Published on April 7, 2008
ODINAFRICA/GLOSS Training Workshop on Sea-Level Measurement and Interpretation. Oostende, Belgium, 13-24 November 2006: ODINAFRICA/GLOSS Training Workshop on Sea-Level Measurement and Interpretation. Oostende, Belgium, 13-24 November 2006 Tanzania Fisheries Research Institute P.O. Box 9750 Dar es Salaam TANZANIA EMAIL: [email protected] 20 NOVEMBER 2006 SHIGALLA MAHONGO IMPACTS OF SEA LEVEL CHANGE The Greenhouse Effect: The Greenhouse Effect Infrared (IR) active gases, principally water vapor (H2O), carbon dioxide (CO2) and ozone (O3), naturally present in the Earth’s atmosphere, absorb thermal IR radiation emitted by the Earth’s surface and atmosphere. The atmosphere is warmed by this mechanism and, in turn, emits IR radiation, with a significant portion of this energy acting to warm the surface and the lower atmosphere. As a consequence the average surface air temperature of the Earth is about 30° C higher than it would be without atmospheric absorption and re-radiation of IR energy. This phenomenon is popularly known as the greenhouse effect, and the IR active gases responsible for the effect are likewise referred to as greenhouse gases. The Greenhouse Effect: The Greenhouse Effect The rapid increase in concentrations of greenhouse gases since the industrial period began has given rise to concern over potential resultant climate changes Greenhouse Gases and Global Climate Change: Greenhouse Gases and Global Climate Change The principal greenhouse gas concentrations that have increased over the industrial period are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). The observed increase of CO2 in the atmosphere from about 280 ppm in the pre-industrial era to about 364 ppm in 1997 has come largely from fossil fuel combustion and cement production. Of the several anthropogenic greenhouse gases, CO2 is the most important agent of potential future climate warming because of its large current greenhouse forcing, its substantial projected future forcing, and its long persistence in the atmosphere. Recorded Worldwide Temperatures: -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1880 1900 1920 1940 1960 1980 2000 Year D Mean Temperature (°C) Recorded Worldwide Temperatures 2005 Temperature Changes Compared to 1951-1980: 2005 Temperature Changes Compared to 1951-1980 Ozone Layer Depletion and Climate Change: Ozone Layer Depletion and Climate Change The ozone layer absorbs harmful ultraviolet-B radiation from the sun. Over the past 30 years ozone levels over parts of Antarctica have dropped by almost 40% during some months and a 'hole' in ozone concentrations is clearly visible in satellite observations. Ozone is been damaged mainly by: 1. Chlorofluorocarbons (CFCs) that are used in refrigerators, aerosols, and as cleaners in many industries. 2. Halons that are used in fire extinguishers. 3. Aircraft emissions of nitrogen oxides and water vapour. As Ozone is considered to be a greenhouse gas, a depleted ozone layer may partially dampen the greenhouse effect. This may therefore lead to increased global warming. Conversely, efforts to tackle ozone depletion may result in increased global warming! Some Impacts of Climate Change: Some Impacts of Climate Change The hydropower-dependent energy sector in Tanzania has been seriously affected by drought. The country is turning to coal and natural gas as new sources of energy. Some cities in Europe and USA experienced power shortages during summer of 2006 due to effects of increased temperatures (The infrastructures failed to cope with the record heat - unsuitable wires, pipes, etc not designed for higher temperatures). About 82% of the icecap on mount Kilimanjaro in 1912 is now gone. If recession continues at the present rate, the majority of the mountain glaciers could vanish in the next 15 years. The area covered by glaciers on the Rwenzori Mountains halved between 1987 and 2003, expected to disappear in the next 20 years. The Melting Snows of Mt Kilimanjaro: The Melting Snows of Mt Kilimanjaro RELATIVE SEA LEVEL CHANGE: RELATIVE SEA LEVEL CHANGE Sea level varies as a result of processes operating on a great range of time-scales, from seconds to millions of years, so that current sea level change is also related to past climate change. The local change in sea level at any coastal location as measured by a tide gauge depends on the sum of global, regional and local factors and is termed relative sea-level change. It is so called because it can come about either by movement of the land on which the tide gauge is situated or by the change in the height of the adjacent sea surface. Relative sea levels are also measured by dating buried coastal vegetation (salt marshes, mangroves, etc.). Most of the tide gauges are located in mid-latitude northern hemisphere, few in middle of oceans, and contaminated