4.4. Coastal Zones and Marine Ecosystems
Global climate change will result in increases in sea-surface temperature
(SST) and sea level; decreases in sea-ice cover; and changes in salinity, wave
climate, and ocean circulation. Some of these changes already are taking
place. Changes in oceans are expected to have important feedback effects on
global climate and on the climate of the immediate coastal area (see TAR
WGI). They also would have profound impacts on the biological production
of oceans, including fish production. For instance, changes in global water
circulation and vertical mixing will affect the distribution of biogenic elements
and the efficiency of CO2 uptake by the ocean; changes in upwelling
rates would have major impacts on coastal fish production and coastal climates.
[6.3]
If warm events associated with El Niños increase in frequency, plankton
biomass and fish larvae abundance would decline and adversely impact fish, marine
mammals, seabirds, and ocean biodiversity (high confidence). In addition
to El Niño-Southern Oscillation (ENSO) variability, the persistence of
multi-year climate-ocean regimes and switches from one regime to another have
been recognized since the SAR. Changes in recruitment patterns of fish populations
have been linked to such switches. Fluctuations in fish abundance are increasingly
regarded as biological responses to medium-term climate fluctuations in addition
to overfishing and other anthropogenic factors. Similarly, survival of marine
mammals and seabirds also is affected by interannual and longer term variability
in several oceanographic and atmospheric properties and processes, especially
in high latitudes. [6.3.4]
Growing recognition of the role of the climate-ocean system in the management
of fish stocks is leading to new adaptive strategies that are based on the determination
of acceptable removable percentages of fish and stock resilience. Another
consequence of the recognition of climate-related changes in the distribution
of marine fish populations suggests that the sustainability of many nations'
fisheries will depend on adaptations that increase flexibility in bilateral
and multilateral fishing agreements, coupled with international stock assessments
and management plans. Creating sustainable fisheries also depends on understanding
synergies between climate-related impacts on fisheries and factors such harvest
pressure and habitat conditions. [6.3.4,
6.6.4]
Adaptation by expansion of marine aquaculture may partly compensate for
potential reductions in ocean fish catch. Marine aquaculture production
has more than doubled since 1990, and in 1997 represented approximately 30%
of total commercial fish and shellfish production for human consumption. However,
future aquaculture productivity may be limited by ocean stocks of herring, anchovies,
and other species that are used to provide fishmeal and fish oils to feed cultured
species, which may be negatively impacted by climate change. Decreases in dissolved
oxygen levels associated with increased seawater temperatures and enrichment
of organic matter creates conditions for the spread of diseases in wild and
aquaculture fisheries, as well as outbreaks of algal blooms in coastal areas.
Pollution and habitat destruction that can accompany aquaculture also may place
limits on its expansion and on the survival success of wild stocks. [6.3.5]
Many coastal areas already are experiencing increased levels of sea flooding,
accelerated coastal erosion, and seawater intrusion into freshwater sources;
these processes will be exacerbated by climate change and sea-level rise.
Sea-level rise in particular has contributed to erosion of sandy and gravel
beaches and barriers; loss of coastal dunes and wetlands; and drainage problems
in many low-lying, mid-latitude coastal areas. Highly diverse and productive
coastal ecosystems, coastal settlements, and island states will continue to
be exposed to pressures whose impacts are expected to be largely negative and
potentially disastrous in some instances. [6.4]
Low-latitude tropical and subtropical coastlines, particularly in areas
where there is significant human population pressure, are highly susceptible
to climate change impacts. These impacts will exacerbate many present-day
problems. For instance, human activities have increased land subsidence in many
deltaic regions by increasing subsurface water withdrawals, draining wetland
soils, and reducing or cutting off riverine sediment loads. Problems of inundation,
salinization of potable groundwater, and coastal erosion will all be accelerated
with global sea-level rise superimposed on local submergence. Especially at
risk are large delta regions of Asia and small islands whose vulnerability was
recognized more than a decade ago and continues to increase. [6.4.3,
6.5.3]
High-latitude (polar) coastlines also are susceptible to climate warming
impacts, although these impacts have been less studied. Except on rock-dominated
or rapidly emerging coasts, a combination of accelerated sea-level rise, more
energetic wave climate with reduced sea-ice cover, and increased ground temperatures
that promote thaw of permafrost and ground ice (with consequent volume loss
in coastal landforms) will have severe impacts on settlements and infrastructure
and will result in rapid coastal retreat. [6.4.6]
Coastal ecosystems such as coral reefs and atolls, salt marshes and mangrove
forests, and submergered aquatic vegetation will be impacted by sea-level rise,
warming SSTs, and any changes in storm frequency and intensity. Impacts
of sea-level rise on mangroves and salt marshes will depend on the rate of rise
relative to vertical accretion and space for horizontal migration, which can
be limited by human development in coastal areas. Healthy coral reefs are likely
to be able to keep up with sea-level rise, but this is less certain for reefs
degraded by coral bleaching, UV-B radiation, pollution, and other stresses.
Episodes of coral bleaching over the past 20 years have been associated with
several causes, including increased ocean temperatures. Future sea-surface warming
would increase stress on coral reefs and result in increased frequency of marine
diseases (high confidence). Changes in ocean chemistry resulting from higher
CO2 levels may have a negative impact on coral reef development and
health, which would have a detrimental effect on coastal fisheries and on social
and economic uses of reef resources. [6.4.4,
6.4.5]
Few studies have examined potential changes in prevailing ocean wave heights
and directions and storm waves and surges as a consequence of climate change.
Such changes can be expected to have serious impacts on natural and human-modified
coasts because they will be superimposed on a higher sea level than at present.
Vulnerabilities have been documented for a variety of coastal settings,
initially by using a common methodology developed in the early 1990s. These
and subsequent studies have confirmed the spatial and temporal variability of
coastal vulnerability at national and regional levels. Within the common methodology,
three coastal adaptation strategies have been identified: protect, accommodate,
and retreat. Since the SAR, adaptation strategies for coastal zones have shifted
in emphasis away from hard protection structures (e.g., seawalls, groins) toward
soft protection measures (e.g., beach nourishment), managed retreat, and enhanced
resilience of biophysical and socioeconomic systems, including the use of flood
insurance to spread financial risk. [6.6.1,
6.6.2]
Integrated assessments of coastal zones and marine ecosystems and better understanding
of their interaction with human development and multi-year climate variability
could lead to improvements in sustainable development and management. Adaptation
options for coastal and marine management are most effective when they are incorporated
with policies in other areas, such as disaster mitigation plans and land-use
plans.
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