1.3.2.7 Coastal ecosystems

In this section we consider ecosystems bordering the land and ocean, covering the ‘littoral’ intertidal and subtidal zones. These zones include some of the most biodiverse and human-depended ecosystems on Earth, despite occupying globally tiny areas: warm-water coral reefs, mangrove forests and seagrass meadows. However, all face increasing pressures from increasingly frequent climate change-induced extremes compounded by habitat destruction, pollution and sea level rise.

Warm-water coral reefs

Warm-water coral reefs span the Earth’s tropical and subtropical ocean, and are estimated to support over half a billion people for their livelihoods and over a quarter of marine species for part of their lifecycle (Wilkinson et al., 2004; Plaisance et al., 2011) (Figure 1.3.15). They can cross a threshold of ecosystem collapse when they cease to have sufficient cover (typically ~10 per cent) and diversity of hard corals to support the wide diversity of species taxa and ecological interactions typical of a coral reef (Bland et al., 2018; Darling et al., 2019; Sheppard et al., 2020; Perry et al., 2013; Vercelloni et al., 2020). Coral reef collapse is an ecological phenomenon at local scales; here we explore where localised coral reef collapse aggregates to the scale of regions, potentially irreversibly, and potentially to a global scale.

Thermal stress, driven by increasingly warmer oceans and superimposed El Niño extreme events, is the primary driver of regional-scale mortality of hard corals (Hughes et al., 2017; Houk et al., 2020). Coral ‘bleaching’ occurs when thermal stress causes corals to expel the symbiotic algae that provides them with food (resulting in a characteristic loss of colour), and can result in death if it occurs frequently enough to prevent recovery (Hughes et al., 2018a, 2018b, Obura et al., 2022). However, a wide variety of interacting and synergistic threats co-occur (e.g. ocean acidification, overfishing, pollution, invertebrate predators and sea level rise), generally lowering the thermal threshold for bleaching and/or mortality, bringing forward timing of collapse, or even surpassing thermal stress in local importance (Ban et al., 2013; Edmunds et al., 2014; Darling et al., 2019; Cramer et al., 2020; Dixon et al., 2022; Setter et al., 2022). Coral mortality may play out over weeks to a few months (for e.g. thermal stress-induced bleaching, for example), or years (for chronic threats such as diseases and land-based impacts), but prolonged failure to recover over a decade or more is necessary to qualify a coral reef as ‘collapsed’.

Localised coral responses to increasing stressor magnitude and intensity are now aggregating at scales exceeding 1,000km and manifesting as regional die-offs (e.g. western and central Indian Ocean, Great Barrier Reef, Mesoamerican Reefs) (Le Nohaïc et al,.2017; Amir, 2022; Muñiz-Castillo et al., 2019; Obura et al., 2022), with most reef regions having experienced multiple mass coral bleaching and die-off events (Darling et al., 2019; Cramer et al., 2020) (Figure 1.3.16). Around 50 per cent of global coral reefs are estimated to have been lost over the past 50-150 years (IPBES 2019), with estimated loss of 16 per cent in 1998 (Wilkinson et al., 1999) and measured loss of 14 per cent from 2009-2018 (Souter et al., 2020), but with high variance among regions.

Projected loss of coral reefs has been estimated in varied ways. Dominant projections are of 70-90 per cent loss of coral reefs at 1.5°C and ~99 per cent at 2°C warming (Cooley et al., 2022). The average year for projected global annual severe bleaching under SSP2-4.5 (a trajectory close to current projections) is 2045, which is delayed 30 years if corals can adapt to an additional 1°C of warming (UNEP 2020). A shift occurs from 84 per cent of reefs globally having ‘good’ thermal regimes in 1986-2019 to 0.2 per cent in 2100 at projections of 1.5°C, and 0 per cent at 2°C warming (Dixon et al., 2022). Finally, the proportion of reefs facing ‘unsuitable conditions’ increases from 44 per cent in 2005 to, under worst case scenarios, 100 per cent by 2055 under any one of several stressors, but by 2035 for cumulative stressors (Setter et al., 2022). Continued ocean warming over several decades (due to lagged ocean heat uptake) and sea level rise over centuries to millennia (due to thermal expansion and ice sheet melt, see 1.2.3) mean some reefs and other coastal ecosystems (see also Mangroves and Seagrassees) may be committed to eventually passing tipping thresholds even if emissions ceased soon (Abrams et al,. accepted).

