Author: Ghulam Mustafa
Introduction
Sea level rise refers to the long-term increase in the average global sea level due to various factors, primarily the thermal expansion of seawater as it warms and the melting of polar ice caps and glaciers. This rise in sea level can have significant impacts on coastal areas. Coastal areas are regions located near the coastline, often including beaches, estuaries, and cities.
An increasing body of evidence suggests that, within the near future, global warming due to the greenhouse effect could lead to a substantial rise in sea level. As a consequence, since a large portion of the world’s urban population lives in low-lying areas near the sea, such a phenomenon would have an important impact on human society. The expected eustatic or global sea-level rise is forecasted to be between 14 cm and 80 cm by the end of the twenty-first century, the best estimate being around 44 cm.
Therefore, the present (2002) rate of mean sea-level rise from 1 mm y–1 to 2 mm y–1 could amplify to a speed of 4 mm y–1 in the coming century. In that case, the vulnerability of coastal areas will be increased because sea-level rise will enhance the efficiency of natural hazards. An increase in mean surface temperature in tropical oceans could also lead to a greater frequency of hurricanes that already cause significant damage to property and human life. If sea-level rise may be compensated for or even reversed in rapidly uplifting areas, such as the isostatic emerging shorelines of Scandinavia and eastern Canada, it can be increased by natural subsidence. Accelerated subsidence in many low-lying urbanized coastal areas due to excessive withdrawal of water as well as oil or gas also promises to exacerbate the situation by amplifying the magnitude of sea-level rise locally. In such a situation, the combined global sea-level rise and continental subsidence will certainly have the greatest impact on human societies in the densely populated coastal cities and residential areas. What is already happening in diverse locations, such as Venice, Shanghai, and Bangkok, all sinking cities situated near a seashore, clearly shows the serious impacts to be expected from an accelerated sea-level rise. It also suggests the policy and the engineering responses that may be considered if the adverse effects of local sea-level rise are to be counteracted.
From now on, it would be wise to revise current aims and attitudes in coastal development and management. As a general rule, planners and developers should be aware of the hazards threatening low-lying coastal areas, such as deltaic plains, lagoons surroundings, and coral islands. Broadly speaking, coastal areas are experiencing a fast growth in population and a rapid trend of urbanization. Most of the present and projected megacities, defined as cities with a population exceeding 8 million people, are located in coastal settings where they are susceptible to the adverse impacts of an accelerated sea-level rise, Given the large scale of coastal urbanization and its expected rapid growth in developing countries during the next decades, this means that the problems related to the projected sea-level rise are a common concern that should be addressed in seaside planning, bearing in mind the concept of sustainable development.
Causes of Sea Level Rise
Past, present, and future changes in global sea level are mainly caused by two fundamental processes: (1) the thermal expansion of existing water in the world’s ocean basins as it absorbs heat and (2) the addition of water from land-based sources mainly ice sheets and glaciers, but also other smaller sources. Geological processes (subsidence and uplift), ocean circulation changes, and other processes are important for determining local and regional rates of sea level rise, but the total volume of the world’s oceans and hence global average sea level is essentially controlled by thermal expansion and addition of water from land-based sources.
Responses to adverse impacts of climate change and sea-level rise
Responses to climate change fall into two broad measures, mitigation and adaptation. Mitigation is to stabilize the climate system through the reduction of GHG emissions and sequestration of GHGs by forests, etc. As mitigation aims to keep climate change within the level to which human society and the ecosystem can adapt, it can be considered to avoid an “unmanageable situation” induced by significant climate change. This unmanageable situation includes a large-scale irreversible change including the disintegration of the GIS and WAIS.
The objective of adaptation, in turn, is to reduce the adverse impacts of climate change using a range of measures such as disaster risk reduction and increased resilience of food production and freshwater supplies. Even if mitigation succeeds in achieving the goal of stabilization, climate change will still proceed to a certain extent, resulting in some impacts on sectors and regions. Therefore, adaptation is considered a “measure to prepare for an unavoidable impact”. Planning for, and implementation of, adaptation is urgent, particularly for developing countries, for they will face larger impacts because of their inadequate infrastructure and low adaptive capacity. Given the uncertainty surrounding international agreements for mitigation, the importance of adaptation is increasing.
Countermeasures against coastal impacts are a part of adaptation. As coastal impacts are expected to be significant, from an early stage many studies have been carried out, focusing on coastal adaptation. The IPCC examined the impacts and responses in coastal zones by establishing the Coastal Zone Management Subgroup (CZMS) during its first assessment.
The IPCC (Intergovernmental Panel on Climate Change), and CZMS proposed the three strategies of protection, accommodation, and retreat. Protection implies a strategy to defend inland areas using coastal structures such as seawalls and breakwaters. Accommodation is a concept to continue using coastal zones by changing their use, e.g., raising houses on poles to prepare for possible inundation and converting rice paddies to fishponds. The last strategy, retreat, is to regulate the land use of vulnerable areas and relocate houses and economic activities to safer areas. After the IPCC First Assessment Report, these three strategies have evolved to propose concrete measures.