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Global and regional sea-level changes and the hydrological cycle
Global and Planetary Changes 32 (2002) vi – viii www.elsevier.com/locate/gloplacha
Editorial
Global and regional sea-level changes and the hydrological cycle The issues confronting investigations of modern sea-level rise can be grouped in terms of Measurement, Mechanisms, and Impacts. Measurement of sea-level change in the 20th century has traditionally been based on tide gauge records, corrected for coastal epeirogeny in response to glacial re-equilibration. More recently, satellite radar altimeter observations of sea level have made it possible to record variations in the open ocean as well as along the coastline, but these records exist only for the last decade or so making it difficult to extract secular sea-level trends. Short-term mechanisms driving sea-level change include ocean volume changes due to thermal expansion, and ocean mass changes due to mass flow between the ocean and other reservoirs such as ice sheets. The impacts of sea-level rise are of greatest concern to the general population, particularly in and near coastal communities. Direct hydrologic consequences include shoreline erosion, flooding of coastal wetlands, salinization of coastal aquifers, alterations of estuarine steam flow, and exacerbation of storm damage to both ecosystems and human-built structures (e.g., houses, roads, bridges, etc.). A better documentation of the rate of sea-level rise (measurement) and understanding of the causes (mechanisms) will place policy-makers and others in coastal communities in a better position for rational decision-making regarding local responses. The papers in this volume emerged from a conference held in Sardinia, Italy in October, 1999. The objective of the conference was to provide a forum for critical discussion of sea-level variability in relation to the hydrological cycle both at global and regional scales. The main focus was on the identification of problem areas which need to be investigated and key
parameters which need to be monitored. Specific research needs were identified as follows: (1) Measurement (a) Tide gauges. The tide gauge record is invaluable because of its longevity. State-of-the-art tide gauges in the record-poor, but globally important southern ocean, will be critical in the coming years. Vertical crustal motions (epeirogeny) can now be measured with the Global Positioning System (GPS). Real-time measurement of the vertical motion of tide gauge benchmarks can be used to separate local relative sea-level variations from the global signal. (b) Satellites. Global ocean surface height records from TOPEX-POSEIDON and other radar altimeters with greater high-latitude coverage (Geosat, ERS-1, and ERS-2) have provided a quantum leap in our ability to measure the topography of the global ocean. Additional satellite altimeters with greater high-latitude coverage are operational or planned for the near future by NASA, the US Navy, CNES and ESA (e.g., GFO-1, JASON-1, ENVISAT), making the future bright for a global satellite operational network. (2) Causes (a) Continental hydrologic budget. In what seemed at first as a surprising conclusion from a primarily oceanographic and geodetic research community, the most critical need (highest priority) was identified as better quantification of the hydrologic budget of the continents. Large and variable water fluxes are being altered by climate change, land use and water resource utilization. A comprehensive data set is necessary for global information combining river hydrographs, ground water levels, lake and reservoir levels, soil
0921-8181/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 ( 0 1 ) 0 0 1 4 4 - 8
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Editorial
moisture, and land cover for at least the present time, and if possible for the last century as well. (b) Ice budgets. The balance between increased melting as a result of atmospheric warming, and increased precipitation (as snow) in Antarctica plays a critical role in the global hydrologic budget. As such, further research is needed in observation of ice accumulation rates on the basis of surface elevations, monitoring of meteorology in and around the southern ocean and coastal Antarctica, and modeling of the relationship of sea surface temperatures to Antarctic atmospheric circulation and precipitation patterns. (c) Ocean temperature. Better monitoring of ocean temperature will increase our ability to assess the present and future contribution of thermal expansion to sea-level rise. The thermal structure of the mixed layer is very poorly known. However, detailed gravity data may provide the necessary constraints to determine the density profile of the mixed layer. (d) Anthropogenic influences. It will not be possible to accurately account for past (20th century) sea-level changes, nor to project future changes without some simple analysis of human intervention in the hydrologic cycle. Activities such as ground water mining, deforestation, and draining of wetlands contribute to sea-level rise, while construction of new dams ameliorates sea-level rise caused by other factors. In particular, by impounding water at a significant rate globally, we may be masking the sea-level rise being caused by other factors. If the rate of major new dam construction slows or stops, then we should expect the observed rate of sea-level rise to increase accordingly in the 21st century, even if other factors remain unchanged from the 20th century. (3) Impacts The policy sector is most concerned with the impacts of sea-level rise because they could have significant economic, social, and therefore political consequences. The assessment of the impacts of sealevel rise over the next century is hindered by a 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. The issue of how involved the scientific community should become in the political process of national
