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 change impacts.

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 structures.