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Read this essay to learn about the rise in sea level as a result of global warming.
Over the last 100 years, the global sea level has risen by about 10 to 25 cm. It is likely that much of the rise in sea level has been related to the concurrent rise in global temperature over the last 100 years. On this time scale, the warming and the consequent thermal expansion of the oceans may account for about 2-7 cm of the observed sea level rise, while the observed retreat of glaciers and ice caps may account for about 2-5 cm.
Other factors are more difficult to quantify. The rate of observed sea level rise suggests that there has been a net positive contribution from the huge ice sheets of Greenland and Antarctica, but observations of the ice sheets do not yet allow meaningful quantitative estimates of their separate contributions. The ice-sheets remain a major source of uncertainty in accounting for past changes in sea level because of insufficient data about these ice-sheets over the last 100 years.
Sea level is obviously directly related to extremes of climate change. As a gross example, at the beginning of the last de-glaciation 18,000 years ago sea level was everywhere about 100 meters lower than it is now. The more subtle question is, can we observe future small (fractional mm) changes of sea level quickly enough, and also interpret them, to provide a useful index of smaller but important changes of climate?
Using estimates of eustatic sea level rise as an indicator of climate change faces the difficulty that sea level rise is an output combining many individual effects. Some of these effects can offset others, so that the exact response of global sea level to climate change remains somewhat uncertain.
It is necessary to work out a hydrological and geophysical budget for the various contributors to local and global sea level change. For example, global warming will cause both expansion of the ocean and changes of circulation. In addition, the melting of small glaciers, while difficult to quantify, is also significant. These two effects together can account for about 1 mm per year of sea level rise over the last century, assuming that a global warming of about 0.5°C has occurred.
Large sections of Chinese coastal regions gradually disappear under rising sea levels because of global warming, severely impairing the country’s social and economic progress. According to the latest observations from domestic tide stations, the sea level along China’s coastline has maintained a rapidly rising speed over the past five decades.
The elevation even accelerated in recent years with an annual increase of 2.6 millimeters. Meteorologists predict that in the next 30 years, the sea level will continue to rise by one to 16 centimeters. By 2050, it will be 6 to 26 centimeters higher. The increase will probably reach 30 to 70 centimeters by the end of the 21st century.
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Ding Yihui, a climate expert with the China Meteorological Administration (CMA), said, “With comparatively-advanced social, economic and cultural developments, China’s off-shore regions will suffer great losses if the sea level doesn’t cease rising.” Ding also attributed a series of potential ecological problems, such as the deterioration of shoals and marshes and the salinization of the groundwater-bearing layers, to the sea level increase.
“It will ruin the ecological and environmental balance along the coastal areas.” China’s long coastline is the base for about 70 percent of the large cities, over a half of the domestic population and nearly 60 percent of the national economy. Du Bilan, a researcher with the National Bureau of Oceanography (NBO), said that the Yangtze River Delta, Pearl River Delta and Yellow River Delta—regions located along the coast with the country’s most developed economies—may all in part be flooded if the sea level kept rising at the current speed.
Statistics from the simulation experiments show that only 65 centimeters more over the highest historical tide level will lead to a submergence of about 3,400 sq. km. in the Pearl River Delta, causing an economic loss of 180.8 billion yuan (about 21.9 billion U.S. dollars).
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CMA director Qin Dahe said that not only China, but the whole world is threatened by rising sea levels. In the next 100 years, the average global temperature will rise by 1.4 to 5.8°C, which will result in a sea level rise ranging from nine to 88 centimeters. It is predicted that the consequent annual economic loss will amount to over 300 billion U.S. dollars.
Facing the growing menace of the sea level rise, meteorologists advise to take more scientific and active preventive measures, including strengthening the construction of protection embankments, enhancing the design criteria of littoral projects as well as strictly limiting groundwater exploitation.
Coastal areas at or below one metre of elevation constitute much of Vietnam’s 3,000 km coastline. Sea level rise presents a serious threat to these coastal areas in particular, to the two low-lying deltaic areas of the north and south. Even a limited rise in sea level overcoming decades could seriously affect the people and nation of Vietnam.
Sea Level Rise will Result in:
i. Loss of land;
ii. Increased vulnerability to flooding, including storm events;
iii. Accelerated erosion along the coasts and in river mouths;
iv. Increased salinization; and
v. Changes in the physical characteristics of tidal rivers.
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The most fertile agricultural lands, together with 50% of the population, are centred on the low-lying Red River and the Mekong delta regions. Sea level rise could have drastic consequences for the livelihoods and socio-economic well-being of the inhabitants of these areas. It is likely that valuable arable land would be lost. Shrimp and crab farms may have to be relocated and coastal fisheries might disappear.
The biophysical characteristics of neighbouring regions not permanently inundated by sea water could be affected and this may render these areas unsuitable for agriculture. For example, the irrigation of paddy rice may be seriously affected as a result of the increased intrusion of saline or brackish water. Estuarine and riverine areas could be affected by changes in the tidal regime and in river currents.
Biological resources in coastal areas provide an important base for socio-economic development. They are significant for the ecological balance and for scientific research and provide an important service of coastal protection. Vietnam’s rich diversity of coastal flora and fauna might be substantially reduced and unique habitats may disappear.