and international decision-making is complex and can lead to awkward situations for individual scientists and research organizations. Nevertheless, the application of scientific results to pressing current problems is a critical aspect of global change research. While this application has traditionally been skirted for fear of misinterpretation by the lay public and popular press, it is beginning to be generally recognized that the sea level research community should make every reasonable effort to make research results available to the public and to the decision-makers within the policy sector. In this special issue, Harvey et al. study the various influence on tide gauge data using southern Australia as an example. They distinguish the contribution to relative sea-level changes over the last century from local/regional tectonic factors, glacio-isostasy, and anthropogenic activities such as ground water withdrawal. From this, they back out a global eustatic signal. In the absence of tide gauge data, Mason and Jordan use geologic methods to construct a sea-level curve based on the unglaciated Chukchi Sea coast of the Seward Peninsula of Alaska. The relative sea-level curve so established indicates a slower rate of late Holocene sea-level rise than those emerging from low latitude localities, as would be expected in light of isoglaciostatic models. Chen et al. use satellite altimetry to constrain the balance of water in the global hydrologic cycle, and the storage of water on the continents relative to the ocean at interannual timescales. The contributions of land-based vs. floating ice in Antarctica is considered, as well as the performance of hydrologic models with regard to Topex/Poseidon data. At the regional level, Stanev examines the interannual to decadal variations of the Black Sea. The level of the Black Sea is controlled by the interplay between hydrology over a broad area of Europe and its climate variability, wind and evaporation over the Black Sea, and water exchange with the Mediterranean through the Strait of Bosporus. Thus, European climate variability is related in the paper with the formation of deep water masses within the Mediterranean. Thomson and Shafer explore the interplay between sea-level rise, sedimentation, and hydrology of the US Gulf coast. The prolonged drought of recent years has altered the sediment and water balance so that the
Editorial
ecosystem is no longer supported by equilibrium conditions of salinity, sediment delivery, and biomass accumulation within the system. Changing conditions are exacerbated by sea-level rise and subsidence due to ground water withdrawal. Finally, Gornitz explores the impacts of sea-level rise on the New York metropolitan area. In such an urbanized environment, unprecedented stress is placed on coastal ecosystems. Also, with a vast tourist trade, energy- and capital-intensive mitigation measures are commonly employed to replenish beaches and maintain the current shoreline position with the entrenchment of structures (buildings, roads, etc.) in the face of sea-level rise. While the papers in this special issue reflect a great deal of progress in understanding the interaction between the global hydrologic cycle and sea level, there are many unresolved questions that will require considerable effort to successfully address. As such, it will
viii
be crucial that future research regarding sea level and its relationship to coastlines and hydrologic processes be conducted in the context of the many interacting physical, biological and social processes that bear on coastal systems.
Acknowledgements The conveners thank the US National Science Foundation (EAR, SBE), and the European Geophysical Society (EGS) for travel support for many of the participants in the Sea-level conference. We are also grateful to IGBP/GAIM and to the section of Geodesy of the EGS for organizational support of this first Vening – Meinesz Conference. Dork Sahagian Susanna Zerbini
Editorial
Global and regional sea-level changes and the hydrological cycle The issues confronting investigations of modern sea-level rise can be grouped in terms of Measurement, Mechanisms, and Impacts. Measurement of sea-level change in the 20th century has traditionally been based on tide gauge records, corrected for coastal epeirogeny in response to glacial re-equilibration. More recently, satellite radar altimeter observations of sea level have made it possible to record variations in the open ocean as well as along the coastline, but these records exist only for the last decade or so making it difficult to extract secular sea-level trends. Short-term mechanisms driving sea-level change include ocean volume changes due to thermal expansion, and ocean mass changes due to mass flow between the ocean and other reservoirs such as ice sheets. The impacts of sea-level rise are of greatest concern to the general population, particularly in and near coastal communities. Direct hydrologic consequences include shoreline erosion, flooding of coastal wetlands, salinization of coastal aquifers, alterations of estuarine steam flow, and exacerbation of storm damage to both ecosystems and human-built structures (e.g., houses, roads, bridges, etc.). A better documentation of the rate of sea-level rise (measurement) and understanding of the causes (mechanisms) will place policy-makers and others in coastal communities in a better position for rational decision-making regarding local responses. The papers in this volume emerged from a conference held in Sardinia, Italy in October, 1999. The objective of the conference was to provide a forum for critical discussion of sea-level variability in relation to the hydrological cycle both at global and regional scales. The main focus was on the identification of problem areas which need to be investigated and key
parameters which need to be monitored. Specific research needs were identified as follows: (1) Measurement (a) Tide gauges. The tide gauge record is invaluable because of its longevity. State-of-the-art tide gauges in the record-poor, but globally important southern ocean, will be critical in the coming years. Vertical crustal motions (epeirogeny) can now be measured with the Global Positioning System (GPS). Real-time measurement of the vertical motion of tide gauge benchmarks can be used to separate local relative sea-level variations from the global signal. (b) Satellites. Global ocean surface height records from TOPEX-POSEIDON and other radar altimeters with greater high-latitude coverage (Geosat, ERS-1, and ERS-2) have provided a quantum leap in our ability to measure the topography of the global ocean. Additional satellite altimeters with greater high-latitude coverage are operational or planned for the near future by NASA, the US Navy, CNES and ESA (e.g., GFO-1, JASON-1, ENVISAT), making the future bright for a global satellite operational network. (2) Causes (a) Continental hydrologic budget. In what seemed at first as a surprising conclusion from a primarily oceanographic and geodetic research community, the most critical need (highest priority) was identified as better quantification of the hydrologic budget of the continents. Large and variable water fluxes are being altered by climate change, land use and water resource utilization. A comprehensive data set is necessary for global information combining river hydrographs, ground water levels, lake and reservoir levels, soil
