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Thresholds of mangrove survival under rapid sea level rise

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Science  05 Jun 2020:
Vol. 368, Issue 6495, pp. 1118-1121
DOI: 10.1126/science.aba2656

Mangroves under sea level rise

The rate of sea level rise has doubled from 1.8 millimeters per year over the 20th century to ∼3.4 millimeters per year in recent years. Saintilan et al. investigated the likely effects of this increasing rate of rise on coastal mangrove forest, a tropical ecosystem of key importance for coastal protection (see the Perspective by Lovelock). They reviewed data on mangrove accretion 10,000 to 7000 years before present, when the rate of sea level rise was even higher than today as a result of glacial ice melt. Their analysis suggests an upper threshold of 7 millimeters per year as the maximum rate of sea level rise associated with mangrove vertical development, beyond which the ecosystem fails to keep up with the change. Under projected rates of sea level rise, they predict that a deficit between accretion and sea level rise is likely to commence in the next 30 years.

Science, this issue p. 1118; see also p. 1050

Abstract

The response of mangroves to high rates of relative sea level rise (RSLR) is poorly understood. We explore the limits of mangrove vertical accretion to sustained periods of RSLR in the final stages of deglaciation. The timing of initiation and rate of mangrove vertical accretion were compared with independently modeled rates of RSLR for 78 locations. Mangrove forests expanded between 9800 and 7500 years ago, vertically accreting thick sequences of organic sediments at a rate principally driven by the rate of RSLR, representing an important carbon sink. We found it very likely (>90% probability) that mangroves were unable to initiate sustained accretion when RSLR rates exceeded 6.1 millimeters per year. This threshold is likely to be surpassed on tropical coastlines within 30 years under high-emissions scenarios.

The rate of relative sea level rise (RSLR) in tropical and subtropical locations is projected to accelerate from current trends of ~3.4 mm year−1 to a mean estimate of ~5 mm year−1 under low-emissions scenarios and ~10 mm year−1 under high-emissions scenarios by 2100 (1, 2). Modeling of feedbacks between RSLR, vertical accretion, root mass formation, and tidal marsh and mangrove vulnerability under sustained high rates of RSLR is vital for the survival of these ecologically and economically important coastal environments (2). Some tidal marshes have been projected to survive RSLR of more than 10 mm year−1 where supported by high available suspended sediment concentrations (3), on the basis of accretion data that span annual to decadal time scales. Reconstructions using paleoenvironmental proxies, however, have suggested that UK marshes were vulnerable to retreat at RSLR of 7 mm year−1 (4).

Mangroves grow in sheltered intertidal environments that are exposed to the effects of RSLR (5). They support among the highest rates of carbon burial of all ecosystems (6), and a growing body of evidence suggests that this efficiency is enhanced by RSLR (7). However, empirical data on mangrove response to high rates of RSLR are lacking, given the limited observation period (1 to 16 years) of real-time measurements (5).

Here, we assess the mangrove response to RSLR from the paleorecord of mangrove vertical accretion preserved in the sedimentary archives of continental shelves and coastal lowlands. Mangrove forests have established and drowned in association with variability in the rate of RSLR after the onset of deglaciation ~26,000 to 20,000 years ago (8). Before the Holocene, long-term RSLR rates of >12 mm year−1 (8) exceeded the capacity of mangroves to maintain position in situ through vertical accretion, and mangroves were displaced landward (9), with few exceptions (10, 11). During the early to mid-Holocene, RSLR slowed in association with the final phase of deglaciation of the Laurentide ice sheet (12). The rate of RSLR varied across the globe owing to glacio-isostatic adjustment, the response of the solid Earth and gravity field to ice mass redistribution during a glacial cycle (13). Far-field sites, those distal from regions of former ice sheet extent and incorporating most of the tropics, exhibit a decline in the rate of RSLR (14) from 10,000 e 1 in Fig. 1A) to a mid-Holocene highstand, followed by stable or falling sea level to near the current position from ~6000 years ago to the present (phase 2 in Fig. 1A). By contrast, at intermediate-field sites closer to centers of glaciation (for example, the Gulf of Mexico and the Caribbean), sea level rose continuously throughout the Holocene owing to isostatic response to melting of the Laurentide ice sheet. These coastlines have experienced a decelerating rate of RSLR from 10,000 years ago to the present (Fig. 1B and figs. S5 and S6).

