Bilal U. Haq

Division of Ocean Sciences,

National Science Foundation

　

Bilal U. Haq 博士

米国科学財団

海洋科学部門

　

Arlington, VA 22230

　　Natural instabilities in the Earth's climatic, hydrographic and tectonic regimes cause major sea-level fluctuations over the longer time scales (tens of thousands of years). In the recent geological past, since the end of the last glacial period (about 17,000 years ago), the sea level has risen by about 110 m following the melting of continental ice sheets that covered much of the mid to high latitudes during the ice age. Rates of sea-level rise during the deglaciation at times exceeded 20 mm/year. Global sea level reached a highstand nearly 6000 years ago and has been largely stable for the past four millennia - a period that spans much of the recorded human history. Thus, any instability in sea level caused by anthropogenic activities could have dramatic consequences for maritime communities.

　　Since the time of the industrial revolution human activities have altered the global environment in an increasingly significant manner. Especially in the 20^th century, the rapid burning of fossil fuels continued to alter atmospheric composition by increasing the emissions of greenhouse gases. Unmanaged land and water use and resource exploitation have also significantly modified natural rates of subsidence in coastal regions that accelerate relative sea-level rise locally, thereby increasing threat to large populated coastal areas. Although the expected magnitude of global sea-level change remains a guesstimate, the trend toward increase in greenhouse warming that could lead to appreciable sea-level rise seems obvious. But the rate at which the sea might advance across the shoreline remains uncertain. Current best estimates suggest that thermal expansion of ocean waters and the melting of the ice fields due to increased surface temperature are most likely to raise the sea level between 0.5 to 1.0 m by the end of 21^st century.

　　Thus, it is vital that various scenarios of sea-level rise be evaluated so that we can determine how, and over what time scale, sea-level rise might impact coastal communities where a large and ever-growing population of the world resides. The projected half to one meter rise in mean sea level may result in major shifts of coastlines, with important socio-economic implications for maritime nations.

　　Existing data suggest that greenhouse gases in the atmosphere have been increasing since the mid 1700s, and especially since the mid 1950s, threatening to enhance the atmosphere's greenhouse potential and increase surface temperatures. Recent climatic record also contains indications of rise in global temperatures, of waning mountain glaciers and enhanced cloudiness in mid latitudes. Empirical data shows that human influence has led to warming of both the atmosphere and the ocean during the past century, especially in the last 50 years.

　　In the recent geological past there is clear evidence that when greenhouse gases such as CO2 and methane increase in the atmosphere from natural sources, such as volcanoes, gas hydrates and ocean degassing, it leads to increases in surface temperatures. Air bubbles from ancient atmospheres trapped in ice on Antarctica and Greenland when analyzed show that in the last 220,000 years CO2 and methane increases in the atmosphere occurred in tandem with increases in surface temperatures.

　　At the current rates of emissions worldwide, CO2 contributes a cumulative 60% of total atmospheric greenhouse forcing. The Intergovernmental Panel on Climate Change (IPCC) has estimated that CO2 concentrations in the atmosphere have steadily increased by over 26% from the pre-industrial level to 1990s, with a particularly rapid increase occurring since the 1950s.

　　Of the other important greenhouse gases, over the same period, methane has increased by over 53%, while nitrous oxide increased by over 7% in the last few decades largely due to the use of chemical fertilizers in agriculture. An estimated 20% of the methane comes from fossil fuels, and a smaller percentage from agriculture. Man-made chloroflorocarbons (CFCs) that are potent greenhouse gases, invented in the 1930s, have only been in extensive use since the 1950s, but their concentrations in the atmosphere have increased exponentially, by as much as 4% per year.

　　Physical evidence of warming also comes from the shrinking mountain glaciers throughout the world and signs of oceanic warming. Episodes of anomalously high temperatures are leading to rapid bleaching of the reef corals, which are forecast to potentially die out by the end of 21^st century if the present trends continue.

　　Thus, trends exists which points to appreciable increase in greenhouse gases in the atmosphere in recent decades. When this trend is projected into the future it argues convincingly for a doubling of CO2 to pre-industrial levels by the mid 21^st century. Estimates of global mean surface temperature increase following the doubling of CO2 are within the range of 1.5° and 4.5℃.

