Sea-level driven land conversion and the formation of ghost forests

Abstract

Ghost forests created by the submergence of low-lying land are one of the most striking indicators of climate change along the Atlantic coast of North America. Although dead trees at the margin of estuaries were described as early as 1910, recent research has led to new recognition that the submergence of terrestrial land is geographically widespread, ecologically and economically important, and globally relevant to the survival of coastal wetlands in the face of rapid sea level rise. This emerging understanding has in turn generated widespread interest in the physical and ecological mechanisms influencing the extent and pace of upland to wetland conversion. Choices between defending the coast from sea level rise and facilitating ecosystem transgression will play a fundamental role in determining the fate and function of low-lying coastal land. A review of the phenomenon of low-lying ‘ghost forests’, and the physical and ecological mechanisms that control their occurrence in the context of sea level rise, with a focus on the Atlantic Coast of North America.

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https://doi.org/10.1038/s41558-019-0488-7
1Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA, USA. 2George Washington University, Washington, DC, USA.
*e-mail: kirwan@vims.edu
Sea-level rise rates have been accelerating since the end of the
nineteenth century, impacting low-elevation land along coasts
and estuaries around the world1. Sea level rise enhances flood-
ing and saltwater intrusion, and threatens coastal communities,
infrastructure and ecosystems2–4. Ghost forests and abandoned
farmland are striking indicators of sea-level driven land conversion.
Dead trees and stumps surrounded by marshland, for example,
represent relic forestland that has been replaced by intertidal veg-
etation. Similarly, bare soil and wetland plants at the edges of agri-
cultural fields indicate the encroachment of wetlands into formerly
productive farmland. These visual illustrations of land conversion
are common along the North American Atlantic and Gulf of Mexico
coasts, and reflect rapid ecosystem change and the inland migration
of the intertidal zone in response to sea level rise (Fig. 1).
The ongoing conversion of uplands to wetlands is both economi-
cally and ecologically important. Eustatic sea level rise is predicted to
increase by between 0.4–1.2 m by 2100 (ref. 5). More than 600 mil-
lion people live in low-lying coastal areas (<10 m elevation)3, and
approximately 50 million people live on land predicted to be per-
manently inundated with 0.5 m of sea level rise6. In the contermi-
nous United States alone, 1 m of relative sea level rise would convert
approximately 12,000–49,000 km2 of dry land to intertidal land with-
out flood-defence structures7,8. Heavily populated, low-lying regions
including subsiding deltas and island nations will be most affected6.
In Egypt and Bangladesh, sea level rise could cause a 15–19% loss in
habitable land and displace 13–16% of the population9. In the United
States, residential property values may decrease with proximity to
wetlands10, and the conversion of uplands to wetlands is perceived
as highly undesirable by many landowners11. On the other hand, the
marshes and mangroves that replace inundated forests and farmland
are considered among the most valuable ecosystems in the world
because they improve water quality, reduce coastal erosion, protect
against flooding, sequester carbon and support marine fisheries12.
Therefore, coastal sustainability in the face of sea level rise involves
rapidly moving ecosystem boundaries and complex trade-offs
between the direct and indirect values of different land uses13.
Although ghost forests first appeared in the scientific literature
over a century ago14 and are a prominent feature of many coastal
and estuarine landscapes from the Atlantic coast of Canada to the
Gulf Coast of the United States15–22, coastal change research has tra-
ditionally focused on more seaward environments, such as barrier
islands, intertidal wetlands and subtidal ecosystems23,24. Extensive
research into those portions of the coastal landscape has identified a
number of feedbacks between flooding, vegetation growth and sedi-
ment transport, that allows them to resist sea level rise until some
threshold rate is exceeded. For example, marshes, mangroves and
oyster reefs are well known to resist sea level rise by accumulating
sediment and growing vertically24–26. Although more work is needed
to determine if analogous processes allow terrestrial land to resist
sea level rise, observations of widespread land conversion16,17,19 sug-
gest terrestrial ecosystems largely lack mechanisms to engineer ver-
tical soil growth. Therefore, forests and other terrestrial ecosystems
are potentially more sensitive to sea level rise than better studied
intertidal and subtidal portions of the coastal landscape20.