Fig. 1 Holocene RSLR and associated mangrove development in far-field estuarine and intermediate-field calcareous settings.

(A) Far-field estuarine settings. (B) Intermediate-field calcareous settings. As the rate of RSLR decelerated, development of organic-rich mangrove sediments was initiated, which facilitated maintenance of vertical position with respect to sea level, forming extensive mangrove forests. When RSLR stabilizes, ongoing sedimentation infills estuaries, building deltas and replacing mangrove with freshwater wetlands, terrestrial forest, or saltpan, depending on climatic setting. CI, credible interval.

The spatially variable deceleration of RSLR from 10,000 to 8000 years ago across tropical and subtropical latitudes coincided with the initiation of vertically continuous, organic-rich mangrove sediments several meters thick, as rising seas flooded shallow continental shelves (phase 1 in Fig. 1, A and B) (15). This deceleration provides an opportunity to explore whether (i) the rate of mangrove vertical accretion responds to changes in RSLR; (ii) ice sheet proximity (intermediate versus far field), geomorphic setting, or tidal range constrains the capacity of mangroves to accrete in relation to RSLR; (ieii) upper thresholds of mangrove vertical accretion can be detected; and (iv) mangrove development and vertical accretion correspond in timing to changes in the global atmospheric carbon budget.

We present empirical data from 122 reconstructions of the timing and rate of mangrove vertical accretion associated with Holocene RSLR in cores collected from 78 tropical and subtropical locations [Fig. 2; (14)]. We independently estimate rates of RSLR for each of the 78 locations before and for the duration of mangrove vertical accretion from a glacio-isostatic adjustment model using an ensemble of different Earth parameters (16) (Fig. 3 and figs. S5 and S6).

Fig. 2 Sampling locations and corresponding rates of mangrove vertical accretion.

(A and B) Mangrove sedimentary units initiating between 10,000 and 6500 years ago (A) and mangrove sedimentary units initiating after 6500 years ago (B). Contemporary mangrove shorelines are indicated in bright green, representing mangrove distribution in 2012 (30).

Fig. 3 Rates of mangrove vertical accretion and RSLR.

(A and B) Accretion rates are derived from the depth of mangrove organic sediment between calibrated 14C dates in individual cores, and rates of RSLR (median and 95% CI) were derived from the glacio-isostatic adjustment (GIA) model ensemble for the same sites for far-field (A) and intermediate-field (B) locations. (C) The probability of initiation of sustained mangrove accretion at or above associated rates of RSLR across all sites. The dotted line shows the RSLR rate at which there is a 10% probability that mangroves can initiate accretion and/or, conversely, that there is a 90% probability that they are unable to initiate accretion.

Slowing RSLR during the early to mid-Holocene coincided with the initiation of extensive mangrove forests (Figs. 1A, phase 1, and 3, A and B). Our analysis suggests that sustained mangrove vertical accretion began across far-field regions (Africa, Asia, Australasia, and South America) at ~10,000 to 8000 years ago and intermediate-field regions (Caribbean and Gulf of Mexico) at ~8000 to 6000 yeares ago (Fig. 3 and figs. S1 and S2). Data were discriminated on the basis of ice sheet proximity and geomorphic setting and differentiated by tidal regime to explore differences in the timing of initiation and rates of vertical accretion (14). The intermediate- and far-field classifications, defined as a function of a location’s proximity to areas of major ice sheet retreat during the last deglaciation, act as a surrogate variable for the temporal pattern of RSLR rates through the Holocene, which in turn influences the availability of accommodation space within which mangroves can accrete vertically. In a generalized linear model of several variables, only the proximity to former ice sheets proved to have a significant relationship with accretion rates (14).