　　It is also evident from the tide gauge records that sea level has been rising regionally at a rate between 1 and 2 mm per year our the past century. Although the tide gauges measure only relative change in sea level in any given location, the empirical records from all coastal regions where tide-gauge data are available indicate a general sea-level rise through this century. IPCC has suggested that global sea level has risen by an estimated 15 cm since the early twentieth century, largely due to the thermal expansion of sea water in response to increasing global temperatures. In their "business-as-usual" scenario (which assumes continued increase in greenhouse emissions at current rates), they have projected a high, a low, and a "best-estimate" value of sea-level rise for the next century, with a "best-estimate" value of 0.66 m. This panel has underscored the fact that even if at some future agreed upon date, e.g. the year 2030, greenhouse emissions were to be stabilized and increased no further, the sea level could continue to rise for several decades, or even centuries, due to the time lag in response of the atmospheric, and, particularly, the oceanic systems.

　　One important limiting factor in any meaningful prediction of global sea-level change is the uncertainty concerning the response of the Antarctic ice sheets to increased greenhouse warming. Relatively large amounts of water are locked up in these ice fields. East Antarctic Ice Sheet is by far the largest, containing ice with a meltwater equivalent to nearly 65-m of sea-level rise. Ice sheets on West Antarctic and Greenland are considerably smaller, containing only about 8.5 m and 7 m of sea level rise equivalent ice, respectively. In comparison, the contribution of mountain glaciers and smaller ice sheets to sea-level rise would be less significant, adding only a third of a meter to sea level rise.

　　There is considerable uncertainty about the longer-term behavior of East Antarctic ice sheet. A recent evaluation of the stratigraphic and chronologic data from this ice sheet implies that in the Pliocene time, some 3 million years ago, when global mean temperatures were 2° to 3℃ higher than now, the East Antarctic Ice Sheet was considerably smaller and West Antarctica was largely an open seaway. Pliocene sea levels of that time have been estimated to be 50 to 60 m's higher than at present. This observation, though controversial, becomes relevant to the present discussion because IPCC's "business-as-usual" scenario implies an increase in global mean temperature at the rate of 0.3℃ per decade, a total of about 1℃ by the year 2025, and by as much as 4.5℃ by the end of the next century. How this might affect major ice melting on Antarctica remains uncertain.

　　It seems likely that the major component of the near-future sea-level rise will be largely due to thermal expansion of seawater, with a minor contribution from melting of the ice caps. It needs o be underscored that these global sea-rise rise values, exacerbated by anthropogenic factors, are only a small component of the total relative sea-level rise expected in some coastal zones where human activities may have greatly accelerated local subsidence of the sea floor.

　　The effective shoreline advance (landward) or retreat (seaward) in any location is affected by both the global sea-level component (discussed above), and regional vertical crustal movements along the continental margins (either subsidence or emergence). When the seafloor subsides the shoreline moves landward, producing an effective relative sea-level rise. In many parts of the world, human activities in the coastal areas have significantly altered natural rates of subsidence, so that subsidence may now pose a much greater threat to the health of coastal states than the projected acceleration in global sea-level rise caused by global warming.

　　Data on vertical crustal movements along continental margins shows that with the exception of northern higher latitudes nearly all coastal areas are subsiding at varying rates. Subsidence rates are particularly high in river deltas and some major urban coastal areas. It is clear that the combined effects of global sea-level rise and regional subsidence can magnify the sea-level rise locally, that may be detrimental to some coastal regions and a major issue of societal relevance in the 21^st century.

　　The following brief discussion includes examples from densely populated deltas, coastal cities, and small island nations that may be particularly vulnerable to subsidence and/or global rise of sea level.

　　River deltas have relatively large natural subsidence rates due to continuous loading of sediment supplied by the rivers. This sediment has a large water content whose in situ dewatering under pressure adds to the total subsidence. Normally, high delta subsidence is compensated by new sediment deposition allowing the system to maintain equilibrium. However, when a river is dammed or diverted, its immediate impact is the loss downstream of replenishing sediment and a dramatic reduction in the flow of fresh water to the delta. The loss of sediment supply means that natural subsidence is not compensated by new sediment accumulation. In addition, reduction in the flow of river water may cause extreme stress in the delicate ecosystems of the coastal brackish-water environment that maintain a precarious balance between fresh-water flow and intrusion of saline sea water in the estuaries. Consequently, a delta may experience local sea-level rise and intrusion of salt water further inland, leading to declining biological productivity and increased biodiversity loss in the coastal ecosystems. In addition, if coastal regions are tapped excessively for ground water or if hydrocarbons are withdrawn too rapidly, these can accelerate local subsidence and associated detrimental effects. Two deltas are discussed below, but similar adverse effects of upstream damming are also apparent in other deltas, such as the Indus and the Niger Deltas.