Hans-Peter Plag
Nevada Geodetic Laboratory, University of Nevada, Reno, USA
Presented on June 9, 2008 at the Workshop "Connecting Delta Cities", Stony Brook University, New York City, USA.
© 2009, Hans-Peter Plag
Abstract ... \ECO \BCO 2
This presentation has the following main parts:
Hans-Peter Plag
Nevada Geodetic Laboratory, University of Nevada, Reno, USA
Local Sea Level (LSL) rise is one of the major anticipated impacts of future global warming with potentially devastating consequences, particularly in many low-lying and often subsiding and densely populated coastal areas. Faced with a trade-off between imposing the very high costs of coastal protection and adaptation upon today's national economies and leaving the costs of potential major disasters to future generations, governments and decision makers are in need of scientific support for the development of mitigation and adaptation strategies for the coastal zone. of assessments of future coastal LSL rise as input for policy making for coastal zone management. Low-frequency to secular changes in LSL are the result of a number of location-dependent processes including ocean temperature and salinity changes, ocean and atmospheric circulation changes, mass exchange of the oceans with other reservoirs in the water cycle, and vertical land motion. LSL changes in response to mass exchange with land-based ice sheets, glaciers and water storage are spatially variable due to vertical land motion induced by the shifting loads and gravitational effects resulting from both the relocation of surface water mass and the deformation of the solid Earth under the load. As a consequence, close to a melting ice mass LSL will fall significantly and far away increase more than the global average. The so-called sea level equation is an integral equation, which expresses LSL as a function of present and past mass changes in ice sheets, glaciers, land water storage, and the resulting mass redistribution in the oceans. Mass exchange between oceans and the ice sheets, glaciers, and land water storage has the potential to change coastal LSL in many geographical regions. However, predictions of mass-induced LSL changes exhibit significant inter-model differences, which introduce a large uncertainty in the prediction of LSL variations caused by changes in ice sheets, glaciers, and land water storage. While the sea level equation has been tested extensively in postglacial rebound studies for the viscous (post-mass change) contribution, a thorough validation of the elastic (co-mass change) contribution has yet to be done. Accurate observations of concurrent LSL changes, vertical land motion, and gravity changes required for such a test were missing until very recently. For the validation, new observations of LSL changes, vertical land motion, and gravity changes close to rapidly changing ice sheets and glaciers in Greenland, Svalbard, and other regions, as well as satellite altimetry observations of sea surface height changes and satellite gravity mission observations of mass changes in the hydrosphere are now available.
The complexity of the Earth system and its inherent unpredictability make it difficult to predict Global Sea Level (GSL) rise and, even more so, LSL over the next 100 to 200 years. Humans have re-engineered the planet and changed major features of the Earth surface and the atmosphere, thus ruling out extrapolation of past and current changes into the future as a reasonable approach. The risk of rapid changes in ocean circulation and ice sheet mass balance introduces the possibility of unexpected changes. Therefore, science is challenged with understanding and constraining the full range of plausible future LSL trajectories and with providing useful support for informed decisions. Currently, the range of plausible future sea level trajectories turns out to be too large to be a useful basis for the planning of mitigation and adaptation. In the face of largely unpredictable future sea level changes, monitoring of the relevant processes and development of a forecasting service on realistic time scales is crucial as decision support. Forecasting and "early warning" for LSL rise would have to aim at decadal time scales, giving coastal managers sufficient time to react if the onset of rapid changes would require an immediate response. The social, environmental, and economic risks associated with potentially large and rapid LSL changes are enormous. Therefore, in the light of the current uncertainties and the unpredictable nature of major forcing processes for LSL changes, the focus of scientific decision support may have to shift from projections of LSL trajectories on century time scales to the development of models and monitoring systems for a forecasting service on decadal time scales. The scientific requirements for such a service are improved monitoring of the relevant forcing processes and the development of models with predictive capabilities on decadal time scales. \ECO \EOB \SOB \NUM 1 \BCO 2
The presentation Climate Change and Sea Level Rise: A Challenge to Science and Society was presented at the Workshop Connecting Delta Cities, June 9-10, 2009, New York City. The slides included here are a slightly modified version of the original presentation. \ECO \EOB \SOB \NUM 1 \SUB 2 \BCO 2
The presentation focuses on the challenges that coastal sea level changes caused by climate change pose to science and society. In many coastal areas, preparation for anticipated sea level changes exceeding half a meter or even meters will require severe adaptation strategies. The lead times required in order for taking meaningful actions are long and the costs are extreme. In order to meet this challenge, decision and policy makers turn to science for advice. Science, on the other hand, is challenge by the complexity of the processes forcing coastal sea level changes. Coastal Local Sea Level (LSL) is the output of many Earth system processes. Predictions of future LSL changes are associated with large uncertainties, which need to be communicated in an understandable and actionable way to those who decide on adaptation. After commenting on these mutual challenges in the next few slides, the science of sea level changes will be reviewed by first introducing the forcing processes for LSL changes, and then by looking at the uncertainties for some of these processes. Our ability to give quantitative advice depends to a large extent on the predictability of the individual processes. Unpredictabilities inherent to some of the forcing processes require a careful review of our general approach to climate change adaptation. The final part of the presentation returns to these challenges and sketches a possible way in which science could support the development of adaptation and mitigation strategies. \ECO \EOB \SOB \NUM 2 \BCO 2
When the ice sheets of the last ice age slowly started to disintegrate about 20,000 years ago, Global Sea Level (GSL) started to rise from a level of about 120 m below today's average sea level. Between 14,000 years and 8,000 years BP, GSL rose on average by 1.9 m/century, and in some centuries, rates were much higher. In many locations, coastal LSL started to change rapidly with large drops in LSL close to the melting ice masses and a rise reaching up to 150% of the global average far away from the ice loads.
