In the context of small islands and low-lying areas, the
or human-built barriers it will be difficult for these plant
following discussion identifies some of the sectors, sys-
and animal communities to survive as sea level rises.
tems and localities that are especially vulnerable to sea-
Moreover, impacts on one or more ‘leverage species’ can
level rise. Vulnerability is influenced not only by the na-
result in sweeping community-level changes
59
.
ture of the impacts, but also by the capacity to adapt.
Vulnerability of sediment processes and
Vulnerability of coastal wetlands, mangroves
coastal zones
and biodiversity
Accelerated sea-level rise will exacerbate the problems
Since coastal vegetated wetlands are intimately linked
of coastal erosion which are already widespread globally.
to sea level, these ecosystems are sensitive to long-term
But there is not a simple relationship between sea-level
sea-level change. Modelling of coastal wetlands (exclud-
rise and the retreat of low-lying coasts
60
. For example,
ing sea grasses) suggests that 33 per cent of global wet-
large amounts of sand from the neighbouring open
lands would be lost with a 36 cm rise in sea level from
coast can be transported into estuaries and lagoons due
2000 to 2080 and 44 per cent would be lost with a 72 cm
to sea level rise. As a result, local erosion rates for these
rise in sea level over this period
55
. Losses would be most
coasts can be an order of magnitude greater than simple
severe on the Atlantic and Gulf of Mexico coasts of North
equilibrium models would suggest
61
.
and Central America, the Caribbean, the Mediterranean,
the Baltic and most small island regions, largely reflect-
Changes in sediment supply can influence atoll island
ing their low tidal range.
morphology to at least the same extent as sea-level rise
62,63
.
This is consistent with the view that uninhabited islands
A global assessment of mangrove accretion rates
56
indi-
of the Maldives are morphologically resilient while those
cates that the rate at which mangroves grow in height is
that have been subject to substantial human modifica-
variable but commonly approaches 5 mm per year. This
tion (Figure 6C.9) are inherently more vulnerable
64,65
.
is greater than recent, and even many projected, rates of
increase in global mean sea level. However, many man-
Vulnerability of coral reefs
grove shorelines are subsiding and thus experiencing a
more rapid relative sea-level rise
57
. Sea-level rise could
Healthy coral reefs have kept pace with rapid postglacial
reduce the current half-million hectares of mangroves
sea-level rise, suggesting that the projected rates of sea-
in 16 Pacific Island countries and territories by as much
level rise are unlikely to threaten these reef ecosystems,
as 13 per cent by 2100
58
.
at least over the next few decades
66
. Some Indo-Pacific
reef flats are currently exposed at low tide. Anticipated
Higher relative coastal water levels, and the associated
increases in sea level might well result in their submer-
increasing salinity of estuarine systems, will encourage
gence and subsequent recolonization by corals
67
. How-
the inland migration of coastal plant and animal commu-
ever, other climate stresses, especially rising sea surface
nities. However, if such migration is blocked by natural
temperature threaten many coral reefs worldwide
46
.
168 GLOBAL OUTLOOK FOR ICE AND SNOW