by earth movements. The main source for the uncertainties in using tide gauge records still remain: poor historical distribution of tide gauges, lack of data from Africa and Antarctica, the GIA corrections used, and localized tectonic activity. CLIMATE CHANGE AND SEA LEVEL RISE: CLIMATE CHANGE AND SEA LEVEL RISE Sea-level rise due to global warming occurs primarily because water expands as it warms up. The melting ice caps and mountain glaciers also add water to the oceans, thus rising the sea level. The contribution from large ice masses in Greenland and Antarctica is expected to be small over the coming decades. But it may become larger in future centuries. Sea-level rise can be offset up by irrigation, the storage of water in reservoirs, and other land management practices that reduce run-off of water into the oceans. Changes in land-levels due to coastal subsidence or geological movements can also affect local sea-levels. Average Rate of Sea Level Rise and the Estimated Contributions from Different Processes: 1910 - 1990 : Average Rate of Sea Level Rise and the Estimated Contributions from Different Processes: 1910 - 1990 CLIMATE CHANGE AND SEA LEVEL RISE: CLIMATE CHANGE AND SEA LEVEL RISE About 20,000 years ago during the LGM, large ice sheets melted causing a rise in sea level of about 100m, most of the melting occurred about 6,000 years ago. Over the past 1,000 years and prior to the 20th century, the average global sea level rise was of the order of 0.2 mm/yr. The rate of sea level rise climbed to about 1-2 mm/yr during the 20th century, with a central value of 1.5 mm/yr (IPCC TAR). The most recent estimate during the 20th century is 1.4 -2.0 mm/yr, with a central value of 1.7 ± 0.3 mm/yr (Church & White, 2006). This significant rate of rise in sea level is attributed to global warming caused by industrialization during the second half of the 19th century. CLIMATE CHANGE AND SEA LEVEL RISE: CLIMATE CHANGE AND SEA LEVEL RISE There is no evidence for any acceleration of sea level rise in data from the 20th century data alone. Mediterranean records show decelerations and even decreases in sea level in the latter part of the 20th century. Most records show evidence of a gradual rise in global mean sea level over the last century. However, signals caused by land movements (e.g. uplift or submergence) can mask this signal due to actual changes in sea level. The IPCC has estimated that, if the emission of greenhouse gases continues at the current rate, the level of the sea surface will rise by an additional 8-20 cm by 2030, 21-71 cm by 2070 and 31-110 cm by 2100. Global Sea Level Change Over the Last 140,000 Years (IPCC TAR): Global Sea Level Change Over the Last 140,000 Years (IPCC TAR) THE PROSPECT OF SATELLITE ALTIMETRY IN SEA LEVEL STUDIES: THE PROSPECT OF SATELLITE ALTIMETRY IN SEA LEVEL STUDIES Satellite altimetry provides near-global coverage of the world’s oceans and thus the promise of determining the global-averaged sea level rise, its regional variations, and changes in the rate of rise more accurately and quickly than is possible from the sparse array of in situ gauges. TOPEX/Poseidon satellite altimeter mission with its (near) global coverage from 66°N to 66°S was launched in August 1992. Estimates of the rates of rise from the short T/P record are 2.5 ± 1.3 mm/ yr over the 6-yr period 1993–98 (Church et al, 2004). Using a combination of tide gauge records and satellite altimetry, Jevrejeva et al. (2006) have estimated this rate to be 2.4 mm/yr over the same period. THE PROSPECT OF SATELLITE ALTIMETRY IN MEAN SEA LEVEL STUDIES: THE PROSPECT OF SATELLITE ALTIMETRY IN MEAN SEA LEVEL STUDIES Whether this larger estimate is a result of an increase in the rate of rise, systematic errors in the satellite and/or in situ records, the shortness of the satellite record, or a reflection of the large error bars is not clear. Analysis of TOPEX/Poseidon satellite altimeter data has demonstrated that meaningful estimates of global averaged mean sea level change can be made over much shorter periods than possible with tide gauges because the global satellite data account for horizontal displacements of ocean mass. However, achieving the required sub-millimeter accuracy is demanding and requires satellite orbit information, geophysical and environmental corrections and altimeter range measurements of the highest accuracy. It also requires continuous satellite operations over many years and careful control of biases. Mean Sea Level Variations at Selected Locations [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations at Selected Locations [Data: www.pol.ac.uk/psmsl] Mean Sea Level Variations at Selected Locations [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations at Selected Locations [Data: www.pol.ac.uk/psmsl] Sea Level Stations in the Western Indian