Evidence for tipping dynamics

Failure to recover from mass mortality shows evidence of having crossed a threshold for recovery, which we address for scales above approximately 1,000km, to regional and global scales. A key question is if coral reef decline globally is just an aggregate of regional events, so a linear/chronic decline process (Souter et al., 2021), or if there may be a global tipping point. 

Observations on coral reef tipping points include the following:

• The first reported global bleaching event in 1998 was associated with atmospheric warming of ~0.6^oC (corresponding to c. 350 ppm CO₂) with a strong El Niño on top (Veron et al., 2009), past which more frequent, intense and widespread coral bleaching and mortality has occurred. 
• A very high risk of impact to corals was assessed by the IPCC as global mean warming levels crossed around 1.2°C (IPCC SR1.5 2018).
• Thermal bleaching tipping points are already being passed in the majority of coral reef regions (Cooley et al., 2022 – see Figure 1.3.17).
• The risk of ecosystem collapse is already predicted at high levels in all coral reef regions assessed. The MesoAmerican Barrier Reef is Endangered (Bland et al., 2018) and Western Indian Ocean coral reefs are Vulnerable to collapse, with two thirds of subsidiary ecoregions being Endangered or Critically Endangered due to projected warming (Obura et al., 2022).

Elevated summer ocean heat maxima (over 1-2°C above site-specific individual coral acclimation thresholds) for weeks to months, and larger acute temperature spikes for several days, cause severe coral bleaching and mass mortality. Mass-mortality bleaching thresholds have been proposed at eight “Degree Heating Weeks” (a measure of how long and how much ocean temperatures are above normal) which is likely by ~2°C global warming (McWhorter et al., 2021), or at two bleaching events per decade (likely by ~1.5°C) (Frieler et al., 2013). Mass coral mortality repeated more than twice per decade and over hundreds to thousands of kilometres and larger, is increasingly recognised as giving insufficient time for recovery of impacted populations, and of ecological interactions (Hughes et al., 2018a, 2018b, Obura et al., 2022). However, estimating globally consistent warming thresholds is challenging given variation from individual corals to species and across all spatial scales in acclimation and adaptation ability. Other stressors reduce the ability of corals to resist thermal stress, thus bringing down tipping thresholds. 

Increasing frequency and intensity of regional-scale coral mortality events past 1°C warming are suggestive that these coral reef regions have already passed regional bleaching tipping points (Cooley et al., 2022). The potential for thermal refuges for corals under likely future scenarios is doubtful (Beyer et al., 2018; Dixon et al., 2022; Setter et al., 2022) as very few or no reef areas are projected to remain below tipping thresholds of key stressors. The existence of putative refuges at greater depths (Bongaerts and Smith 2019) or higher latitudes (Yamano et al., 2011; Setter et al., 2022) are not strongly supported by recent work (Hoegh-Guldberg et al., 2017; Cooley et al., 2022).

Ecological and biogeographical (spatial) positive/amplifying feedback loops prevent local recovery of coral reefs and promote expansion of reef collapses from local to regional scales when surviving corals and coral patches become too spatially separated for successful reproduction of adults, and supply of larvae from surviving to damaged reefs (Hock et al., 2017). 

Coral reef decline does not substantially feedback to the climate system on policy-relevant timescales. However, localised surface cooling may arise through increased low level cloud albedo induced by sulphur compounds released by reef metabolism. Consequently, extensive coral die-offs could amplify local warming (Jackson et al., 2020).