We found a strong relationship (p < 0.001, generalized linear model) between rates of mangrove vertical accretion and RSLR rate across all sites (51% of the variation in the accretion rate can be explained by the RSLR variation; Fig. 3). Mangrove vertical accretion first initiated ~9800 years ago in the Ganges-Brahmaputra River Delta, as RSLR decreased from ~9 to ~6 mm year−1, and continued for ~650 years at high rates until replaced by subtidal deposits as RSLR increased again to >7 mm year−1 (14, 17). Elsewhere in far-field locations, mangrove vertical accretion initiated as RSLR decreased below ~7 mm year−1 starting 8800 years ago (Fig. 3). Rates of vertical accretion of >6 mm year−1 were sustained for more than 1000 years in mangrove forests in Australia, Africa, South America, Central America, and Asia (Fig. 2 and data S1), irrespective of geomorphic setting (fig. S1).

Mangrove vertical accretion initiated in intermediate-field regions later than in far-field locations, first in Belize ~7800 years ago as RSLR fell below ~5 mm year−1 and in other Caribbean and Gulf of Mexico sites between 7500 and 6000 years ago (Figs. 2 and 3 and table S1). This later initiation, in comparison to far-field regions, may be related to the limited allochthonous sediment supply available in the carbonate (reef) settings, although in Belizee, rates of accretion of more than 6 mm year−1 were observed at two sites (data S1), driven by strong authochthonous inputs.

We used a Bayesian framework to estimate the probability of initiation of mangrove accretion conditional on rates of RSLR within the Holocene dataset (14). The empirical Holocene data (data S1), which span a wide range of geomorphic settings and tidal regimes (fig. S2), suggest that when RSLR rates exceed 6.1 and 7.6 mm year−1, respectively, mangroves are very likely (>90% probability) and extremely likely (>95% probability) to be unable to initiate sustained accretion. We found lower RSLR thresholds for intermediate-field sites and higher thresholds for far-field sites (Table 1).

Table 1 Probability that mangroves are unable to initiate sustained vertical accretion at rates of RSLR.

See (14). RSLR rates at all sites (global) and intermediate- and far-field sites at which it is very likely (>90% probability) and extremely likely (>95% probability) that mangroves are unable to initiate sustained accretion and the associated 95% uncertainty interval (UI). For example, at all sites, there is a 94.3 to 95.4% probability (95% UI) that mangroves are unable to initiate at rates that exceed 7.6 mm year−1.

View this table:

Our database also reveals spatial variability in the duration of mangrove accretion (figs. S1 and S2). In only 3 of 50 far-field locations was there indication of drowning of mangroves during a marine transgression (i.e., mangrove sediments overlain by tidal or subtidal deposits; data S1). In most far-field locations (30 of 50), accretion and progrodation of the fluvial delta led to the replacement of mangrove by saltmarsh (characteristic of upper intertidal elevations), freshwater wetland, or terrestrial forest, often by the mid-Holocene when relative sea level stabilized (phase 2 in Fig. 1A and data S1). For this reason, contemporary mangrove extent is highly contracted compared with the early- and mid-Holocene mangrove development in many major river deltas such as the Ord River, Australia; the Red River, Vietnam; and the Mekong River, Vietnam and Cambodia (15, 1820). Mangrove accretion persisted significantly longer at intermediate-field locations (fig. S2) because accommodation space was enhanced by glacio-isostatic adjustment.

The extensive development of mangrove environments under RSLR has exerted an influence on global carbon cycles over geologic time scales (21), including, we suggest, during the early to mid-Holocene (Fig. 4). Carbon balance modeling based on stable carbon isotope signatures suggests that the 5 parts per million by volume reduction in atmospheric CO2 in the early Holocene was driven by increases in the uptake of carbon by the land biosphere on the order of 290 Pg C (22, 23) before 7000 years ago. The timing and volume of this uptake have been attributed to the northward expansion of boreal vegetation after ice sheet retreat [~110 Pg C (24)] and organic soil development [180 Pg C (23)], of which circumArctic peatlands may have sequestered 20 to 60 Pg C based on the depth of peat formation at the time (24). We conservatively estimate a somewhat larger contribution (~85 Pg C) from mangrove carbon sequestration and burial over the period 8600 to 6000 years ago (14). The extensive development of mangrove forests over this period largely replaced methanogenic environments {freshwater wetlands and floodplains [data S1; (25)]} and corresponds to declining rates of methane emission, particularly in the Southern Hemisphere (26). The δ13CH4 signals in the Southern Hemisphere for this period show a 1.5 per mil depletion consistent with a replacement of vegetation using the C4 photosynthetic pathway (tropical grasslands and saltmarsh adapted to low atmospheric CO2), with mangroves using the C3 pathway (26, 27).