0921-8181/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 8 1 8 1 ( 0 1 ) 0 0 1 4 4 - 8
vii
Editorial
moisture, and land cover for at least the present time, and if possible for the last century as well. (b) Ice budgets. The balance between increased melting as a result of atmospheric warming, and increased precipitation (as snow) in Antarctica plays a critical role in the global hydrologic budget. As such, further research is needed in observation of ice accumulation rates on the basis of surface elevations, monitoring of meteorology in and around the southern ocean and coastal Antarctica, and modeling of the relationship of sea surface temperatures to Antarctic atmospheric circulation and precipitation patterns. (c) Ocean temperature. Better monitoring of ocean temperature will increase our ability to assess the present and future contribution of thermal expansion to sea-level rise. The thermal structure of the mixed layer is very poorly known. However, detailed gravity data may provide the necessary constraints to determine the density profile of the mixed layer. (d) Anthropogenic influences. It will not be possible to accurately account for past (20th century) sea-level changes, nor to project future changes without some simple analysis of human intervention in the hydrologic cycle. Activities such as ground water mining, deforestation, and draining of wetlands contribute to sea-level rise, while construction of new dams ameliorates sea-level rise caused by other factors. In particular, by impounding water at a significant rate globally, we may be masking the sea-level rise being caused by other factors. If the rate of major new dam construction slows or stops, then we should expect the observed rate of sea-level rise to increase accordingly in the 21st century, even if other factors remain unchanged from the 20th century. (3) Impacts The policy sector is most concerned with the impacts of sea-level rise because they could have significant economic, social, and therefore political consequences. The assessment of the impacts of sealevel rise over the next century is hindered by a 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. The issue of how involved the scientific community should become in the political process of national
and international decision-making is complex and can lead to awkward situations for individual scientists and research organizations. Nevertheless, the application of scientific results to pressing current problems is a critical aspect of global change research. While this application has traditionally been skirted for fear of misinterpretation by the lay public and popular press, it is beginning to be generally recognized that the sea level research community should make every reasonable effort to make research results available to the public and to the decision-makers within the policy sector. In this special issue, Harvey et al. study the various influence on tide gauge data using southern Australia as an example. They distinguish the contribution to relative sea-level changes over the last century from local/regional tectonic factors, glacio-isostasy, and anthropogenic activities such as ground water withdrawal. From this, they back out a global eustatic signal. In the absence of tide gauge data, Mason and Jordan use geologic methods to construct a sea-level curve based on the unglaciated Chukchi Sea coast of the Seward Peninsula of Alaska. The relative sea-level curve so established indicates a slower rate of late Holocene sea-level rise than those emerging from low latitude localities, as would be expected in light of isoglaciostatic models. Chen et al. use satellite altimetry to constrain the balance of water in the global hydrologic cycle, and the storage of water on the continents relative to the ocean at interannual timescales. The contributions of land-based vs. floating ice in Antarctica is considered, as well as the performance of hydrologic models with regard to Topex/Poseidon data. At the regional level, Stanev examines the interannual to decadal variations of the Black Sea. The level of the Black Sea is controlled by the interplay between hydrology over a broad area of Europe and its climate variability, wind and evaporation over the Black Sea, and water exchange with the Mediterranean through the Strait of Bosporus. Thus, European climate variability is related in the paper with the formation of deep water masses within the Mediterranean. Thomson and Shafer explore the interplay between sea-level rise, sedimentation, and hydrology of the US Gulf coast. The prolonged drought of recent years has altered the sediment and water balance so that the
Editorial
ecosystem is no longer supported by equilibrium conditions of salinity, sediment delivery, and biomass accumulation within the system. Changing conditions are exacerbated by sea-level rise and subsidence due to ground water withdrawal. Finally, Gornitz explores the impacts of sea-level rise on the New York metropolitan area. In such an urbanized environment, unprecedented stress is placed on coastal ecosystems. Also, with a vast tourist trade, energy- and capital-intensive mitigation measures are commonly employed to replenish beaches and maintain the current shoreline position with the entrenchment of structures (buildings, roads, etc.) in the face of sea-level rise. While the papers in this special issue reflect a great deal of progress in understanding the interaction between the global hydrologic cycle and sea level, there are many unresolved questions that will require considerable effort to successfully address. As such, it will
viii
be crucial that future research regarding sea level and its relationship to coastlines and hydrologic processes be conducted in the context of the many interacting physical, biological and social processes that bear on coastal systems.
Acknowledgements The conveners thank the US National Science Foundation (EAR, SBE), and the European Geophysical Society (EGS) for travel support for many of the participants in the Sea-level conference. We are also grateful to IGBP/GAIM and to the section of Geodesy of the EGS for organizational support of this first Vening – Meinesz Conference. Dork Sahagian Susanna Zerbini