Here, we review the natural and human mediated processes that
influence sea-level driven land conversion. Although this Review
considers a variety of land types, we emphasize the conversion of
forests to marshes because it is the most common and well-studied
conversion, and because it produces ghost forests that are a strik-
ing visual indicator of sea-level driven land conversion. The first
section illustrates that historical land submergence is geographically
widespread and has impacted terrestrial forests, agricultural fields
and developed landscapes alike. The second section discusses the
ecological processes linking sea level rise and land conversion, such
as plant population demography and community reorganization,
that shape the environmental consequences of land conversion.
The third section argues that drowning of uplands is potentially the
most important process determining future wetland area; and the
fourth section considers the extent to which humans will prevent
or facilitate coastal land submergence. The Review ends with impli-
cations for land management, and highlights uncertainty in local
flood defence strategy as the key knowledge gap limiting our ability
to predict future sea-level driven land conversion and its impact on
coastal ecosystems.
Extent and control of historical land submergence
Ghost forests, abandoned agricultural fields and other indicators
of historical land submergence occur throughout low-lying and
Sea-level driven land conversion and the
formation of ghost forests
Matthew L. Kirwan 1* and Keryn B. Gedan2
Ghost forests created by the submergence of low-lying land are one of the most striking indicators of climate change along
the Atlantic coast of North America. Although dead trees at the margin of estuaries were described as early as 1910, recent
research has led to new recognition that the submergence of terrestrial land is geographically widespread, ecologically and
economically important, and globally relevant to the survival of coastal wetlands in the face of rapid sea level rise. This emerg-
ing understanding has in turn generated widespread interest in the physical and ecological mechanisms influencing the extent
and pace of upland to wetland conversion. Choices between defending the coast from sea level rise and facilitating ecosystem
transgression will play a fundamental role in determining the fate and function of low-lying coastal land.
Corrected: Author Correction
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gently sloping portions of the Atlantic and Gulf Coasts of North
America15–20 (Fig. 1). Land submergence is most extensive within
the mid-Atlantic sea-level rise hotspot that stretches from North
Carolina to Massachusetts, where relative sea level is rising three
times faster than eustatic rates27. For example, 400 km2 of uplands in
the Chesapeake Bay region have converted to tidal marsh since the
mid-1800s19, and large tracts of hardwood and cedar forest death
have been observed in Delaware Bay16. However, ghost forests are
not confined to the sea level rise hotspot; they have also been doc-
umented throughout the Florida Gulf Coast17,18, the St. Lawrence
estuary in Canada15, and tidal freshwater forests in South Carolina,
Georgia and Louisiana21,28. There has been 148 km2 of forest conver-
sion over 120 years along the Florida Gulf Coast17, and near com-
plete loss of pine forests in the Lower Florida Keys29. Surprisingly,
the phenomenon has not been widely documented on coastal plains
outside of the United States. There are no reports of ghost forests
from low-lying tropical regions where the phenomenon would be
predicted, such as the Yucatan Peninsula, Mexico30, or from the
Pacific Ocean’s western margin, such as along eastern China, due to
the prevalence of seawalls there31.
Observations of historical land submergence indicate that topog-
raphy and relative sea level rise are the two most important controls
on the rate of lateral forest retreat32. Migration rates are substantially
lower in United States Pacific coastal regions and New England
estuaries (<10 cm yr–1)33,34 than in the mid-Atlantic coastal plain
(up to 7 m yr–1)16,19,35, where rates of land conversion are inversely
correlated with slope16,19. Although mortality of canopy trees may
depend on punctuated disturbance events, such as storms, and
therefore lag behind sea level rise30,36, land conversion is tightly tied
to sea level over decadal timescales16,18,30. For example, the elevation
of coastal treelines has increased in parallel with late-Holocene sea
level rise, and lateral rates of forest retreat are 2–14 times higher
than pre-industrial rates20,35 (Fig. 2).
The conversion of agricultural fields and residential lawns to wet-
lands is less visually striking than ghost forests, as one herbaceous
plant community is replaced by another, but is much more eco-
nomically damaging. Marshes migrate rapidly into urban and sub-
urban lawns, where mowed marshes look similar to mowed lawns37.