Assessment and knowledge gaps

Warm-water coral reefs have localised tipping points (high confidence) and are now experiencing regionally clustered tipping points (high confidence). Based on the evidence collected here, we suggest that the critical threshold of 1.5°C (range 1-2°C) (Armstrong McKay et al., 2022) should be adjusted, narrowing and lowering the range to 1-1.5°C, with a middle estimate of 1.2°C, marked by the multi-year global coral reef bleaching events of 2015-2017 (Cooley et al., 2022; Hoegh-Guldberg et al., 2018; Dixon et al., 2022; Setter et al., 2022). The co-occurence of additional synergistic drivers also support lowering the critical threshold (Willcock et al., 2023) and there is evidence of accelerating collapses at increasing spatial scales (Cooper et al., 2020).

The combined effects of long-term warming, sea level rise, ocean acidification and other stressors bears more investigation to identify the lower critical threshold for the coral reef tipping point. The potential for coral adaptation to warming is a critical but poorly known factor, and subject to high levels of variation locally. The potential effectiveness of restoration for coral reefs at scale, and with enhanced capacity to resist future threats, are both currently poor. The effect of climate migration on coral recovery is not known, with potentially positive effects at higher latitude (with in-migration), but negative at lower latitudes (with out-migration, but no replacement; Herbert-Read et al., 2023).

Mangroves and seagrasses 

Mangroves and seagrasses play vital roles in coastal societies and economies. They provide fundamental and hard-to-substitute ecosystem services such as support to fisheries, nutrient cycling, coastal protection and sediment trapping (Malik et al., 2015; Nordlund et al., 2016; Menéndez et al., 2020; Nabilah Ruslan et al., 2022; doAmaral-Camara et al., 2023, James et al., 2023). Located between the sea and the land, their unique dual nature exposes mangroves and seagrasses to climate drivers that arise in both systems (Lovelock et al., 2017a; Duke et al., 2017a, 2019), making them particularly vulnerable to climate change (Duke et al., 2022). Recent attention has focused on their climate mitigation services (‘blue carbon’) linked to their high productivities and long-term (millennia) storage of organic matter in their sediments, which positions them among the most dense carbon sinks on Earth (Donato et al., 2011; Alongi et al., 2016; Macreadie et al., 2021; Serrano et al., 2021).

While they occupy small areas (c. 140,000 sq km and uncertain c. 266,562 sq km for mangroves and seagrasses respectively in 2020; Bunting et al., 2022; McKenzie et al., 2020; Figure 1.3.18 and 1.3.19), they store up to 12.3 GtC and 3.8 GtC respectively (Macreadie et al., 2021). These ecosystems are natural sinks of CO₂, but when degraded they can release CO₂, NO₂ and CH₄, adding to the emissions of the estuaries they are embedded in (Rosentreter et al., 2022). Emissions derive from carbon stored long-term in sediments, which cannot be recovered in a lifespan and is therefore additional to the current atmospheric balance (Lovelock et al., 2017b; Schorn et al., 2021; Romero-Uribe et al., 2022).

Mangroves and seagrasses are historically among the most human-threatened ecosystems in the world (Valiela et al., 2001; Waycott et al., 2009), with 35-50 per cent of mangroves’ original cover now lost, mainly to aquaculture and agriculture (Richards and Friess, 2016, Goldberg et al., 2020; Hagger et al., 2022), while other factors including nutrient overload, invasive species, and ocean warming have led to a 19-30 per cent decrease of the original seagrass surveyed area (Waycott et al., 2009; Dunic et al., 2021). 

In spite of this, the magnitude of their past and current feedback to global warming remains uncertain (Rosentreter et al., 2022). Under current rates of deforestation, estimates of global mangrove emissions by the end of the century range between 0.24 to 0.34 Gg CO₂e if foregone soil carbon sequestration is also included (Adame et al., 2021), which is comparable to the European Union’s emissions in 2022. Southeast and South Asia (West Coral Triangle, Sunda Shelf and the Bay of Bengal) are projected to lead the emissions, followed by the Caribbean (Tropical Northwest Atlantic), the Andaman coast (West Myanmar), and northern Brazil (Adame et al., 2021)

Evidence for tipping dynamics

In spite of major historical habitat loss and degradation, there are not yet generalised signs of irreversible global transitions of mangroves towards alternative states such as tidal flats, and the remaining systems have so far retained large-scale stability in the tropics. Bistability is, however, observed in northern subtropical distributions with mangrove encroachment over tidal marshes where freezing events are now rarer (Feller et al., 2017; Hesterberg et al., 2022). Observational data also suggests rainfall-induced bistability of mangroves and salt marshes (Duke et al., 2019).

