are estimated to be exposed to relative sea-level rise of at least 7 millimetres per year.⁹⁵ Losses of this habitat would be accompanied by losses of associated ecosystem services, including wave-energy attenuation, habitat provision for biodiversity, food production and carbon storage.

One of the most certain impacts of climate change over the course of this century will be increases in the frequency of extremes of sea level, such as king tides and storm surges being superimposed on a rising baseline.^629,1685 Intertidal coastal habitats are at risk from exposure to physical damage from these events.¹⁶⁸⁶ More frequent coastal inundation will also contribute to salinisation of coastal wetlands and shifts in intertidal species distributions.¹⁶⁸⁷ Year-to-year sea level fluctuations linked to the El Niño–Southern Oscillation (Box 6.1) can cause widespread dieback of mangroves, such as occurred in the Gulf of Carpentaria during the 2015–2016 summer.^1688,1689 The potential for future increases in the intensity of El Niño–Southern Oscillation events (Section 6.3.1) therefore has important implications for intertidal coastal ecosystems, such as mangroves and saltmarshes.¹⁶⁹⁰

Altered weather patterns and increases in the intensity and duration of severe weather events are likely to bring both acute and chronic impacts to the Region’s ecosystems. For example, increasingly extreme rainfall events ^540,554 are likely to be associated with greater pulsed delivery of sediments, nutrients and pesticides in land-based runoff.⁵³⁵ Projected increases in the frequency of droughts in southern catchments will likely affect the condition of wetlands and other coastal habitats and lead to greater sediment delivery during drought-breaking floods.⁵³⁵ The loss of 2.3 per cent of mangroves in parts of the Mackay Whitsunday region from 2009 to 2019, attributed to storms and precipitation variation,¹⁶⁹¹ illustrates the sensitivity of these ecosystems to changes in weather patterns. Seagrass meadows in coastal areas are vulnerable to physical damage from severe storms and cyclones as well as to protracted impacts caused by declines in water quality from rainfall and discharge.⁵³⁵ Likely increases in flooding and strong winds are linked to enhanced coastal erosion but may also deliver sediments needed for vertical growth of wetlands.⁵³⁵

Projected changes to the character of cyclones (Sections 3.2.2 and 6.3.1) are a key focus because of their pivotal role as drivers of ecosystem structure, diversity and change.^{1692,1693,1694,1695} The coastal effects of cyclones depend on their intensity, size, track and speed of travel.¹⁵⁷⁵ Coastal fringing and intertidal habitats, and shallow areas of the lagoon floor, as well as coral reefs, are all susceptible to damage, with flow-on effects for associated species.⁵³⁵ The attribution to cyclones of 48 per cent of losses in coral cover between 1985 and 2012,¹⁶⁹⁶ and 41 per cent between 2008 and 2020,¹⁶⁹⁷ underscores the central importance of cyclones in driving disturbance dynamics on the Reef. Although cyclones are forecast to increase in average intensity,¹⁵⁷⁵ a critical knowledge gap is whether cyclone damage to ecosystems above and below water will also increase.^535,549  Rises are projected in sea level, average cyclone intensity and rainfall rates; each generally acts to further elevate storm surge and freshwater flooding.¹⁵⁷⁵ Since damage increases exponentially with wind speed, an increase in peak wind speeds is likely to produce greater overall wind damage.^535,1575 Underwater cyclone damage depends on wave generation, with large, slow-moving intense cyclones causing the most damage because they maximise wave height.⁵⁴⁹ Wave barriers, such as upwind reefs, reduce damage. Trends in determinants of damage other than peak wind speeds remain uncertain.^549,1575

Damage can be widespread, for example coral damage from cyclone Yasi extended up to 250 kilometres from the cyclone track.⁵⁴⁴ However, damage to reefs tends to be highly variable at local scales, depending on the intensity and duration of wave exposure.^{544,1698,1699} The extent of damage to corals is species-specific, with branching and plate corals more vulnerable than massive and encrusting corals.^1700,1701 It is certain that cyclones will continue to be a major source of disturbance in the Region and will contribute to increasing cumulative pressure on ecosystems.⁵³⁵ Changing patterns of cyclone activity will also have interactive effects with temperature, since the cooling influence of cloud cover associated with cyclones has been shown to moderate the intensity of marine heatwaves at regional scales.⁵⁵⁵ Predictions of the determinants of damage, as well as the spatial patterns of future changes in cyclone distributions, are critical knowledge gaps.⁵⁴⁹

Altered ocean currents will have flow-on effects on the entire marine ecosystem, from planktonic production to pelagic and reef fish distribution.^356,1572,252 Intensification of the offshore East Australian Current has been linked to increased nutrient supply and hence increased ocean productivity.¹⁷⁰² As the East Australian Current strengthens and moves warmer water further south, some tropical species are expanding southwards along the south-east coast of Australia.^{252,1030,1703} As southern waters continue to warm, particularly during winter, these species may persist and establish new populations.^1704,1705 Different species compositions could result in a reorganisation of ecosystem functions, particularly if displaced species are habitat forming and lose their dominant position in the ecosystem.^1706,1707

Local scale changes to stratification and upwelling are likely to affect the locations of thermal refugia ²⁴⁶ and reduce the availability of nutrients on which, for example, Halimeda meadows depend,²⁰³ but key knowledge gaps remain in predictive modelling at this scale.⁵³⁵