Abandonment of agricultural land due to salinization is prevalent
in low elevation coastal regions around the world, including large
a b
c d
Fig. 1 | Geographic distribution of sea-level driven land conversion in North America. a, Red spruce ghost forest and buried stumps, New Brunswick,
Canada. b, Atlantic white cedar ghost forest in New Jersey (indicated by dashed line). c, Salt damaged agricultural field in Virginia, where white and grey
areas indicate bare ground, and yellow-red colours represent stressed crops. d, Palm tree ghost forest in Florida. Credit: David Johnson (a),
Kenneth W. Able (b), USDA Farm Service Agency (c) and Amy Langston, Virginia Institute of Marine Science (d)
012345
Forest retreat rate (m yr–1)
Long Shoal
River, NC
Cedar
Creek, MD
Goodwin
Island, VA
Hell
Hook, MD
Cedar
Island, NC
Pre-1875
Post-1875
Fig. 2 | Accelerating forest retreat rates. Lateral forest retreat rates for
five US mid-Atlantic sites, where gold bars represent late-Holocene rates
(pre-1875) inferred from sediment cores and historical maps, and green
bars represent modern rates (post-1875) inferred from historical maps
and aerial photographs. 1875 was chosen to approximate the initiation of
accelerated sea level rise on the Atlantic coast1. Modern forest retreat rates
are 2–14× higher than late-Holocene rates, and generally increase through
time. Adapted from ref. 20, Virginia Institute of Marine Sciences.
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areas of North Carolina38,39, Italy40, Mexico41 and Bangladesh42. In
Bangladesh, saltwater intrusion has salinized 10,000 km2 of land in
the last four decades, including an estimated 3,000 km2 of arable
land42. Bangladeshi farmers responded by increasing fertilizer appli-
cations to compensate for losses in yields, switching crops and con-
verting 1,380 km2 of farmland to shrimp ponds42. Sea level rise is
forecasted to result in major losses in agricultural area over the next
century in nations with agricultural production in deltaic or coastal
regions (for example, 1,000 km2 will be lost within the Pearl River
Delta region of China)43. The Mekong Delta of Vietnam stands to
be one of the most affected areas in the world, where losses in rice
production threaten global food supply44.
Processes linking sea level and land conversion
Dead trees underlain by wetland vegetation are a striking final
indicator of uplands that have been displaced by sea level rise and
saltwater intrusion (Fig. 3). However, the creation of ghost forests
and the wholesale reorganization of ecosystems begins with more
subtle changes that can be anticipated with a deeper understanding
of the ecological processes that link sea level rise and land conver-
sion. In the early stages of groundwater salinization, live trees may
exhibit reduced sap flow45 and annual growth46,47, although reduced
growth is not always observed34,48. During the next phase of ghost
forest formation, forest distress becomes more visible. Young trees
die conspicuously and tree recruitment ceases30,46 (Fig. 3a). Because
recruitment ceases prior to the death of mature trees30,46,48, tree age
distributions skew towards older trees at lower elevations36,46, and
relict trees stand as ghost forests in-waiting18. Salt-tolerant species
establish in the understory as adult trees die30, aided by increased
light penetration and seed delivery from storm wrack deposits49.
Shrubs often dominate the transition from forest to tidal wet-
land18,21,50 (Fig. 3b). Of the 148 km2 of converted forest land in Big
Bend, Florida, 55% converted to marsh while 45% converted to a
shrub-dominated habitat17 that persisted for 20 years18. These areas
may be particularly persistent in formerly agricultural areas, where
land is graded flat during cultivation. Finally, dead tree trunks and
stumps persist in tidal marshes for decades, a lasting remnant of the
forests displaced by sea level rise and saltwater intrusion (Fig. 3c).