Mangrove and cajeput forests—important ecosystems in low-lying areas—may be reduced in extent or lost completely. Marshy areas in river estuaries are habitats and resting places for birds and these will be threatened by sea level rise.
Likewise, sandy stretches where sea turtles lay their eggs may be flooded. The development of coral reefs could be affected. Research currently being undertaken in Vietnam indicates that significant impacts due to sea level rise may already be occurring. Data from the past decade show that, in the Cau Mau coastal region, more than 600 hectares of land has been eroded from the mouth of the Bo De River with strips of land 200 m wide lost in some locations.
Observations also indicate that increased salt intrusion is causing a gradual change in species distribution in the mangrove forests. The more that the mangrove forest area is reduced, the greater the impact from salt water intrusion and erosion on the neighbouring land and the greater the vulnerability to storm-induced flooding. The social and economic consequences of sea level rise could well be wide-ranging. Port facilities may have to be re-engineered.
Coastal industries may be lost. Transportation will be disrupted. The provision of drinking water may be affected as saline water penetrates aquifers. Communities living in coastal areas vulnerable to increased flooding may have to be relocated. This would increase pressure on the remaining land and exacerbate problems associated with forest destruction and ecosystem degradation as new agricultural tracts are created. Biodiversity would be degraded, land erosion would increase and flooding may worsen as a result.
In response to the impact of sea level rise, increased expenditure will be necessary on flood protection and the planning and zoning of activities in coastal areas, including agriculture, industry, transportation and tourism, may have to be rethought. Sea level rise will have significant implications for all activities in low- lying areas.
Given the planning timescales involved, it is important that serious and prompt consideration be given to suitable responses in relevant policy areas such as coastal protection, agriculture, industry and land use zoning. Sea level could rise 40 to 65 cm by the year 2100, due to predicted greenhouse-gas-induced climate warming.
Such a sea level rise would threaten coastal cities, ports, and wetlands with more frequent flooding, enhanced beach erosion, and saltwater encroachment into coastal streams and aquifers. Therefore, it is important to study records of how sea level has been changing.
Sea level has fluctuated dramatically in geologic times. It was 2-6 m above the present level during the last interglacial period, 125,000 years ago, but 120 m below present during the last Ice Age, 20,000 years ago. In the last 100 years it has increased by 10-25 cm. However, future sea level is very difficult to predict, because not enough is known about how the ice-sheets in Greenland and Antarctica will react to global warming.
Furthermore, local sea level is affected by many regional processes, including tides, ocean currents, and geographically-varying land movements. These Earth motions are caused by ongoing adjustments of Earth’s crust to the removal of the former ice sheets, tectonic deformation, subsidence of river deltas under sediment loads, and extraction of underground water, oil, or natural gas near the coast.
Vivien Gornitz has compared tide-gauge records and radiocarbon- dated geologic data from four widely separated regions, spanning a broad range of geologic settings, in order to remove any long-term geologic trends present in the recent instrumental records. Modern sea- level trends are found to be consistently 1-1.8 mm/yr higher than those derived from long-term geologic data.
This result implies a recent acceleration of sea-level rise relative to the last few thousand years. This finding is compatible with other evidence for recent global warming, such as land and marine air temperature measurements, the worldwide retreat of mountain glaciers, subsurface borehole temperature profiles, and a northward migration of boreal forest-tundra boundary in northern Canada and Siberia.
To establish effective countermeasures against global warming, it is essential to predict the future global warming by using model calculation, etc. The WMO and the United Nations Environment Programme (UNEP) established together the Intergovernmental Panel on Climate Change (IPCC) in 1988, and are seeking the participation of scientists from many countries to undertake a scientific and technical evaluation of global warming.
IPCC members include many Japanese researchers from related official agencies and universities as well as the Japan Meteorological Agency. The first evaluation report issued by IPCC in 1990 provided the scientific basis for the UN Framework Convention on Climate Change, the signing of which took place at the United Nations Conference on Environment and Development (Earth Summit) in Rio de Janeiro in 1992.
IPCC released its second report in 1995, which forecasts that the average surface temperature of the Earth will rise about 2°C with a corresponding 50 cm rise of the sea water level by the year 2100, as a most probable case when we do not make effective countermeasures. These estimations were based on the results of climate model calculations carried out by major research organizations in various countries.
These values, however, are less than certain, so it is necessary to make efforts to improve their accuracy in the future. Although only 2°C increase in the average temperature is estimated by them, this value should be considered as very serious, because the temperature difference between ice age and now is only 5°C.
Also, the IPCC concluded that the temperature rise in local areas seriously diverges from the average temperature rise of 2°C. In other words, the temperature rise in polar areas would be higher than average, and less in the tropic areas. If this estimation is correct, granary areas spread in the middle latitude will be critically affected, and the desertification will be accelerated in the areas having a desertification tendency.
The forest distribution will also be affected, because it is impossible to move their territory to more comfortable areas in such a short period. Furthermore, it is worried that some island countries and some big cities developed near the shoreline could be below the sea level, because of the rise of the sea water level following the global warming.