Saintilan et al. studied ancient mangrove
sediments to infer a threshold of relative
SLR for mangrove tolerance. The term “rela-
tive” is used because the rate of SLR is deter-
mined not only by the increase in water vol-
ume of the oceans but also by subsidence or
uplift of coastal land. Subsidence and uplift
result, in part, from glacial isostatic adjust-
ment, which describes a complex set of geo-
physical processes that vary geographically.
The authors used sediment cores to collate
the dates when mangrove vegetation first ap-
peared in a variety of locations over the past
10,000 years. Mangroves can be detected
from their distinctive, highly organic sedi-
ments, which reflect mangrove ecosystems’
great productivity and conditions that favor
preservation of vast quantities of organic
carbon. The data obtained from sediment
cores, sampled by a range of researchers
from 78 sites on five continents, indicate that
mangrove ecosystems did not develop unless
relative SLR was less than 6 to 7 mm/year.
The global mean rate of SLR is now
3.4 mm/year and is projected to exceed
the threshold and reach ~10 mm/year by
2100 under “business-as-usual” scenarios
(greenhouse gas concentration trajectory
Representative Concentration Pathway 8.5)( 3 ). However, SLR could stabilize at ~5 mm/
year by the year 2100 under moderate-emis-
sions scenarios in which countries reduce
carbon dioxide (CO 2 ) emissions ( 3 ). Clearly,
reducing CO 2 in the atmosphere is a first
line of defense against passing the relative
SLR threshold for mangrove persistence.
However, in many mangrove locations, rates
of relative SLR are already higher than 6 to 7
mm/year. Now, in the Anthropocene, subsid-
ence of coastal land is also caused by extrac-
tion of oil, gas, and water ( 4 ). For example,
the Mekong Delta of Vietnam is subsiding at
a rate of 6 to 20 mm/year ( 5 ) and the Ganges-
Brahmaputra Delta by 1 to 7 mm/year ( 6 ),
which is accompanied by land erosion and
saltwater intrusion. At the same time, sedi-
ment supply to the coast, which is vital to
counteracting the effects of relative SLR as
it contributes to raising the seafloor, has de-
clined; this is because rivers are dammed
and, in some cases, sediment has been mined
and exported, all of which further increase
the vulnerability of mangroves to SLR ( 7 ).
Saintilan et al. also found variation
around the mean threshold value of 6 to 7
mm/year, indicating that local factors, such
as subsidence and sediment supply, drive
location-specific relative SLR thresholds for
mangroves. Reduction in extraction of water,
oil, and gas from floodplains and enhanc-
ing sediment supply might avoid crossing
the SLR threshold for mangrove persistence.
Dam removal and the delivery of nourishing
sediments and water to the coast are prac-
ticed in Europe and North America but have
little uptake in the tropics ( 8 ). Improvement
of water quality and reduction of overex-
ploitation will help maintain mangrove
health by facilitating strong root growth,
which also contributes to vertical accretion
( 9 ). Balancing the freshwater and sediment
needs of mangroves that provide natural
ecosystem services against water storage,
irrigated agriculture, and hydropower is an
emerging series of trade-offs that policy-
makers must face ( 10 ).
One key consideration in these trade-offs
is that under moderate SLR, mangroves miti-
gate CO 2 in the atmosphere by storing it in
the plants and sediment of the ecosystem
(so-called blue carbon) ( 11 ). Saintilan et al.
used their sediment core data to estimate the
amount of CO 2 sequestered over ~1400 years
of mangrove development and reached a
conservative estimate of 85 Pg of stored car-
bon, which is larger than the estimated car-
bon stored in boreal peats as they developed
through the same Holocene period. These
historical estimates verify that sequestrationSCIENCE sciencemag.org
Sea level rise threatens coastal mangrove forests.
Disruption of these ecosystems endangers coastal
communities and decreases carbon sequestration.School of Biological Sciences, The University of Queensland,
St Lucia, QLD 4072, Australia. Email: [email protected]
5 JUNE 2020 • VOL 368 ISSUE 6495 1051
Published by AAAS