Upland ecosystem mortality is driven by the synergistic impacts
of salinity and inundation, which are more challenging for plants
than either stress alone21,51–53. Generally, plants that tolerate flooding
are more resistant to low-level salinity stress21. Variation in stress
tolerances between plant species can explain differences in the rate
of transition of different forest types. For example, in Delaware Bay,
Atlantic white cedar (Chamaecyparis thyoides) forests died back
at faster rates than hardwood forests (typical species: red maple
(Acer rubrum), sweetgum (Liquidambar styraciflua), blackgum
(Nyssa sylvatica)). Eastern red cedar (Juniperus virginiana) is among
the most tolerant tree species; the species outlasted loblolly pine
(Pinus taeda), winged elm (Ulmus alata) and Florida maple (Acer
floridanum) during forest dieback in Florida54. Trees and crops are
most vulnerable to salinity stress during germination and as seed-
lings51,55,56. Mortality of relatively salt-tolerant tree seedlings occurs
when salinity exceeds about 5 ppt (ref. 57), and most crops cannot
tolerate sustained salinities over 2 ppt (refs. 58,59).
The transition of uplands to wetlands can be either gradual or
punctuated by disturbance events, such as hurricanes, fires and
insect outbreaks. Pulses of high salinity water during storms often
trigger mortality60,61. Although storm waters recede in hours, salin-
ity effects can linger for years to decades in the groundwater62,63,
and individual storms have lasting impacts on tree growth47. Storm
floods can reach tens of kilometres inland and are accompanied by
wind, erosion and wrack disturbances. Correspondingly, shifts in
upland land cover occur suddenly, when storm-related disturbance
destroys an upland ecosystem61. In the absence of major disturbance,
change may occur more gradually, as elevated groundwater salini-
ties slowly take their toll on a plant community that is intolerant to
salinity. Moreover, the impact of storms increases with sea level rise,
leading to the progressive inland retreat of upland ecosystems over
time15. Terrestrial water budgets can also affect the rate of change,
as saltwater intrusion resulting from sea level rise is exacerbated by
drought54, surface and groundwater withdrawals41, and hydrological
connectivity from dams, ditching and canals64.
Ecosystem transitions affect the provision of ecosystem services,
though the exact nature of these shifts varies based on trade-offs in
services between upland and wetland ecosystems13,65. Tidal wetlands
a b c
Fig. 3 | Stages of ghost forest creation. a–c, Photos show forest-to-marsh conversion in the Chesapeake Bay region (MD, USA) characterized by (a) death
of tree saplings, (b) opening of canopy and invasion of Phragmites and shrubs, and (c) adult tree death and conversion to marsh, indicated by stumps in
foreground and ghost forest in background. Image in c courtesy of Lennert Schepers, UAntwerpen.
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exhibit much higher areal rates of carbon sequestration and storage
than terrestrial environments7. Therefore, the conversion of forests
and croplands to tidal wetlands will increase total carbon seques-
tration of a region, provided that gains are not offset by concur-
rent losses in tidal wetland area (see ‘Implications for the survival
of adjacent wetlands’). Similarly, upland conversion of agricultural
lands (a nutrient source to adjacent waterways and estuaries66) to
tidal wetlands (a nutrient sink12) should ultimately increase nutri-
ent uptake. During transition, however, salinization of uplands can
result in short-lived21 releases of massive amounts of legacy nutri-
ents that have accumulated in cultivated soils over prior decades.
In North Carolina, saltwater intrusion into former farmland is pre-
dicted to release 18×106 kg of ammonium, or approximately half
the annual ammonium flux of the Mississippi River to the Gulf of
Mexico39. In Maryland, high releases of phosphate occur during
saltwater intrusion into agricultural land67. These nutrient releases
contribute to coastal eutrophication and associated algal blooms
and dead zones68.
Upland conversion may reduce biodiversity provisioning, as wet-
land migration represents an opportunity for invasive species expan-
sion. In Delaware Bay, 30% of converted forest area became native
tidal marsh habitat, while 60% became dominated by the invasive
common reed (Phragmites australis)16. The conversion of uplands
to the invasive common reed during ghost forest formation is of
particular concern for Atlantic tidal marsh endemic species with
narrow habitat requirements, such as the diamondback terrapin
(Malaclemys terrapin)69 and the saltmarsh sparrow (Ammodramus
caudacutus), predicted to go extinct by 2030 due to sea level rise70.
In Florida, the invasive Brazilian pepper (Schinus terebinthifolius)
inhabits a similar niche to the common reed in that it outcompetes
native species in the ecotone and exhibits wide salinity tolerance,
and is also expected to spread during upland conversion18,71. Thus,
sea-level driven land conversion will affect both the composition
and function of the coastal landscape.
