You wake up abruptly as police sirens begin blaring outside of your home. As you stumble to turn on the light, you hear something being said on a bullhorn about evacuation. A minute later, air raid sirens join in the frenzy. You turn on the television to catch the news, and there are images of Antarctica, and ice, and enhanced satellite images. What could possibly be the connection between the peaceful and serene images of the frozen continent and all of this noise? You listen carefully to the live news report. “At 11:53 PM yesterday evening—slightly more than two hours ago, a large portion of the West Antarctic Ice sheet slid violently into the adjoining polar ocean.” You think, “Well that’s great news, but what does it have to do with me, and why all of this fuss?” The news anchor continues, “All residents of coastal states living within 200 miles of the ocean or coastal bays are requested to evacuate immediately. The National Oceanographic and Atmospheric Administration has predicted that the water displacement generated by the slide of this ice sheet into the ocean will send a tsunami wave of 10 meters along the edge of the Atlantic Ocean basin. It is predicted that the South Atlantic regions of Florida, Georgia, and South Carolina will be impacted by this wave in roughly 16 to 17 hours. This evacuation will be permanent because the global sea level after the wave passes will be 6 meters above current sea level. We now transfer you to the press room of the White House for an emergency address by the President.” You pinch yourself hard on the cheek thinking this must be a dream, this is impossible! But is it?
It is thought that water began to accumulate on Earth’s surface between 4.2 and 4.4 billion years ago, after the crust cooled below the boiling point of water. Water that now covers 71% of the earth’s surface was derived from two main sources, outgassing of water-laden volcanic gases, and from fragments of comets impacting the upper atmosphere. Compared to the early Earth’s history, the rates of input of water onto the earth’s surface has slowed, and it is now thought that the volume of water (in all its states) on the surface has been relatively constant for several billion years. This implies that there are also mechanisms causing a slow loss of water away from Earth’s surface. These losses could be due to recycling of Earth’s crust and water-laden ocean sediments, as well as the slow escape of water vapor from the upper atmosphere into space.
This relative constancy of water mass on the planet over the last several billion years does not, however, mean that sea level has remained constant. Still, the 220-meter range of sea level variation over this period is small compared to the mean depth of the ocean—3,800 meters.
Sea Level Changes
Changes in sea level, which have occurred over most of the earth’s geologic history, are due to two processes. Eustatic processes change the absolute amount of liquid water within the ocean basins. The main mechanism driving eustatic changes in sea level is the reproportioning of water between liquid and solid phases (ice) due to changes in global climate. Isostatic processes change the underlying topography of the sea floor. These changes can occur on either regional scales, as in the rebound of crust after deglaciation, or in the slow subsidence of deltas at passive continental margins, or on global scales, as in periods of marked increases in sea floor spreading rates. This increases the height of ridge and rise features throughout the global ocean basin, in turn causing displacement of water upward onto the coastal continental landscape.
Changes in sea level have been implicated directly and indirectly as contributing to mass extinction events that have occurred within the earth’s geologic past. Rapid sea level decline has even been hypothesized to cause changes in atmospheric oxygen levels. In this case, rapid decay (oxidation) of shallow, newly exposed organic-rich marine deposits would remove oxygen from the atmosphere.
Sea level has been rising since the end of the last glaciation about 15,000 years ago. The rate of global sea level rise for the last 100 years has been 2 mm/year and 15–20 cm total (0.08 in/year, 6–8 in total). Estimates for global sea level rise for the next century suggest that this rate will double to 4 mm/year and 40–45 cm total (0.16 in/year, 16–18 in total). The observed rate of coastal sea level rise varies from region to region because of the variability in isostatic crustal movement (Figure 1). If future increases in sea level become more rapid, shallow water intertidal or subtidal communities may not be able to keep pace with sea level change and will die off as environmental conditions change beyond their limits of tolerance. Such a change is now occurring in intertidal salt marsh environments around the Mississippi River delta in Louisiana, as the coastal landscape subsidence combined with global sea level rise exceeds the rate of vertical growth of the marsh community.
Trends of sea level rise within coastal city regions of the United States. Note the higher rate of sea level rise for Galveston, Texas which is experiencing coastal subsidence, whereas in Sitka, Alaska sea level appears to be falling because of coastal uplift. (NOAA National Ocean Service)
Impacts of Global Warming
The newfound concern about global warming impact stems from the virtual consensus among climatologists that the planet is experiencing a period of global warming.
