Geography
Sea Level Change
Sea level change refers to the rise or fall in the average level of the Earth's oceans. It is influenced by factors such as melting ice caps, thermal expansion of seawater, and tectonic activity. Sea level change has significant implications for coastal regions, including increased risk of flooding, erosion, and loss of habitat.
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10 Key excerpts on "Sea Level Change"
eBook - ePub- Michael A. Summerfield(Author)
- 2014(Publication Date)
- Routledge(Publisher)
17 Sea-Level Change
DOI: 10.4324/9781315841182-1717.1 Global and regional sea-level change
Sea-level change is a central concern of geomorphologists. Not only does the sea surface determine the base level for erosion, but relative vertical movements of land and sea can greatly alter the area of land exposed to exogenic sub-aerial geomorphic processes. Indeed, over the past 100 Ma the relative proportion of land and sea has changed dramatically. The study of sea-level change is also important because, as we have seen in previous chapters, it can provide key evidence of climatic change and can also give us a benchmark for estimating rates of tectonic uplift.Satellite technology now enables us to measure the form of the sea surface (the ocean geoid – see Section 2.1.1 ) to a precision of a few millimetres. These accurate altimetric measurements can be used to monitor changes in sea surface elevation due to tides and currents which occur over short periods (hours, days or months). Sea-level change in a geological sense, on the other hand, occurs on a much longer time scale of thousands and millions of years. These changes in sea level through time can only be determined in terms of movements of the sea surface with respect to adjacent land areas. Consequently, it is shorelines, marking the interface between land and ocean, which are the focus of sea-level change studies. In many cases sea-level changes, which of course involve vertical movements of the sea surface, have to be inferred, often indirectly, from horizontal shoreline movements.Transgressions occur when the shoreline advances landward, whereasregressions represent the retreat of shorelines from the land.A relative change in sea level is a rise or fall in the mean level of the sea surface with respect to the landsurface (or the sea bed). Either the sea surface, or the landsurface, or the two in combination, may rise or fall during a relative sea-level change. A relative rise of sea level is an apparent rise in the mean level of the sea surface with respect to a landsurface and can result from:
eBook - ePub- Simon Haslett(Author)
- 2008(Publication Date)
- Routledge(Publisher)
5 Sea level and the changing land-sea interfaceSea-level change is one of the main factors in stimulating coastal change, and in the long term it has been controlled by changes in the volume of landlocked ice sheets. If sea level rises, then coasts must be able to change dynamically to keep up, or drown. On the scale of a human lifetime we may consider sea level to be unchanging, but even through the twentieth and early twenty-first centuries it appears that sea level has been rising due to human activity, and may continue to rise well into the future. This chapter covers:- the principal mechanism for sea-level change
- methods employed in constructing records of sea-level change
- the various responses of coastlines to sea-level change
- twentieth-century sea-level rise, that has been attributed to our enhancement of the Greenhouse Effect, and future predictions
- ways of managing sea-level rise
5.1 Introduction
The level of the sea is not constant, it is always rising and falling, whether through the passing of waves (including tides), meteorological influences, or gravitational effects in the form of the earth’s geoid. Long-term and significant sea-level changes, however, reflect changing levels of both land and sea; eustasy refers to absolute changes in global sea level, and isostasy refers to the vertical movement of land due to local geological factors. It is the balance between these two processes at a given coastline that produces observed changes in sea level, referred to as relative sea-level change, because an absolute (eustatic) rise in sea level may not be required to allow the sea to rise relative to the land. Figure 5.1
eBook - PDF- Gerd Masselink, Roland Gehrels, Gerd Masselink, Roland Gehrels(Authors)
- 2014(Publication Date)
- American Geophysical Union(Publisher)
For this reason, relative sea level will be the focus of this chap- ter and so, hereafter, the term ‘sea level’ will be used to mean relative sea level. A key theme in this book is the influence of climate change on coastal evolution, and an important issue of concern in this regard is our ability to predict future changes in a warming climate. Measurements of sea-sur- face height play an important role in understanding recent climate-driven sea-level change and underpin our ability to predict future changes. Therefore, absolute sea-level changes have an important place in this chapter, albeit secondary to relative changes. 2.1.2 Processes affecting sea level From the definitions of sea level given in section 2.1.1, it follows that any process that affects a height shift in either the sea surface or the sea floor at a given location will pro- duce a change in relative sea level. Only those processes that influence the sea surface will significantly impact absolute sea level. There are a large number of processes operating over a range of spatial and temporal scales that will deflect either, or both, the upper and lower bounding surfaces of the ocean (Fig. 2.3). The aim of this section is to provide an overview of these processes and to empha- size those that operate over century and longer timescales, as these have the greatest influence on coastal evolution during the Quaternary (in terms of defining the location of the coastal zone during this period). Over relatively short timescales (seconds to days), ver- tical deflections of the ocean surface are driven mainly by interactions of the ocean with the atmosphere. The rapid- ity of these changes reflects the fact that the atmosphere and the ocean can flow relatively quickly (compared to other components of the climate system, such as ice and the solid Earth).