Implications for the survival of adjacent wetlands
The conversion of uplands to wetlands is a primary mechanism for
wetland survival in the face of sea level rise, and counterintuitively
leads to predictions that wetlands may expand with sea level rise
under certain conditions2,8,17. At the most basic level, wetlands must
migrate to higher elevations faster than they erode laterally and
drown vertically in order to maintain their size32. Although marshes
and mangroves build soil vertically, there are limits to the rate of sea
level rise that wetlands can survive in place. Numerical models pre-
dict that maximum possible vertical accretion rates overlap with the
range of predicted sea level rise rates for 2100 (generally 5–30 mm
yr–1)72, and observations of wetland drowning indicate that these
limits have already been exceeded in some places73,74. When these
threshold rates of sea level rise are exceeded, wetlands must migrate
laterally into submerging uplands to survive.
Historical observations and simple analyses of coastal topogra-
phy indicate that upland drowning has the potential to create large
areas of new wetlands that are comparable in size to existing wet-
lands. For example, historical maps of the Chesapeake Bay suggest
that approximately 1/3 of all marshland today formed as a result of
migration into drowning uplands since the mid-nineteenth century,
and that upland drowning compensated for historical erosion of
marshes in the region19. On the Florida Gulf Coast, marsh forma-
tion in submerging uplands has outpaced historical loss, and led to
net marsh expansion17. More work is needed to infer how future sea
level rise will alter the timescales associated with wetland loss and
migration, but these historical trends together with observations of
ghost forests underlain by marsh vegetation, suggest that wetland
migration can occur on the decadal–century timescales relevant
to wetland loss. Across the conterminous United States, there are
~26,000 km2 of saline wetlands75, and sea level rise of 1.2 m would
inundate ~12,000–49,000 km2 of uplands7. Thus, the formation of
new wetlands in drowning uplands has the potential to compensate
for even large losses of existing wetlands.
Rates of marsh migration generally increase in parallel with sea
level rise20,35,37, but existing marsh is relatively resistant to sea level
rise because enhanced flooding leads to faster vertical accretion76.
Upland migration, therefore, allows marshes to potentially expand,
rather than contract, in response to sea level rise13,76. Numerical
modelling suggests that marshes adjacent to gently sloping uplands
will expand under moderate increases in sea level rise, followed by
inevitable contraction when high rates of sea level rise lead to wide-
spread drowning of existing marshland76. The particular rate of sea
level rise that leads to a transition from marsh expansion to marsh
contraction depends principally on the slope of adjacent uplands32,
anthropogenic barriers to migration77, and factors such as tidal
range and sediment supply that control the resistance of existing
marsh to sea level rise and edge erosion24. Nevertheless, numerical
models that consider both dynamic marsh accretion and the poten-
tial for marshes to migrate inland suggest that many marshes will
expand under moderate rates of sea level rise, and then contract
under higher rates13,76,78,79 (Fig. 4).
These types of simple landscape models based on topography and
land use have thus far assumed a binary response of land types to sea
level rise (for example, complete conversion of inundated forestland
and no conversion of inundated urban land; see ‘Opportunities and
barriers to coastal submergence’), and that wetlands will migrate into
uplands as soon as they become sufficiently inundated (for example,
without ecological lags; see ‘Processes linking sea level and land
conversion’). Other work identifies additional caveats. For example,
the response of low-lying land to sea level rise will vary both within
and across regions19,80–82, where regions with steep upland topogra-
phy and anthropogenic barriers to migration may see near complete
loss of marshes81. In places where marshes persist, the proportion of
flood-tolerant vegetation types will increase78,79,81 and newly created
wetlands may themselves be vulnerable to sea level rise10. Salt water
intrusion into freshwater soils increases organic matter decomposi-
tion rates so that soil elevation loss could limit wetland migration
and/or survival in submerging forests with organic-rich soils4,28.
Finally, interactions between multiple facets of climate change and
socioeconomic factors (for example, changing hurricane frequen-
cies and flood protection strategies) may influence sea-level-driven
land conversion in unanticipated ways. Nevertheless, recent global
modelling suggests wetland migration into submerging uplands is
the single biggest factor influencing wetland area through time, and
that global wetland area could increase by up to 60% by 2100 for a
1.1 m sea level rise (Fig. 5)83.