Ocean (with PSMSL RLR data): Sea Level Stations in the Western Indian Ocean (with PSMSL RLR data) Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl] Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl] Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl] Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl]: Mean Sea Level Variations in the Western Indian Ocean [Data: www.pol.ac.uk/psmsl] Some Observations on Sea Level Trends in East Africa: Some Observations on Sea Level Trends in East Africa Tide gauge records in the region are not long enough to give any conclusive evidence on sea level rise. Fairly long records of at least 50 years are needed because of the influence of natural variability in the climate system (Douglas, 1992). Some of the stations, including the oldest ones such as Mombasa (1932) and Port Louis (1942) have significant record gaps, making it difficult to examine the trends with certainty. From the available data, sea level has been rising in some stations whereas in others it has been observed to fall (e.g. Zanzibar at - 3.9 mm/yr), probably due to decadal variability. The decadal variability can be explained by the Aberdeen record (1862-1965). There is a consistent fall in sea level during the first 23 yrs, then a consistent rise during the remaining 77yrs. The East African region is of special interest in the aspect of sea level rise. Whilst there is worldwide trend of rise in sea level, both tide gauge and satellite altimetry (e.g. Church et al, 2006) indicate a falling trend in sea level in some parts of the region. Some Observations on Sea Level Trends in East Africa: Some Observations on Sea Level Trends in East Africa The falling trend in sea level, though a rare occasion, is not unique to the East African region. The sea level at Helsinki has been declining at an average rate of 2mm/yr over the past century (1879 and 2001). There are records from a number of sea level stations which are not in the PSMSL database but can be used to observe the trend in sea level (e.g. Port Victoria, Saint Paul, Dar es Salaam, Lamu, Dzaoudzi, Reunion, etc.). Comparison cannot be made on sea level trends at different periods. E.g., the sea level at Port Louis (1942-1947) showed a rising trend (+2.8mm/yr), that of nearby Port Louis II (1986-2003) indicates a falling trend of 1mm/yr. Some stations have longer records than those in the PSMSL database. However, use of such data requires reducing the data to a common datum. Recommended data sets of the PSMSL are the monthly and annual “Revised Local Reference” (RLR) means. Some Observations on Sea Level Trends in East Africa: Some Observations on Sea Level Trends in East Africa SLPR2 software can be used to convert data from the UHSLC into generic format to observe the rate of sea level rise or acceleration. For long records, small, non-consecutive data gaps cannot alter observations of the sea level trends (e.g. Brest). Station records should be examined before rushing into conclusions. E.g. at Takoradi, “data from 1966 onwards looks irregular and unreliable”,. The entire records of Durban and Rodriguez are also “flagged for attention”. At Maputo, a benchmark was destroyed 1999 and replaced by another one nearby. No direct relationship between the two as they did not exist at the same time. While the region is embarking on expansion and upgrade of the existing sea level network, there is an urgent need to build capacity in satellite altimetry so that current trends in sea level can be monitored by both methods. PHYSICAL IMPACTS OF SEA LEVEL RISE: PHYSICAL IMPACTS OF SEA LEVEL RISE PRIMARY IMPACTS Inundation and displacement of wetlands and lowlands Increased vulnerability to coastal storm damage and flooding Shoreline erosion Saltwater intrusion into estuaries and freshwater aquifers SECONDARY IMPACTS Altered tidal ranges in rivers and bays Changes in sedimentation patterns Decreased light penetration to benthic organisms Increase in the heights of waves Inundation and displacement of wetlands and lowlands: Inundation and displacement of wetlands and lowlands This, the most obvious impact of sea level rise, refers both to the conversion of dryland to wetland and the conversion of wetlands to open water. In salt marsh and mangrove habitats, rapid sea-level rise would submerge land, waterlog soils, and cause plant death from salt stress. Increased vulnerability to coastal storm damage and flooding: Increased vulnerability to coastal storm damage and flooding Sea level rise would increase the impact of tropical cyclones and other storms that drive storm surges. The effects would be disastrous on small island States and other low-lying developing countries, such as the Maldives, where 90 per cent of the population lives along the coast. Flooding due to storm surges will increase under conditions of higher sea