No longer available |Learn more- (Author)
- 2014(Publication Date)
- The English Press(Publisher)
________________________ WORLD TECHNOLOGIES ________________________ Overview of sea-level change Local and eustatic sea level Water cycles between ocean, atmosphere, and glaciers. Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as Sea Level Changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL. “Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin. Short term and periodic changes There are many factors which can produce short-term (a few minutes to 18.6 years) changes in sea level.
eBook - PDF- Ivan Valiela(Author)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
The ice on land in Greenland is more exposed to warming atmospheric tem-peratures, and is hence losing volume. In Antarctica it is thought that ice volumes may be still expanding, but sufficient data are lacking. SEA LEVEL RISE 61 about 0.5 cm to global sea level rise during the 20th century, a small contribution compared to the larger global effects of thermal mountain gla-cier melt and thermal expansion. Meteorologically driven changes A variety of weather-related factors may alter sea level. We have already mentioned waves and tides, only to add that these are short-term influ-ences. There are meteorological disturbances that may be somewhat longer term, lasting years. Examples of such weather-driven features are the effects of ENSO events. Multiyear gauge records off Rabaul, Papua New Guinea show a trend to decreases in relative sea level, owing to uplifting of the land (Fig. 3.11). Note, however, the large drops and rises in sea level during 1982–1983 and 1986–1987. These were years with pronounced El Niño events. Another example is the increased frequency with which the Thames Barrier en-gineering works, which protect London from flooding, have had to be closed (Fig. 3.12). These periodic high floods are a combination of more frequent unusual meteorological events and high tides. Changes in atmospheric pressure and winds can therefore significantly alter sea level, not only in localities such as New Guinea and London, but across large sections of entire oceans, usually for months to a few years. Crustal deformations All land surfaces on earth are subject to vertical shifts, prompted by post-glacial rebound, plate movement by tectonic forces, accumulation of sediments, as well as other minor mechanisms. Huge ice sheets, up to 3,000 m thick, covered the northern third of the world’s continents at the peak of the Wisconsin glaciation, 18,000 years ago 15 (Fig. 3.13 top panels).
- Das, Aditya Kishore, Mira Das(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
Mechanism of Sea Level Rise due to Climate Change Sea level can rise by two different mechanisms with respect to climate change. First, as the oceans warm due to an increasing global temperature, the volume of seawater increases taking up more space in the ocean basin and causing a rise in water level. The second mechanism is the melting of ice over land, which then adds water to the ocean. The Intergovernmental Panel on Climate Change (IPCC 4 th Assessment Report, 1997) predicts that the total global-average sea level rise from 1990 to 2100 will be 110 to 770 millimeters (4.3 to 30.3 inches) under different warming scenarios. The expansion will be different for different greenhouse gas emission scenarios. Unwarned and temporary sea level rise might occur due to tsunami and tectonic plate slip as per latest findings. The tragic loss of life associated with the Indian Ocean tsunami of December 2004 followed by sea level rise in certain coasts is the latest example. The nature and extent of impact due to a tsunami is related to the mechanism responsible for its generation like seismic event, mass movement, etc. Consequently, the materials deposited and the events responsible for these deposits assume significance. This requires the linkage of terrestrial and marine records, and involves observation of palaeo-environmental reconstruction and computer model simulation. Model predictions for the Paradip coast of Orissa in east coast was analyzed for the area 19º to 22º N and 84.9º to 88.5º E with its southern open boundary located at latitude 19º N due to natural hazards like cyclones. Although the principal component is surge, the sea-surface elevation may either be decreased or increased with respect to surge value, depending on its phase with astronomical tide. However, the mutual interaction of tide and surge is nonlinear, and both processes must be considered simultaneously.
eBook - PDFQuaternary Sea-Level Changes
A Global Perspective
- Colin V. Murray-Wallace, Colin D. Woodroffe(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
Indeed, it has been suggested that humans have had a measurable impact on the environment since the Industrial Revolution, and that this should be recognised by assigning the period we live in to a new geological epoch called the Anthropocene (Zalasiewicz et al., 2010). Among the human-induced climate-change impacts in the Anthropocene it appears that sea level has also been affected. The extent to which the sea is already rising, whether it has been influenced by human activities, and how sea level will vary in the future, remains incompletely understood. There are major uncertainties about emission (and hence atmospheric greenhouse gas concentration) scenarios, but there is also a lack of detailed understanding of processes by which many other factors contributing to relative sea-level change will evolve in future. These concerns with present sea-level trends and future sea-level trajectories involve several factors explored in preceding chapters in relation to Quaternary sea-level changes. However, there are also other components operating at shorter timescales or that result from human activity. In order to better understand current and future sea-level changes there are new, sophisticated techniques which provide insights into aspects of contemporary sea-level change, and offer considerable potential to understand changes at a scale that can better inform human endeavours in the coastal zone. In the following sections, reconstruction of sea-level changes from tide gauges is reviewed. Extension of tide-gauge records back in time using geological proxies is examined; corroboration using sophisticated satellite and other monitoring systems is outlined; and modelling future scenarios is explored. Complications related to land movements are summarised, together with potential impacts of sea-level rise and the implications for adaptation measures that will be needed.