Opportunities and barriers to coastal submergence
Although there is abundant land that could be inundated by sea level
rise, anthropogenic structures and coastal development may prevent
land conversion in many regions of the world. Ghost forests, aban-
doned farmland and other indicators of land submergence are most
common in the south-eastern and mid-Atlantic United States, in part
because these coastal regions are largely rural and devoid of large, sys-
tematic flood control structures outside of major cities. In contrast,
ghost forests are rare in western Europe and China because extensive
seawalls and dykes protect uplands from sea level rise and coastal
flooding31,84. Large flood control structures are less common in the
United States, but migration of wetlands into submerging uplands
may instead be prevented by local barriers including berms, bulk-
heads, roads, ditches with floodgates and impervious surfaces80,85.
For example, 42% of all land less than 1 m above spring high water is
currently developed along the United States Atlantic coast, whereas
less than 10% is currently protected against development86.
Human impacts are typically perceived as barriers to wet-
land migration, but people also facilitate sea-level driven land
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conversion, and the net-impac t can be difficult to discern. Historical
marsh migration rates likely decrease with the degree of coastal
development in the Chesapeake Bay region, but the relationship is
weak and highly site specific19,85. Elsewhere, suburban lawns convert
to marsh as quickly as adjacent forests37, and reclaimed agricultural
areas are particularly susceptible to salinization and land conver-
sion40. Wetland restoration projects commonly remove berms to
reconnect agricultural fields and other land types with tidal flood-
ing87,88. Barrier removal has mixed effects. Since barriers enhance
land subsidence and limit sedimentation, the land behind the barri-
ers may require substanti al restoration to be suitable for wetlands87–90.
Indeed, accidental or poorly planned breaches after significant
subsidence can rapidly drown wetlands89. In other cases, large
levees are carefully removed or moved further inland to create
wetlands that contribute to natural flood protection, in a concept
known as nature-based engineering or managed realignment84,90
(Fig. 5a). Finally, human actions sometimes unintentionally accel-
erate land submergence by increasing rates of saltwater intrusion
via groundwater withdrawal and/or subsidence4,91, or building
canals that input saltwater64. Nevertheless, anthropogenic barri-
ers block substantial wetland migration today in many regions80,81,
and wholesale submergence and abandonment of low-lying coastal
land is unlikely because in most cases the cost of conventional
flood control structures is far less than the cost of economic dam-
ages associated with flooding92.
The United States Gulf Coast represents an interesting case study
for how population growth and flood-control structures might
interact to determine the extent of upland land conversion (Fig. 6a).
This region contains approximately 50% of United States saline
wetlands75, high variability in human population densities and rates
of relative sea level rise, and the most extensive flood protection
system in the United States77,82,93. Analysis of topography and land
use across the entire Gulf Coast indicate that 19,572 km2 of land
is vulnerable to submergence under a 1.2 m sea level rise, and that
barriers projected under population growth will prevent conversion
in an additional 4,056 km2 (ref. 77). This work highlights that there
are strong spatial gradients in both opportunities and barriers to
migration within the Gulf Coast region, such that the absence of
land conversion in highly urbanized areas may result in large reduc-
tions in local wetland area77,82. Nevertheless, these analyses have
three fundamental implications at the regional scale. First, current
and projected barriers to wetland migration are small relative to
the total amount of land available for migration (~20%), such that
the total area of land that will be inundated will be large regard-
less of protection of urban areas. Second, only 35% of land avail-
able for migration is currently owned by government and private
conservation organizations77, suggesting that most land conversion
will take place on private land and depend on local decisions not
fully considered in analyses based on urbanization and levee con-
struction. Finally, the area of land potentially available for saline
wetland migration (19,572 km2)77 is larger than the area of land cur-
rently occupied by saline wetlands on the Gulf Coast (13,600 km2)75
and similar to the current extent of saline wetlands in the entire
conterminous United States (26,000 km2). Together, these obser-
vations emphasize that sea-level-driven land conversion will be
cba
Expansion
Slope
0.0005
0.001
0.002
0.01
0.2
dMW/dt (m yr–1)
SLR (mm yr–1)
2
15
No migration
Drowning
10
5
0
–5
–10
4 6 8 10 12 14
Fig. 4 | Effect of topographic slope and human impacts on marsh size. a, Model simulations showing change in marsh width (dMW/dt) for different rates
of sea level rise (SLR) and slopes of adjacent land (coloured lines). For gently sloping, natural coasts, marshes expand with increasing SLR rates until a
threshold rate is exceeded. Marshes inevitably decline in size when uplands are steep or protected by anthropogenic barriers (black line represents a case
with no migration). b, Steep uplands prevent landward marsh migration and favour small and/or shrinking marshes (Bay of Fundy, Nova Scotia, Canada).