GSL became rather stable around 8,000 years ago with average rates of changes dropping down to about 0.1 m/century or less. In many areas, where no large subsidence took place, humans could finally settle in the coastal zone. Rowley et al., (2007) speculate that human civilizations may have been able to develop as rapidly as they did over the last few millenniums because sea level was largely stable and allowed human settlements to persist over a long time in the coastal zone. \ECO \EOB \SOB \NUM 2 \SUB 2 \BCO 2
Even in the times of relatively stable sea level, coast lines kept shifting and settlements had to move along with the changing coast lines. The example shows the Danish coast around 900 and 2000. Large areas of the western part of what is today Denmark and Northern Germany has been inundated and partly eroded by the encroaching sea. Even today, coastal erosion is a serious problem in some of these (and other) coastal areas, threatening a lot of coastal high-value property. \ECO \EOB \SOB \NUM 2 \SUB 3 \BCO 2
Even in the times of relatively stable sea level, coast lines kept shifting and settlements had to move along with the changing coast lines. The example shows the Danish coast around 900 and 2000. Large areas of the western part of what is today Denmark and Northern Germany has been inundated and partly eroded by the encroaching sea. Even today, coastal erosion is a serious problem in some of these (and other) coastal areas, threatening a lot of coastal high-value property.
Remains of coastal infrastructure submerged today under sea level, or high above the present-day coast line demonstrate that settlements had to adapt to slow changes in LSL. Other indicators document major disasters caused by sea-level related hazards such as storm surges and tsunamis.
In the course of settling in coastal areas, humans got acquainted to the perils of the sea. \ECO \EOB \SOB \NUM 3 \SUB 1 \BCO 2
During the last millennium and more, humans have suffered many disasters caused by the sea: storm surges erased whole settlements, for example, at the German coasts, and humans temporarily moved out of the coastal zone for many decades. Tsunamis destroyed cities, for example, the earthquake and tsunami in Lisbon in 1755. The scale of the event becomes clear from the shock it brought to Europe and its noticeable impact on culture and philosophy. The background picture is one of the many art pieces that depicted the horror associated with this event. \ECO \EOB \SOB \NUM 3 \SUB 2 \BCO 2
Over time, humans learned to built coastal protections that would offset changes in sea level, coastal erosion, and the impact of storm surges.
Thus, humans no longer had to move out of the hazardous zones as the only option for adaptation. An area of command and control of the environment started: Humans developed skills in protecting coasts against the power of the sea, at least temporarily, and by that control the natural processes in the coastal zone. Settlements expanded in more and more hazardous areas prone to inundation and extreme flooding. \ECO \EOB \SOB \NUM 3 \SUB 3 \BCO 2
With these new capabilities and a mindset of control and power over the environment, humans no longer had to adapt to slowly changing sea levels or move out of hazardous areas. More and more settlement developed in the coastal zone utilizing the many economic advantages of the sea. \ECO \EOB \SOB \NUM 3 \SUB 3 \BCO 2
But when the coastal protections failed, for example, during the North Sea storm surges on January 31 and February 1, 1953, ... \ECO \EOB \SOB \NUM 3 \SUB 4 \BCO 2
But when the coastal protections failed, for example, during the North Sea storm surges on January 31 and February 1, 1953, disaster was unavoidable. 185x people were killed in the Netherlands, 3xx in the U.K. and ... \ECO \EOB \SOB \NUM 3 \SUB 5 \BCO 2
Disasters often are the cause for change. The storm surge of 1953 was not exception. In the decades that followed, policymakers in the North Sea countries most prone to storm surges decided on elaborated programs for coastal protection. Massive dikes were built ... \ECO \EOB \SOB \NUM 3 \SUB 6 \BCO 2
... Engineers developed magnificent gates that allowed rivers to flow through the dikes and that could be close with high reliability to fend off an approaching surge. Large parts of cities, like in London, were developed below sea level. One-third of the Netherlands are today below sea level, protected by dikes, barriers and pumps, and two-third of the Gross National Product of the country are earned in the area below sea level, indicating that much of the country's assets are in the area exposed to the hazard of being flooded if the protections fail.
Upper left: The Thames Barrier; taken from .... Lower right: the Rotterdam barrier; taken from http://www.deltawerken.com. \ECO \EOB \SOB \NUM 4 \BCO 2
But recent disasters have shown limitations of coastal protection:
But recent disasters have shown limitations of coastal protection:
But recent disasters have shown limitations of coastal protection:
But recent disasters have shown limitations of coastal protection:
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