Sea levels, oceans, and coastal areas
The global average sea level has risen by 10 to 20 cm over the past 100 years. The rate of increase
has been 1 – 2 mm per year – some 10 times faster than the rate observed for the previous 3,000
years. It is likely that much of this rise is related to an increase of 0.6±0.2°C in the lower
atmosphere's global average temperature since 1860. Related effects now being detected include warming
sea-surface temperatures, melting sea ice, greater evaporation, and changes in the marine food web.
Models project that sea levels will rise another 9 to 88 cm by the year 2100. This will occur due to
the thermal expansion of warming ocean water and an influx of freshwater from melting glaciers and ice. The
rate, magnitude, and direction of sea-level change will vary locally and regionally in response to coastline
features, changes in ocean currents, differences in tidal patterns and sea-water density, and vertical
movements of the land itself. Sea levels are expected to continue rising for hundreds of years after
atmospheric temperatures stabilize.
Coastal zones and small islands are extremely vulnerable. Coasts have been modified and intensively
developed in recent decades and thus made even more vulnerable to higher sea levels. Developing countries
with their weaker economies and institutions face the gravest risks, but the low-lying coastal zones of
developed countries could also be seriously affected. Already over the past 100 years, 70% of sandy
shorelines have been retreating.
Flooding and coastal erosion would worsen. Salt-water intrusion will reduce the quality and quantity
of freshwater supplies. Higher sea levels could also cause extreme events such as high tides, storm surges,
and seismic sea waves (tsunami) to reap more destruction. Rising sea levels are already contaminating
underground fresh water supplies in Israel and Thailand, in small atolls scattered across the Pacific and
Indian oceans and the Caribbean Sea, and in some of the world’s most productive deltas such as
China’s Yangtze Delta and Vietnam’s Mekong Delta.
Sea-level rise could damage key economic sectors . . . A great deal of food is produced in coastal
areas, making fisheries, aquaculture, and agriculture particularly vulnerable. Other sectors most at risk are
tourism, human settlements, and insurance (which has already suffered record losses recently due to extreme
climate events). The expected sea-level rise would inundate much of the world's lowlands, damaging
coastal cropland and displacing millions of people from coastal and small-island communities.
. . . and threaten human health. The displacement of flooded communities, particularly those with
limited resources, would increase the risk of various infectious, psychological, and other illnesses. Insects
and other transmitters of disease could spread to new areas. The disruption of systems for sanitation,
storm-water drainage, and sewage disposal would also have health implications.
Valuable coastal ecosystems will be at serious risk. Coastal areas contain some of the world's
most diverse and productive ecosystems, including mangrove forests, coral reefs, and sea grasses. Low-lying
deltas and coral atolls and reefs are particularly sensitive to changes in the frequency and intensity of
rainfall and storms. Coral will generally grow fast enough to keep pace with sea-level rise but may be
damaged by warmer sea temperatures.
Ocean ecosystems may also be affected. In addition to higher sea levels, climate change will reduce
sea-ice cover; decreases of up to 14% have been measured in the Arctic during the past two decades, and a
decline of 25% has been recorded in the Antarctic from the mid-1950s to early 1970s. Climate change will also
alter ocean circulation patterns, the vertical mixing of waters, and wave patterns. These changes can be
expected to affect biological productivity, the availability of nutrients, and the ecological structure and
functions of marine ecosystems. Changing temperatures could also cause geographical shifts in biodiversity,
particularly in high-latitude regions, where the growing period should increase (assuming light and nutrients
remain constant). Any changes in plankton activity could affect the oceans' ability to absorb and store
carbon. This could "feedback" into the climate system and either moderate or boost climate change.
Various natural forces will influence the impact that higher sea levels will have. Coastal areas are
dynamic systems. Sedimentation, physical or biotic defenses (such as coral reefs), and other local conditions
will interact with rising sea-water. For example, freshwater supplies in coastal zones will be more or less
vulnerable depending on changes in freshwater inflows and the size of the freshwater body. The survival of
salt marshes and mangrove forests will depend in part on whether the rate of sedimentation is greater than or
less than the rate of local sea-level rise. Sedimentation is more likely to exceed sea-level rise in
sediment-rich regions such as Australia, where strong tidal currents redistribute sediments, than in
sediment-starved environments such as the Caribbean.
Human activities will also play a role. Roads, buildings, and other infrastructure could limit or
affect the natural response of coastal ecosystems to sea-level rise. In addition, pollution, sediment
deposits, and land development will influence how coastal waters respond to, and compensate for, climate
Many policy options are available for adapting to sea-level rise. Sensitive environmental, economic,
social, and cultural values are at stake, and trade-offs may be unavoidable. Possible response strategies
include protection (dikes, dune restoration, wetland creation), accommodation (new building codes, protection
of threatened ecosystems), and planned retreat (regulations against new coastal development). Some countries,
including Australia, China, Japan, the Netherlands, the UK, and the US, have already designated withdrawal
corridors where buildings will be removed to allow precious wetlands to move inland. Other specific responses
are dredging ports, strengthening fisheries management, and improving design standards for offshore