Scarce global monitoring prevents robust analyses of seagrass trends, but transitions (>50 sq km) towards unvegetated sediments have intensified in many coastal regions in the last two decades (e.g. Europe, Australia, US, Caribbean) (Waycott et al., 2009; Carr et al., 2012; Arias-Ortiz et al., 2018; Duarte et al., 2018; Kendrick et al., 2019; Cooley et al., 2022; MacLeod et al., 2023) (Fig 1.3.19). For temperate regions, bistability and tropicalisation of temperate seagrass species are observed in edge-of-range meadows, with uncertain stability trends (Bartenfelder et al., 2022). For tropical seagrasses, local resilience after disturbance has been observed when enough time and reduced pressures apply (MacLeod et al., 2023).

While the resilience of these systems (particularly mangroves) does not yet seem compromised at the global scale, there is increasing evidence of region-dependent declines in resilience for both seagrasses (Dunic et al., 2021; Turschwell et al., 2021) and mangroves (Bergstrom et al., 2021, Friess et al., 2022; Amaral et al., 2023, Duke et al., 2023 in press). These responses relate to: 

1. An increased exposure to more frequent and intense extreme events such as hazardous cyclonic activity (Figure 1.3.20), more frequent and intense El Niño (Figure 1.3.21) and marine heatwaves (Fig 1.3.19), which add to the long existing human pressures (nutrient overloads, land use changes, sedimentation rates, etc) and to the long-term environmental impacts that promote mangrove and seagrass mortality (including sea level rise, ocean acidification, ocean/atmosphere warming, regional drought, salinity, hypoxia, diseases and invasive species) (Waycott et al., 2009; Krauss et al., 2014; Lovelock et al,.2015; Feller et al., 2017; Duke et al., 2021; Friess et al., 2022; MacLeod et al., 2023).
2. Shortened recovery times below re-establishment needs. Post-disturbance recovery has been reported to take ca. 10-20 years depending on the ecosystem service considered (Lugo 1980; Jimenez et al., 1985; MacLeod et al., 2023), with mangrove recovery taking c. 20 years (more on arid climates), and c.10 years for seagrasses. A decade has been considered the absolute minimum successful re-establishment time for both systems, if pre-disturbance conditions (hydrological stability and seed sources) were retained (Lugo 1980; Teutli-Hernandez et al., 2020, Duke et al., 2023 in press; MacLeod et al., 2023). Revisiting times are currently below these thresholds in many regions,
3. Unprecedented increases in compound extreme events that precede, succeed, or coincide in time and space and amplify ecosystem responses (Allen et al., 2021). Along this line, magnified mangrove mortality due to drought-hurricane duos has already been reported in the Caribbean (Taillie et al., 2020; Amaral et al., 2023). 
4. Exposure to multivariable extreme pressures (Fig 1.3.22). While models frequently focus on a few independent-forcing variables, in reality multiple amplifying, synergistic or antagonic effects occur among stressors. As an example, El Niño combines multiple variables such as heat, drought, flooding, more extreme oscillations in sea level (e.g. Taimasas in the Indo-Pacific), and marine heatwaves, whose combined interaction amplifies mangrove and seagrass mortality.

Decreasing resilience enhances damages in coastal habitats, including severe losses of biodiversity, collapse of regional fisheries and aquaculture, and reduced capacity of habitat-forming species to protect shorelines, preventing re-establishment (Cooley et al., 2022). These make mangroves and seagrasses likely candidates for regional tipping points, with major social and economic consequences. Additionally, lagged ocean warming (over decades) and sea level rise (over centuries) mean coastal ecosystems will continue to face increasing pressure after atmospheric warming stabilises, meaning tipping can be committed decades before it is realised (see warm-water coral reefs above).