Fig. 4 Timing of mangrove vertical accretion in relation to greenhouse gas concentrations.

(A and B) Atmospheric CO2 concentrations [from Antarctic ice cores (23)] (A) and mangrove organic soil formation (B) [the sum of all observations spanning each century weighted by the vertical accretion rate of each observation (14)]. Light colored curves in (A) represent ± 1 standard error for CO2 (B), as presented in the original datasets. ppmv, parts per million by volume.

As RSLR increases in the 21st century, an increase in the rate of mangrove vertical accretion, coupled with landward expansion as sea level rises, can be expected to drive increases in the rate of carbon sequestration and preservation in mangrove environments, providing a negative feedback on radiative forcing, as suggested more broadly for coastal wetlands (7). Although our results demonstrate that accretion in mangroves increases in response to RSLR, we found that it was very likely (>90% probability) that mangroves were unable to initiate sustained accretion when RSLR rates exceeded 6.1 mm year−1 in any but the most sediment-laden settings. RSLR is projected to remain below 5 mm year−1 under low-emissions scenarios [Representative Concentration Pathway (RCP) 2.6] throughout the 21st century (1). However, RSLR is expected to exceed 5 mm year−1 by 2030 and 7 mm year−1 by 2050 under high-emissions scenario RCP8.5 in low-latitude mangrove settings where rates of RSLR are expected to be higher than the global average (1, 2).

Where a deficit commences between vertical accretion and RSLR, time to submergence will be a function of the position of the mangrove within the tidal frame. In settings of low tidal range, mangroves are more likely to be situated at elevations close to the threshold of submergence from the outset. In settings of high tidal range, mangroves are more likely to be situated at elevations well above this threshold and tolerate a deficit between the rates of accretion and RSLR for decades to centuries (5). Geomorphic setting will also influence vulnerability to submergence, because allochthonous sediment contributions in tide- and river-dominated estuaries may provide an elevation subsidy not available in environments receiving low sediment supply, such as coral reefs. In this context, sediment retention in catchments affected by water resource development (i.e., trapped behind dams) and local sediment controls may decrease mangrove resilience to RSLR in river estuaries (5). The natural response of mangrove encroachment across flooded coastal lowlands is therefore the main determinant of future extent (28), although this is already greatly impeded by coastal developments along many coastlines (29). Our findings therefore emphasize the importance of (i) mitigating the magnitude of rapid RSLR and (ii) ensuring that coastal adaptation measures allow for the expansion of mangrove across coastal lowlands.

Supplementary Materials

science.sciencemag.org/content/368/6495/1118/suppl/DC1

Materials and Methods

Figs. S1 to S6

Tables S1 to S2

References (30124)

MDAR Reproducibility Checklist

Data S1

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: We thank J. Mitrovica of Harvard University for providing the GIA model. T. A. Shaw of Nanyang Technology University assisted with the preparation of the figures. Figure 1 used the image library of the Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/). Funding: N.S. was supported by an Outside Studies Program grant from Macquarie University and AINSE. B.P.H. is supported by the Singapore Ministry of Education Academic Research Fund MOE2018-T2-1-030, the National Research Foundation Singapore, and the Singapore Ministry of Education, under the Research Centers of Excellence initiative. This article is a contribution to PALSEA2 (Palaeo-Constraints on Sea-Level Rise), a working group of the International Union for Quaternary Sciences (INQUA), and International Geoscience Program (IGCP) Project 639, “Sea-Level Changes from Minutes to Millennia.” This work is Earth Observatory of Singapore contribution 294. K.R. received funding from the Australian Research Council (FT130100532). Author contributions: N.S. conceived the study. N.S., N.S.K., and B.P.H. assembled the contributing mangrove sediment data. E.A. extracted RSLR estimates from GIA models. N.S., E.A., and J.J.K. conducted data analyses. N.S., N.S.K., E.A., and K.R. prepared the figures. All authors contributed to the writing of the manuscript. Competing interests: The authors declare no competing interests. Data and materials availability: All data are available in the main text or the supplementary materials.
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