c, Gently sloping uplands facilitate landward marsh migration and favour large and/or expanding marshes (Chesapeake Bay, MD, USA). Panel a reproduced
from ref. 76, John Wiley & Sons. Image in c courtesy of Lennert Schepers, UAntwerpen.
a b
Global wetland area (103 km2)
2010
400
350
300
250
200
150
100
50
0
2040 2070
Nature-based
engineering
Business
as usual
2100
Breached
levee
Narrow
marsh
Farmland
Levee
New
marsh
Fig. 5 | Effect of flood defence strategy and land conversion on wetland
size. a, Nature-based engineering to create marsh in front of leveed
agricultural fields in the Wash Estuary, UK. The levee was intentionally
breached in 2002, marsh vegetated colonized naturally, and now
protects the more inland levee. b, Modelled global wetland area for
the Intergovernmental Panel on Climate Change RCP 8.5 sea-level rise
scenario. Colours represent different flood-defence scenarios, where the
model assumes no landward wetland migration where the projected human
population in the 100-year floodplain exceeds 5–20 people km−2 (red,
reflecting business as usual), 20–150 people km−2 (pink), and 150–300
people km−2 (yellow, reflecting extensive nature-based engineering).
Credit: Anglian Coastal Monitoring Programme (a). Panel b adapted from
ref. 83, Springer Nature Ltd
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widespread and a fundamental determinant of wetland area at
regional scales, even in the presence of urban barriers.
Moving beyond static models based on topography and land use
is difficult because adaptation to coastal flooding depends not only
on the rate of sea level rise, but also on a variety of human deci-
sions influenced by complex socio-economic factors. There are
strong landowner attitudes against wetland migration11 (Fig. 6b),
growing coastal populations94, and it is economically rational to
build flood defence structures for most of the world’s coasts92.
On the other hand, rising sea level and energy costs suggest that
building and maintenance costs will increase through time, such
that conventional engineering may be unsustainable in the long
term95,96. Rising costs may especially prevent engineering solutions
in developing countries and poorer regions3. Interestingly, highly
developed deltaic regions, including the Mississippi, Rhone and
East Asian deltas, are the most vulnerable to rising energy costs96.
Therefore, regions with large areas of currently protected land
are also the most likely to incorporate nature-based engineering
approaches that would allow submergence of some land for the
first time in centuries84,90.
While the factors that contribute to flood defence are ultimately
quite complex, levees generally occur where population densities
in the 100-year coastal flood plain exceed 20 people per km2, and
global modelling suggests this threshold represents a key determi-
nant of wetland fate under sea level rise and population growth83.
Lower population thresholds (reflecting nature-based engineering)
lead to wetland expansion, whereas higher thresholds (reflecting
conventional engineering) lead to wetland contraction (Fig. 5b).
Therefore, decisions to defend or abandon portions of the coast
represent a fundamental, if not primary, determinant of coastal sub-
mergence and the migration of wetlands into uplands24,83.
Recommendations for future research
Our Review suggests that widespread sea-level driven submergence
of low-lying land will continue in the future, even under scenarios
of coastal population growth and large-scale defence of urban areas.
However, land conversion will largely take place on privately owned
land82,86, where landowner attitudes and adaptation efforts suggest
local resistance70. We, therefore, pose the following questions to
guide future research and land management decisions.
