Thursday, March 27, 2008

Global Warming and Sea Level Rise

Global Warming and Sea Level Rise
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?
BackgroundIt 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.
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