On the potential tipping dynamics of coastal systems, the IPCC AR6 chapter on ocean and coastal ecosystems (Cooley et al., 2022) noted “irreversible phase shifts with global warming levels >1.5°C, making both systems at high risk this century even in <1.5°C scenarios that include periods of temperature overshoot beyond 1.5°C (high confidence). Mangroves, under SSP1-2.6, are expected to be unable to keep up with sea level rise by 2050, with ecological impacts escalating rapidly beyond 2050”. 

(Saintilan et al., 2020, 2023) found it very likely that mangroves were unable to initiate sustained accretion when relative sea level rise rates exceeded 6.1 (4-7) mm/year. This threshold is likely to be surpassed on low-latitude tropical coastlines within 3-5 decades under high-emissions scenarios (Sweet and Park 2014; Saintilan et al., 2020, 2023). For seagrasses, the IPCC AR6 (Cooley et al., 2022) projects contractions of temperate edge-ranges (e.g. Zostera costera seagrasses in the US would retract by 150-650km under RCP2.6 and RCP8.5, respectively and Posidonia oceanica in the Mediterranean Sea, which might lose as much as 75 per cent of their habitat by 2050 under RCP8.5 and become functionally extinct by 2100). Marine heat waves will escalate seagrass responses, with moderate responses to sea level rise (Cooley et al., 2022).

Assessment and knowledge gaps

We conclude with medium confidence that, under current relative sea level rise projections, subsidence, expected increases in extreme events and coastal development (Cooley et al., 2022), tipping responses for mangroves are likely to be regionally visible by 2080 at temperature thresholds between 1.5-2°C (starting with physiologically stressed regions that host increasing extreme events – also medium confidence). Seagrasses are likely (medium confidence) to show region-dependent die-off responses earlier (by mid century) due to more intense and recurrent marine heatwaves, nutrient pollution and turbidity, at global temperature thresholds closer to 1.5°C (medium confidence).

We have high confidence that tipping responses will be region and site-dependent with diverse timings and degradation speeds. For mangroves, physiologically stressed regions such as arid or highly seasonal climates like the Middle East or the dry corridor of Central America, karstic systems such as the Caribbean, small islands, northern Australia, or the northern Coral Triangle are likely (medium confidence) to show tipping responses earlier than other regions such as the Indo-Pacific, South America or parts of the Indian Ocean, whose systems either have more species, are less exposed, or are less vulnerable to hazard exposure (e.g. there is more space for encroachment, or more refugia). 

For seagrasses, temperate regions are predicted to be more vulnerable to tipping than warmer regions (Turschwell et al., 2021; Green et al., 2021; Cooley et al., 2022) (Fig. 1.3.23). Seagrasses in warm regions that are more exposed to water pollution, turbidity, extreme events (marine heat waves and cyclones), coastal development, salinity or invasive species are expected to tip earlier than seagrasses in other warm regions.

Compared to the IPCC AR6 report (Cooley et al., 2022), we highlight a higher confidence on the directional effects of storms on both mangroves and seagrasses towards regionally synchronous mortality (Carlson et al., 2012; Wilson et al., 2019; Taillie et al., 2020; Amaral et al., 2023; Duke et al., 2023 in press). Evidence also exists on decreased regional resilience in mangroves after cyclones (Amaral et al., 2023) and transitions to mudflat shifts in areas where storms combine with erosion co-stressors (Bhargava and Friess 2022). Similarly, warming responses in mangroves have a clearer directional trend, with extreme El Niño hot-droughts superimposed onto global warming and regional drought leading to well-known extended mangrove mortality in many regions (Jimenez et al., 1985), including recent reports of die-off in Australia (Duke et al., 2017a), Panama (Fig. 1.3.21) and the Maldives (Save Maldives Campaign and Neykurendhoo Island Council, 2020). 

Current modelling does not yet properly cover extreme events or multiple drivers, nor their interactions (Cooley et al., 2022). These gaps are likely leading to an underestimation of their impacts on ecosystems and their long-term resilience thresholds. Resilience responses to enhanced stressors will be region- and site-dependent, but models still need data to properly represent key drivers per region and their interactions, as well as the thresholds of survival of regional ecosystems (Marba et al., 2022).