First, is land-conversion inevitable on privately owned, rural
land? Research in the last five years has identified and mapped large
barriers to wetland migration, such as urban land and publically
owned levees at regional scales77,80–82. However, the majority of vul-
nerable land is located on private property in rural areas38,82,87. Future
research should investigate the efficacy of local and privately main-
tained barriers, such as berms, ditches and secondary roads, and the
probability and consequences of barrier failure. Barriers influence
the adaptive capacity of coastal systems by enhancing land subsid-
ence and limiting sedimentation. Therefore, this research should
quantify key thresholds in the timing of barrier removal/failure that
minimize both the cost of abandoned land and the cost of restora-
tion. Government and conservation organizations are increasingly
preserving wetland ‘migration corridors’ but understanding of if
and how landowners influence land submergence will help priori-
tize conservation efforts.
Second, can transitional land uses and nature-based engineering
compensate for trade-offs between private property and ecosystem
service values? Sea-level-driven land conversion leads to simulta-
neous loss in value for private landowners and gain in ecosystem
services for the general public13,97. Future research should focus on
whether transitional land and water management decisions, such
as planting salt-tolerant crops98, leasing land to hunt clubs, early
harvest of susceptible timber lands and groundwater manipula-
tions4, could significantly offset economic losses and influence the
function of newly forming wetlands. Future research should also
consider the viability of nature-based engineering, where limited
wetland migration could simultaneously enhance natural flood pro-
tection and reduce levee maintenance costs84,91.
Finally, how can polic y incent ives shape the future of coastal upland
conversion? There are few programs in the United States that provide
assistance or recommendations to landowners affected by sea level
rise, and they are harshly criticized for providing perverse subsidies99
and benefiting repeatedly flood damaged and reconstructed proper-
ties100. Programs such as the United States Department of Agriculture’s
Conservation Reserve Program, that subsidize remediating salinity
damage on farm fields, could be repurposed as instruments for adap-
tation to sea level rise. Regional predictions for tidal wetland habitat
gain or loss should set the context for management and policy incen-
tives to either prioritize wetland migration or upland protection100.
a b
0.5m SLR by 2100 1.0m SLR by 2100 1.5m SLR by 2100
0 1,000 2,000 3,000 4,000
0
40
80
120
0.4
0.6
0.2
0.0
0
40
80
120
0
40
80
120
Urban barriers to wetland migration (km2)
Opportunity for wetland migration (km2)
State
AL
AL/FL
LA
LA/MS
MS/AL
TX/LA
Proportion of respondents
SU U N L SL
Fig. 6 | Land conversion in the face of human barriers. a, Projected urban barriers and opportunities for wetland migration for US Gulf Coast estuaries82.
Opportunities for wetland migration are an order of magnitude greater than urban barriers to migration in each estuary, and potential wetland migration
increases with increasing sea level rise (SLR) scenario (top to bottom). b, Preferences of 1,002 landowners regarding conservation easements to allow
marsh migration in the north-eastern United States. Responses are strongly unlikely (SU), unlikely (U), neutral (N), likely (L) and strongly likely (SL).
Adapted from ref. 82, British Ecological Society (a); ref. 11, PNAS (b).
NATURE CLIMATE CHANGE | VOL 9 | JUNE 2019 | 450–457 | www.nature.com/natureclimatechange 455

Review ARticle NATure ClimATe ChANGe
In summary, our Review highlights extensive sea-level-driven
land conversion, marked by ghost forests and abandoned agricul-
tural land that represent relict features of a rapidly submerging coast.
Accelerated sea level rise over the next 80 years could potentially
create new wetlands equivalent in size to current ones, even under
scenarios of coastal population growth and urban levee construc-
tion. These changes will happen disproportionately on rural and pri-
vate lands, where efforts to prevent or promote land conversion are
poorly understood. Given the extent of historical change, the mag-
nitude of forecasted change, and an unpredictable human response,
sea-level-driven land submergence is likely to lead to wholesale reor-
ganization of coastal ecosystems and economies within this century.
Received: 24 September 2018; Accepted: 23 April 2019;
Published online: 27 May 2019
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Acknowledgements
This work was supported by the US National Science Foundation (Coastal SEES
#1426981; LTER #1237733; CAREER #1654374), and the USDA Agricultural and Food
Research Initiative Competitive Program (#2018-68002-27915). SouthWings provided
a flight that helped motivate the work. This is contribution no. 3827 of the Virginia
Institute of Marine Science.
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