Global warming may result in an increase in the rate of sea level rise due to thermal expansion of seawater and the melting of ice in glaciers and polar regions (Figure 2). The coefficient of thermal expansion of seawater is 0.00019 per degree Celsius, meaning that if a volume of seawater occupied 1 cubic meter of water (1000 L, 264.20 gal), after warming by 1°C, it would expand to 1.00019 m3 (1000.19 L, 264.25 gal). Translated over the mean depth of the ocean (3.8 km, 2.4 mi), an increase in temperature of 1°’C will cause a sea level rise of about 70 cm (28 in). Whereas thermal expansion acts upon water already in the basin, contributions from melting ice represent new, added water to the present ocean volume. The melting of ice that is currently perched upon land, as in the ice sheets of Greenland, Iceland, and the Antarctic, has the potential to raise sea level considerably, by about 80 meters (262 ft). Ice that is already floating in the ocean water, as in the Arctic ice mass, Antarctic ice shelves, and much smaller icebergs, may melt but will not contribute to sea level rise since the mass of water contained in these features already displaces its equivalent water volume.
Landsat 1 MSS digitally enhanced image of the Byrd Glacier in Antarctica, where it joins the Ross Ice Shelf. Melting of glaciated areas will directly increase sea level, whereas the melting of shelf ice will not cause a rise in sea level but may be an important indicator of regional climate change. (NOAA Central Library Photo Collection)
There is ample evidence that the increase in melting has begun. Monitoring of mountain glaciers throughout the world over the last two decades indicates that 75% of them are losing mass and that the rate of this loss has almost doubled, from 0.25 m to 0.50 m (0.82 to 1.64 ft) of water equivalent, in the last 20 years. In 1991, NASA reported that the extent of sea ice in the Arctic Ocean declined by 2% between 1978 and 1987. More recently, Norwegian scientist Ola Johannessen, of the Nansen Environmental and Remote Sensing Center, has presented satellite measurements of microwave emissions that show declines in permanent Arctic ice of 7% over each of the past two decades.
In the Antarctic, a series of ice shelves (Wordie, Larsen a, and Larsen b) have collapsed over the last decade spurred by the 2.5°C increase in the average temperature on the peninsula since the mid-1940s. There is some concern that loss of these ice shelves may destabilize continental ice sheets, with the Western Antarctic Ice Sheet being considered as potentially capable of slippage into the surrounding ocean.
Which Areas will be Impacted?
Sea level rise will produce increased flooding risks in areas already below sea level, like New Orleans, Louisiana in the United States and coastal Holland in Europe. The impact of sea level rise will also be severe for certain small island nations. A series of low-lying islands in the Pacific (Marshall, Kiribati, Tuvalu, Tonga, Line, Micronesia, Cook), Atlantic (Antigua, Nevis), and Indian Oceans (Maldives), will be greatly impacted. For example, in the Maldives most of the land is less than 1 meter (3.05 ft) above sea level. A 450-acre seawall recently built to surround the Maldivian capital atoll of Malè cost the equivalent of 20 years of the entire Maldivian gross national product, according to U.N. reports. Coastal regions that possess little geographic relief and regions which are also experiencing subsidence due to sediment accumulation are also at threat.
The largest population at risk are the people living in Bangladesh. About 17 million Bangladeshis live less than 1 meter (3.05 ft) above sea level. In Southeast Asia, a number of large cities, including Bangkok, Bombay, Calcutta, Dhaka, and Manila (each with populations greater than 5 million), are located on coastal lowlands or on river deltas. Particularly sensitive areas in the U.S. include the states of Florida and Louisiana, coastal cities, and inland cities bordering estuaries.
Recent technological advancement in climate research and satellite remote sensing have enabled scientists to more fully understand the mechanisms that cause climate change and sea level response. Paleoclimate tools include an arsenal of fossil, geophysical, and geochemical indicators that are sensitive to changing climate and can be interpreted back in time to provide evidence of past environmental conditions. Such data can then be applied to model the temporal and spatial patterns of future climate change and sea level response. Satellite remote sensing now permits the collection of large-scale regional and global data sets that prior to the 1950s were only a dream. Such an increase in the areal extent of examination allows earlier detection of climate change, as it integrates important areas, previously difficult to study, such as the extreme polar regions. Other useful information is coming from declassified military information on sea ice draft (thickness under water) from the Arctic region. Submarines and permanently moored devices are now continually monitoring this portion of the earth’s ice mass.