eBook - PDFSea Level Rise
History and Consequences
- Bruce Douglas, Mark T Kearney, Stephen P. Leatherman(Authors)
- 2000(Publication Date)
- Academic Press(Publisher)
Chapter 3 Bruce C. Douglas Sea Level Change in the Era of the Recording Tide Gauge 3.1 INTRODUCTION Sea level rise is often regarded as an issue for the future, rather than of the present day. The most alarming scenarios involve partial melting or collapse of a great polar ice sheet a hundred years or more hence due to global warming. The result of such a catastrophe would be flooding of coastal regions of the world, causing massive loss of life and property and breakdown of social order. Fortunately, plausible forecasts (Houghton et al, 1996) of Sea Level Change for the 21st century are far less ominous. But as Chapters 2 and 8 document, the impact of even a moderate rate of sea level rise is severe, especially for island and developing nations. In fact, if global sea level rises in the 21st century at only the 20th-century rate of about 2 mm per year, the economic and social burdens will still be profound, because an increase of sea level significantly increases the impact of storms on heavily populated low-lying coastal areas. It is a matter of practical urgency to determine the amount and causes of global sea level rise so that mitigation activities can begin as soon as possible. The global increase of sea level by as much as 20 cm over the last century, although important, is minor compared to what occurred in the distant past. Sea level rose by approximately 125 meters (over 600 times greater!) as a result of the disappearance of the great glacial ice sheets that began about 21,000 years ago. Melting was complete everywhere by 4000-5000 years ago, at which time relative sea level was within a few meters of its present value for most of the earth. Removal of the ice load also caused the elevation of certain high-latitude locations to increase by hundreds of meters and adjacent areas to subside as the earth adjusted viscoelastically.
eBook - PDFSea-Level Science
Understanding Tides, Surges, Tsunamis and Mean Sea-Level Changes
- David Pugh, Philip Woodworth(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
From [62]. Sea-level changes in time to do with the solid Earth 310 For these reasons, sea level is one of the major controllers of coastal evolution. Sea-level change raises or lowers the level at which wave attack occurs and causes erosion. Sea level (water depth) determines tidal wavelength and hence the spatial pattern of the tides in shallow waters. It modifies the magnitude of storm surges that result in coastal flooding. These different processes in the ocean can interact with each other and with others that affect the coast from the land side, such as river flow and fluvial sediment supply. As a consequence, the modelling and prediction of coastal evolution present a formidable challenge that, never- theless, can be simulated in general terms [73, 74]. The challenge is even greater when coastal constructions, including flood defence schemes, mineral extraction and energy generation, are imposed on natural processes. The role of the tides within a number of coastal processes has been discussed previously [75] and in those processes the role of sea level is implicit for the above reasons. However, the role of sea level in coastal evolution becomes more explicit over longer time- scales. The most obvious concerns the rise of sea level after the Last Glacial Maximum (Figures 10.15 and 11.17) when the continental shelves were flooded and tides were modified throughout the world ocean [76].
eBook - PDF- Jimmy Jiao, Vincent Post(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
284 Sea Level Change and Coastal Aquifers for the year 2100 is 0.28 to 0.98 m (Church et al., 2013), but this could be more by as much as a metre if the Antarctic ice cap enters a state of runaway meltdown (Feldmann and Levermann, 2015; DeConto and Pollard, 2016). A relatively new insight is that ground- water storage depletion by abstraction may also contribute to sea level rise to a certain degree (Konikow, 2011b; Wada et al., 2012), as will be discussed in Section 10.6. It is expected that sea level will rise in 95% of the ocean area, the exception being areas near large ice masses where sea levels will fall when their melting causes them to exert less gravitational pull. About 70% of the world’ s coastlines will experience a change in sea level that will be within 20% of the global average, but locally important deviations will occur depending on other processes that control relative sea level such as isostatic rebound or tectonic movement (Church et al., 2013). 10.3 Impact of Sea Level Rise on Coastal Zones 10.3.1 Shoreline Retreat and Flooding Accelerated sea level rise during the twentieth century (Kopp et al., 2016) has led to a higher incidence of minor flooding events along the coast of the USA (Sweet et al., 2014), and rising temperatures lead to a greater risk of cyclone-related storm surges (Grinsted et al., 2013). It is expected that millions more people will experience more floods as a result of the rise in sea level (IPCC, 2007). Land subsidence exacerbates damage from coastal flooding. Land subsidence can be caused by over pumping of coastal aquifers (Chen Thermal expansion Greenland ice sheet surface mass balance Antarctic ice sheet surface mass balance Ice sheet rapid dynamics Land water storage Total of contributions Glaciers –0.2 0.0 0.2 0.4 m 0.8 0.6 15–55% 30–55% Figure 10.2 Contribution of various processes to projected global sea level rise in the twenty-first century (Church et al., 2013). 10.3 Impact of Sea Level Rise on Coastal Zones 285
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