3. Ecosystem health

Cyclones are intense, large-scale, low-pressure weather systems that form over warm tropical waters and rotate, producing persistent destructive winds that can build extreme seas given sufficient time and fetch. The number of cyclones making landfall is not random over time.⁵³⁷ Rather, the frequency of landfall is higher during La Niña years than El Niño years ^538,539 (Box 6.1). The number of cyclones has decreased across northern Australia since at least 1982, and likely since 1900 ⁵⁴⁰ (Section 6.3.1). A continuing trend of decreasing cyclone numbers, but with an increasing average intensity, is expected along with typically large variations in cyclone numbers from year to year.^540,541

Cyclones can disturb coastal and marine ecosystems through the high winds, waves, storm surge and heavy rain they generate, so they are important drivers of ecosystem change in the Region.¹⁷⁹ Coral reefs, seagrass meadows, mainland and island beaches, and ecosystems on the coastal fringe, such as mangrove forests, are especially vulnerable to such impacts. For instance, mangroves can be damaged by high winds and inundation with salt water, and mangroves at the outer fringe can be wave damaged. 

Observed and projected global increases in the average severity of cyclones ^542,543 (Section 6.3.1) have important implications for  Reef ecosystems (Section 6.3.2). For example, coral reefs can experience damage of widely differing severity, from breakage or removal of individual coral colonies to damage to the reef structure itself, depending on the intensity and duration of wave exposure.⁵⁴⁴ Cyclone-induced wave force and resulting damage varies at local scales due to factors like depth, the shape of the seafloor, the presence of physical barriers and reef orientation.⁵⁴⁵ Some species are more fragile than others and are more likely to break or dislodge under the force of the waves.⁵⁴⁴ In a healthy reef ecosystem, the patchiness of cyclone damage can promote diversity by creating a range of microhabitats and enabling the growth of corals with varying morphologies and rates of growth.^546,547 Reef recovery depends on factors such as the severity and duration of exposure, the pattern and frequency of future disturbances,⁵³⁷ and the availability of suitable conditions for coral regrowth.⁵⁴⁸ If consolidated reef matrix is badly damaged recovery can be very slow.⁵⁴⁴

The extent to which cyclone-induced waves damage coral reefs depends on a combination of the cyclone’s intensity, size and the length of time spent tracking near coral reefs.⁵⁴⁹ Although a trend of increasing intensity (but decreasing frequency) ⁵⁴⁰ appears to be emerging, future trends for the other determinants of damage to reefs (cyclone size and speed of travel) remain uncertain.⁵⁴⁹ Likewise, there is considerable uncertainty surrounding potential future shifts in locations and patterns of areas exposed to cyclones.⁵⁴⁹ While poleward shifts in the latitude of maximum intensity of cyclones have been observed in both northern and southern hemispheres,⁵⁵⁰ and there is evidence showing potential expansion of regions suitable for cyclones,^551,552 it is unclear whether these trends are occurring in the Region.

Cyclones and other low-pressure systems can produce oceanic storm surges that cause serious damage to coastal infrastructure and ecosystems. One of the most certain future impacts from cyclones will be an increase in coastal flooding when storm surges are superimposed on higher sea levels (Section 3.2.5), increasing their destructive power.^540,553 The probability of heavy rain events has also increased in recent decades due to regional warming (as warmer air carries more moisture), and it is projected to increase further as air temperatures continue to rise.^540,554 Conversely, wave action associated with cyclones has been shown to upwell cooler water that can lower sea surface temperatures and reduce the impacts of coral bleaching at regional scales,⁵⁵⁵ while the cooling influence of increased cloud cover can also reduce sea surface temperatures at local scales.⁵⁵⁶

Over the past 5 years, cyclone activity has been relatively low (Figure 3.3). Cyclone Trevor crossed the Region before making landfall on 23 March 2019 near the town of Lockhart River in Cape York as a category 3 system, causing significant damage on land. Impacts to the Reef were found to be minimal at long-term coral reef survey sites in the vicinity, noting most were well south of the cyclone’s track.⁵⁵⁷ Cyclone Kimi tracked through the Region in January 2021 reaching category 1 but not making landfall. Cyclone Tiffany reached category 2 intensity within the Region, before making landfall near Cape Melville on Cape York Peninsula on 10 January 2022 at category 1. Cyclone Jasper crossed the Reef before making landfall at Wujal Wujal, north of Cape Tribulation, on 13 December 2023 as a category 2 system. Cyclone Kirrily crossed through the Region in January 2024, making landfall at Townsville as a category 3 system. The period of relative quiescence before the successive cyclones in late 2023 and early 2024 saw a welcome recovery in coral cover (Sections 2.3.5, 2.4.4 and 8.3.1), highlighting that the Reef remains a resilient and dynamic ecosystem. Maintaining this resilience by addressing pressures amenable to management action is crucial for the long-term health of the Reef in the face of compounding impacts from climate change and ocean acidification.

Winds are an important influence on the Reef, affecting a range of processes, including the shaping of islands and coastlines, the movement and local characteristics of waves and currents, and distribution pathways of marine pollution.⁵⁶¹ Established wind patterns can affect the breeding success and life history traits of reef animals such as corals, fish,⁵⁶² and sea snakes.⁴³⁵ Any changes in wind patterns may affect connectivity within the Region, via the distribution of windborne seeds ⁵⁶³ and larvae of coral, fish and invertebrates.⁵⁶⁴ Climate change may affect wind speeds and patterns in the Region, but the specific changes are likely to be complex and are not yet well understood.^565,566,567 

In the past 5 years, cyclone activity has been relatively low. However, large variation in the numbers of cyclones is expected, and cyclones remain one of the most important drivers of ecosystem change in the Region. A longer-term trend may be emerging of decreasing cyclone numbers and increasing average intensity.