It is difficult to accurately predict future sea levels. There are many factors to consider, and there is not a linear relation to global temperature variation and sea level. However, it is clear that over the past 100 years sea level has risen about 1/2 foot. Scientists are now predicting that this rate will double in the next 50 years.

An important factor to consider is that coastlines are not regular and do not have the same slope. Although a near-term rise of a few inches does not sound like a great concern, those areas with very mild slopes will have a large encroachment with only a small global rise. An accelerated rise in sea level would inundate coastal cities, wetlands, and lowlands. In some parts of the world entire nations are at risk of going under water with rising seas. For example, Bangladesh and the Maldives are lands at terrible risk for loss of life, homes, and businesses due to their low elevation. In the United States, the state of Florida is particularly at risk and state and local government officials are beginning to acknowledge this threat (see Climate Worries Surface in Florida). Nearly half of the US population is expected to live near coastlines in the coming decade (see Figure below). Globally this estimate increases to 80%!

As sea level rises the majority of the world’s population will face critical issues related to their way of life and even their very survival. Damage due to coastal flooding results in economic and habitat loss for humans as well as plant and animal species. Rising seas are also expected to increase the rate of erosion along the shorelines of beaches and marshes, threatening coastal structures on private lands as well as some of our favorite recreational hotspots. Near Recife, Brazil the shoreline has receded more than 8 feet per year from 1985 to 1995. The dramatic land loss was attributed to a combination of sea level rise and loss of sediment supply from human alteration of the landscape (e.g. dam construction, harbor dredging).

Another problem with sea level rise is the mixing of ocean water with fresh water supplies causing an increased salinity of coastal rivers and bays. Estuarine habitats change with a greater influx of salt water and intertidal communities must move to higher elevations. In Bermuda this is already evident as mangrove forests are dying due to salt water inundation. Also, as water tables rise many fresh water aquifers may be in danger. Groundwater is already becoming saline in some locations through salt-water intrusion that accompanies subsidence (a lowering of the land surface due to groundwater depletion). Consequently, cities may have to reduce the rate at which they currently pump water from aquifers, forcing them to seek other sources to supply fresh water.

This graph show the sea level rise expected from 1900 to 2100 under a "business-as-usual" scenario (i.e., no reduction in human-induced greenhouse gas emissions). As with most projections, there is a great deal of uncertainty in the actual amount that sea level will rise in the future. The graph demonstrates that our uncertainty increases the farther ahead in time that predictions are made. Of the factors that contribute to sea level rise, thermal expansion of ocean volume accounts for most of the projected rise. Mountain glaciers account for 49% of the predicted rise, whereas in the past 100 years melting has accounted for only a small portion of the rise in sea level. Melting of the Greenland ice cap is expected to contribute about 12% of the increase.

It is interesting to note that in some models the Antarctic ice cap is predicted to increase, which would counteract some of the melting in other areas. Some scientists predict that warm temperatures should produce more moisture and cause more snow to accumulate at the poles. However, recent calving (the process of icebergs breaking away from larger bodies of ice) of large ice sheets in Antarctica does not lend much support this argument. After 400 years of relative stability, nearly 1,150 square miles of Antarctic ice shelves have collapsed in the last 2 years. Warming on the Antarctic Peninsula is 5 times the global average; since 1945 the region has experienced a warming of about 4.5 degrees Fahrenheit.