THE BASIS FOR EXPECTING A RISE IN SEA LEVEL 



Past Changes in Climate and Sea Level 



Throughout geologic history, sea level has risen and fallen by over three hundred meters 

 (one thousand feet). Although changes in the size and shape of the oceans' basins have played a 

 role over very long periods of time (Hays and Pitman 1973), the most important changes in sea 

 level have been caused by changes in climate. During the last ice age (18,000 years ago), for 

 example, the earth was about five degrees Celsius colder than today, glaciers covered most of the 

 northern hemisphere, and sea level was one hundred meters (three hundred feet) lower than it is 

 today (Donn, Farrand, and Ewing 1962). 



Although most of the glaciers have melted since the last ice age, polar glaciers in Greenland 

 and Antarctica still contain enough water to raise sea level more than seventy meters (over two 

 hundred feet) (Untersteiner 1975). A complete melting of these glaciers has not occurred in the 

 last two million years, and would take tens of thousands of years even if the earth warmed 

 substantially. However, unlike the other glaciers, which rest on land, the West Antarctic Ice Sheet 

 rests in the ocean and is thus more vulnerable. Warmer ocean water would be more effective than 

 warmer air at melting glaciers and could melt the ice shelves that prevent the entire glacier from 

 sliding into the oceans. Mercer (1970) suggests that the West Antarctic Ice Sheet completely 

 disappeared during the last interglacial period (which was one or two degrees warmer than today 

 and occurred 100,000 years ago), at which time sea level was five to seven meters (about twenty 

 feet) above its present level. 



Over periods of decades, climate can influence sea level by heating and thereby expanding 

 (or cooling and contracting) sea water. In the last century, tidal gauges have been available to 

 measure relative sea level in particular locations. Along the Atlantic Coast, sea level has risen 

 about 30 centimeters (one foot) in the last century (Hicks, Debaugh, and Hickman 1983). Studies 

 combining tide gauge measurements around the world have concluded that average global sea 

 level has risen ten to fifteen centimeters (four to six inches) in the last one hundred years (Barnett 

 1983; Gornitz, Lebedeff, and Hansen 1982). About five centimeters of this rise can be explained 

 by the thermal expansion of the upper layers of the oceans resulting from the observed global 

 warming of 0.4 °C in the last century (Gornitz, Lebedeff, and Hansen 1982). Meltwater from 

 mountain glaciers has contributed two to seven centimeters since 1900 (Meier 1984). Figure 1-1 

 shows that global temperature and sea level appear to have risen in the last century. Nevertheless, 

 questions remain over the magnitude and causes of sea level rise in the last century. 



The Greenhouse Effect and Future Sea Level Rise 



Concern about a possible acceleration in the rate of sea level rise stems from measurements 

 showing the increasing concentrations of carbon dioxide (C0 2 ), methane, chlorofluorocarbons, 

 and other gases released by human activities. Because these gases absorb infrared radiation 

 (heat), scientists generally expect the earth to warm substantially. Although some people have 

 suggested that unknown or unpredictable factors could offset this warming, the National 

 Academy of Sciences (NAS) has twice reviewed all the evidence and concluded that the warming 

 will take place. In 1979, the Academy concluded: "We have tried but have been unable to find 

 any overlooked physical effect that could reduce the currently estimated global warming to negli- 

 gible proportions" (Chamey 1979). In 1982, the NAS reaffirmed its 1979 assessment 

 (Smagorinsky 1982). 



A planet's temperature is determined primarily by the amount of sunlight it receives, the 

 amount of sunlight it reflects, and the extent to which its atmosphere retains heat. When sunlight 

 strikes the earth, it warms the surface, which then reradiates the heat as infrared radiation. 

 However, water vapor, C0 2 , and other gases in the atmosphere absorb some of the radiation 



