GLOBAL WARMING AND SEA LEVEL RISE

Questions we would like you to consider in this issue are:

What factors cause changes in sea level?

How can scientists estimate the magnitude of sea level rise given the thermal expansion of seawater and melting of ice associated with sustained global warming?

How can we assess the severity of impact of sea level rise to shallow coastal communities and human populations given the best estimate of sea level rise over the next 100 years?

2:06 am 02/22/2001; 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 Antartica, 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 West Antarctic Ice sheet slid violently into the the adjoining polar ocean". You think well thats 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 embayments are requested to evacuate immediately. The National Oceangraphic 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?

Background

It is thought that water began to accumulate on the earth's surface between 4.2 and 4.4 billion years ago, after the crust cooled below the boiling point of the water molecule. This fluid [water] which 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: solid, liquid and vapor) 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 the earth's surface. These losses could be due to recycling of the 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 range of sea level variation over this period, which is about 220 meters, is small compared to the mean depth of the ocean - 3,800 meters.

Sea Level Changes

Changes in sea level which have occured over most of the earth's geologic history are due to two processes. Processes that change the absolute amount of water (as a liquid) within the ocean basins are called eustatic. 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 occured 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 2mm / year (15-20 cm total). Estimates for global sea level rise for the next century suggest that this rate will at least double to 4mm year (40 - 45 cm total). 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 succumb as environmental conditions change beyond their limits of tolerance. Such a change is now occuring in intertidal salt marsh environments around the Mississippi River delta in Lousaiana, as the coastal landscape subsidence combined with global sea level rise exceeds the rate of vertical growth of the marsh community.

Figure 1. Trends of sea level rise within coastal city regions of the United States. Note the higher rate of sea level rise for Galveston, TX which is experiencing coastal subsidence.

Impacts of Global Warming

Although ample scientific evidence now exists of geo-historical impacts of changing sea level upon biological communities, humans have only recently become concerned with the potential of such processes to change their lifestyles and standards of living. This newfound concern stems from the virtual concensus 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. Thermal expansion of seawater involves increasing the distance between neighboring water molecules. This distance increases with increased temperature above temperatures of 3.98 ° C. The coefficient of thermal expansion of seawater is 0.00019 per degree Celsius (1° C), meaning that if a volume of seawater occupied 1 cubic meter (1m³), after waming that volume by 1° C, it would then expand to 1.00019 m³. Translated over the mean depth of the ocean (3.8 km), a 1 degree increase in temperature will cause a sea level rise of about 70 cm. Whereas thermal expansion acts upon water already in the basin, contributions from melting ice represent a mechanism which will add new water to the present ocean volume. The melting of ice that is currently perched upon terrestrial land, as in the ice sheets of Greenland, Iceland, and the Antarctic have the potential to raise sea level considerably (80 meters total). Ice that is already floating upon the ocean water as in the Artic ice mass, Antarctic ice shelves, and much smaller ice bergs may melt but will not contribute to sea level rise since the mass of water contained in these features has already displace its equivalent water volume.

Figure 2. Glaciated region near McMurdo Sound, Antarctica. The mass of ice associated with the Antarctic ice sheets represents 95% of all ice present on the earth. Although there is evidence that Antarctic ice shelves (over water) have decreased in extent, there is no substantual evidence that ice sheets (over land) have declined in mass.

There is ample evidence that the increase in melting has begun. Monitoring of mountain glaciers throughout the world over the last 2 decades indicate that 75% of them are lossing mass, and that the rate of this loss has almost doubled (from 0.25 m to 0.50 m 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 percent over each of the past two decades (Figure 3).

Figure 3. Recent satellite data shows multiyear (MY) Arctic sea ice declining over time.

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-degree Celsius 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

The impacts of sea level rise will be severe for certain countries. 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 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 people in Bangladesh live less than one meter above sea level (see Issue 1). Bangladesh is already vulnerable to flooding: in 1971, 20 million people were made homeless in a flood. In 1992, 125,000 people died in a cyclone. In Southeast Asia a series of large city megalopises including Bangkok, Bombay, Calcutta, Dhaka, and Manila (each of which have populations greater than 5 million), are located on coastal lowlands or on river deltas. Particlarly sensitive areas in the U.S. include the states of Florida and Lousaiana, 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 which cause climate change and sea level response. Paleoclimate tools include an arsenal of fossil, geophysical, and geochemical indicators which 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 can now permit the collection of large scale regional and global data sets that prior to the 1950's were only a dream. Such an increase in the areal extent of examination allows more robust and earlier detection of climate change, as it integrates important areas, previously difficult to study - like 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.

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