level. As is true at present, damage due to flooding will be most severe when the surges strike during high tide. Shoreline erosion: Shoreline erosion While acknowledging that erosion is also caused by many other factors, Bruun (1962) showed that as sea level rises, the upper part of the beach is eroded and deposited just offshore in a fashion that restores the shape of the beach profile with respect to sea level. A rise in sea level immediately results in shoreline retreat due to inundation. However, a 1 m rise in sea level implies that the offshore bottom must also rise 1 m. The sand required to raise the bottom can be supplied by beach nourishment. Otherwise waves will erode the necessary sand from the upper part of the beach. Saltwater intrusion into estuaries and freshwater aquifers: Saltwater intrusion into estuaries and freshwater aquifers Sea level rise would generally enable saltwater to advance inland in both aquifers and estuaries. In estuaries, the gradual flow of freshwater toward the oceans is the only factor preventing the estuary from having the same salinity as the ocean. A rise in sea level would increase salinity in open bays because the increased the cross-sectional area would slow the average speed at which freshwater flows to the ocean. The impact of sea level rise on groundwater salinity could make some areas uninhabitable even before they were actually inundated, particularly those that rely on unconfined aquifers just above sea level. Generally, these aquifers have a freshwater "lens" floating on top of the heavier saltwater. As sea level rises, the depth of the freshwater lens in the coastal zone is greatly reduced, leading to salinization of water supplies. In extreme cases exacerbated by over-pumping, the aquifer may rapidly become unsuitable for drinking and even for irrigation. Altered tidal ranges in rivers and bays: Altered tidal ranges in rivers and bays Sea level rise could change tidal ranges by: Removing barriers to tidal currents Changing the resonance frequencies of tidal basins. Greater tidal currents would tend to form larger ebb tidal deltas, providing a sink for sand washing along the shore and thereby causing additional erosion. Some of the bathymetric changes that might amplify tides would have the same impact on storm surges. Finally, higher tidal ranges would further increase the salinity in estuaries due to increased tidal mixing. Changes in sedimentation patterns: Changes in sedimentation patterns Under natural conditions, most of the sediment washing down rivers is deposited in the estuary due to settling and flocculation. Settling occurs downstream from the head-of-tide because the slowly moving water characterized by estuaries can not carry as much sediment as a flowing river. Flocculation is a process by which salty water induces easily entrained fine-grained sediment to coalesce into larger globs that settle out. A rise in sea level would cause both of these processes to migrate upstream, and thereby assist the ability of wetlands in the upper parts of estuaries to keep pace with sea level, while hindering their ability in the lower parts. Decreased light penetration to benthic organisms: Decreased light penetration to benthic organisms If sea levels were to rise at a pace faster than corals could build their reefs upward, eventually light conditions would be too low for the zooxanthellae to continue photosynthesis. On reefs near low-lying coastal areas, sea-level rise would likely increase coastal erosion rates, thus degrading water quality and decreasing light penetration, thus reducing the depth to which reefs can grow. Losses of coral reefs would mean losses in the high biodiversity of these systems as well as the fisheries and recreational opportunities they provide. Increase in the heights of waves: Increase in the heights of waves A rise in sea level would also increase the size of waves. In shallow areas, the depth of the water itself limits the size of waves, which could be the most important impact of sea level rise along shallow tidal embayments with steep, muddy shores. The steep slopes imply that inundation would not be a problem. However, with water depths one meter deeper, waves could form large enough to significantly erode the muddy shores. Bigger waves could also increase the vulnerability of lands protected by coral reefs. In many areas, these reefs protect mangrove swamps or sandy islands from the direct attach by ocean waves; but deeper water would reduce the reef’s ability to act as a breakwater. The extent to which this will happen would depend on the ability of the corals to keep pace with sea level rise. Impacts of Sea Level Change: The East African Experience: Impacts of Sea Level Change: The East African Experience The Eastern African coastal zone is very heavily populated today because of