Effects of Global Warming: Sea Level

Arctic Ice Cover in 1979 and 2005

Surface temperatures in the Arctic have risen almost twice as much as the global average. Satellites have made continual observations of Arctic sea ice since 1978. The extent of Arctic sea ice has declined by about 8.5 percent per decade from its size in 1979 (top image). Since 2002 satellite records have revealed unusually early onsets of springtime melting in the areas north of Alaska and Siberia, as well as a smaller recovery of sea ice during the winter season. In 2005 (bottom image) satellites showed the lowest extent of Arctic sea ice on record.

      As the atmosphere warms, the surface layer of the ocean warms as well, expanding in volume and thus raising sea level. The melting of glaciers and ice sheets, especially around Greenland, further swells the sea. Sea level rose 10 to 25 cm (4 to 10 in) during the 20th century. (The range is due to measurement uncertainties and regional variation.) By the end of the 21st century, sea level is projected to rise another 28 to 58 cm (11 to 23 in) if greenhouse gas emissions continue to increase significantly. The projection is somewhat less—a rise of 19 to 37 cm (8 to 15 in)—for a scenario in which greenhouse gas emissions peak around the year 2050 and then decrease. These projections do not incorporate possible large-scale melting of the Greenland or Antarctic ice sheets, which could begin in the 21st century with warming of a few degrees Celsius.

    Rising sea level will complicate life in many island and coastal regions. Storm surges, in which winds locally pile up water and raise the sea, will become more frequent and damaging. Erosion of cliffs, beaches, and dunes will increase. As the sea invades the mouths of rivers, flooding from runoff will also increase upstream.

Small island nations such as Tuvalu and Kiribati, where the highest land is only a few meters above sea level, are already experiencing saltwater intrusion, which is making groundwater undrinkable, and increased impacts from typhoons and heavy surf. These nations could literally cease to exist as the rise in sea level continues, and their governments are negotiating with other nations to transplant their populations.

Even a modest rise in sea level will have huge impacts on coastal ecosystems. For example, a 50-cm (20-in) rise would submerge about half of the present coastal wetlands of the United States and other low-lying areas such as parts of New Orleans and the Louisiana coast. Much of the Florida Everglades would be lost to the sea. New marshes would eventually form in many places, but not where urban areas and developed landscapes block the way.

     Damage can be curbed locally in various ways. Coastlines can be armored with dikes, levies, and other barriers to block encroachment of the sea. Alternatively, governments can assist coastal populations in moving to higher ground, although such a process is extremely costly, especially in heavily populated areas. Some extremely low-lying countries would face rising sea level with huge populations at risk. Wealthy countries like The Netherlands may need to spend huge amounts of money to protect their shorelines, while poor countries like Bangladesh may be forced to simply abandon low-lying coastal regions.