its growing industrial infrastructure. It is estimated that 13% of the 62 million people in East Africa reside along the coast due to rapid development of coastal activities such as fishing, sea ports for imports and exports, coastal tourism and industries. The East African coastal zone is presently experiencing some coastal degradation due to erosion along some sandy and low-lying beaches. In Dar es Salaam, accelerated marine erosion and flooding in the last decade have uprooted settlements and resulted in the abandonment of luxury beach hotels. Coral reefs, especially near Malindi in Kenya, are being damaged due to excessive siltation caused by coastal erosion. In Seychelles, several parts of the country have already experienced coastal flooding, coastal erosion and loss of infrastructure as a result of increased intensity of wave action and probably sea-level-change. Predicted Future Impacts: The East African Perspective: Predicted Future Impacts: The East African Perspective The coastal countries and islands of East Africa are highly vulnerable to sea-level rise, having many low-lying structural developments including major ports and cities, extensive farmlands, settlements and tourist facilities located along low-lying parts of the coast. Sea level rise would cause inundation of the extensive mangroves of Mozambique and Tanzania and these would retreat, thus increasing rates of erosion of the shoreline. In Mauritius, applying the Brunn rule for calculation of future coastal retreat within 2.2 km of the west coast, the whole area from the actual shoreline to the tarred road will disappear following a sea level rise of 1 m (Beebeejaun, 2000). In Tanzania, Mwaipopo (1997) employed a sea-level rise scenario of 1 mm/yr to determine that about 2,117 km2 of land would be inundated, and another 9 km2 of land would be eroded. In Kenya, the most vulnerable sites are the Watamu and Sabaki River estuary. It is also projected that with a 0.3m increase in sea level, about 17% of Mombasa district will be submerged (Oyieke, 2000). Seychelles will be severely affected by sea level rise by virtue of the concentration of economic activities on the coast, uniqueness of the coastal environment, as well as current direct impacts on coastal processes and resources. Predicted Future Impacts: The East African Perspective: Predicted Future Impacts: The East African Perspective Although the region experiences calm conditions through much of the year, sea-level rise and climatic variation may reduce the buffer effect of coral and patch reefs along the east coast, increasing the potential for erosion. Growing coastal activities have attracted large populations to the coast and this has exerted big strains on coastal groundwater resources. For example, Dar es Salaam is heavily populated with over 85% of the industries situated in and around the city. The depth to water-table in the coastal zone is often very shallow and is subject to saline sea water contamination and pollution. An increased global sea level rise is expected to raise the water-table along the coast and result in increased salinity of the groundwater. Many of the island states are already experiencing this phenomenon and the situation is expected to worsen with sea level rise. RESPONSE STRATEGIES: RESPONSE STRATEGIES There are three response strategies to rising sea level and its physical impacts: RETREAT, ADAPT or DEFEND. In practice, many responses may be hybrid and combined elements of more than one approach. Retreat can involve chaotic abandonment of property and cultural investments, or it can be an ordered, planned program that minimizes losses from rising sea level and maximizes the cost-effectiveness of the operation. The operation also seeks to leave surrendered areas as aesthetic looking as possible and to avoid abandoned structures that are an operational hazard to other social and economic activities. Adaptation/Accommodation – all natural system effects are allowed to occur and human impacts are minimized by adjusting human use of the coastal zone. For East African countries, adaptation is the immediate priority to respond to sea-level rise. Defence/Protection – natural system effects are controlled by soft or hard engineering, reducing human impacts in the zone that would be impacted without protection. SETBACKS: SETBACKS The assessment of impacts of sea level rise over the next century is hindered by lack of knowledge of the detailed topography of the near shore. New global elevation maps based on detailed surveys at cm resolution will make it possible to accurately determine the areas which will be inundated by storm surges under conditions of rising sea level. This will require a concerted effort by the satellite altimetry community as well as local ground-based geodetic surveyors in all coastal areas world-wide.