One of the more serious and certain consequences of global warming is an increase in sea level. Intense concern is expressed for coastal regions of the planet where much of the world's population is found, because of the disastrous flooding that would follow from rapid melting or collapse of a major polar ice sheet. There is by contrast far less public consciousness of the rise in sea level that has occurred during the past 100 years and its very significant environmental and economic impacts. This is unfortunate, because even if sea level in the future rises at only the present rate, very severe costs will ensue because of the rapid economic development and population expansion occurring in the world's coastal regions. This "collision course" of social and sea level trends was largely the motivation for this book.
In the scientific community there is a growing consensus that the overall worldwide rate of sea level rise during the 20th century has been nearly 2 mm per year, sharply (10-fold) higher than the average of the past several millennia. This result is based on analyses of tide gauge data taken since the late 19th century, historical land records, and geological evidence from the late Holocene period. Apparently global sea level has a natural temporal variability that ranges from seasonal to millennial unrelated to any additional variations that might arise from anthropogenic changes of the atmosphere.
This book is designed to be suitable for a senior- or first-year graduate-level course in the earth or environmental sciences, including geography, geology, and marine science. Special emphasis is given to the evidence for historical sea level change, and case studies are used to demonstrate the resulting consequences. The emphasis is placed on the concepts involved in the causes and consequences of sea level change. Nevertheless, the presentation is not "watered down" to the merely descriptive. Enough detail is given so that the concepts can be understood and the many references profitably consulted. We make extensive use of graphical presentations, believing that graphics and data are the best teachers. The precise answers to the geophysical questions may change as a result of new data and research, but not the questions that must be asked if the accelerating increase in coastal population and development is to be managed in a reasonable and environmentally sound manner. We hope that this volume will provide a good basis for further research by its readers, and at the same time provide those interested in policy questions an overview of the subject that will serve them well in their work.
The first chapter of this book is introductory. The concept of sea level and its use as an elevation reference are discussed, and examples of the temporal and geographic variations of contemporary sea level change are presented. Sea level and its variations are surprisingly complicated, being affected by a large number of local, regional, and global factors. The impact of sea level change on the coastal landscape is also introduced, particularly in regard to beach erosion and coastal development.
Chapter 2 covers sea level change during the past 8-10 millennia, but before the era of the recording tide gauge. The sea level record during this time is obtained from geological, geochemical, and marine organism analyses and shows what some investigators believe to be significant variations possibly related to climate fluctuations. The chapter considers in much detail the evidence for intercentennial fluctuations of sea level on the order of a few meters that have occurred during the late Holocene and their possible relation to paleoclimate.
Chapter 3 is concerned with monitoring sea level in the era of the recording tide gauge, which began near the middle of the 19th century. A tide gauge is conceptually just a fancy dipstick, but it records, in addition to the global rate of sea level rise, effects of ocean circulation, meteorological forcing, and local or regional uplift or subsidence at the measurement site. In addition, a tide gauge site must provide a highly accurate and precise data record for scores of years in spite of repairs, replacements, and changes in technology. The international effort to provide long and high-quality sea level records has provided one of the essential indicators of contemporary climate change, including the El Niño phenomenon. But the existence of sea level records is not enough. The problems involved in analyzing and interpreting tide gauge records, and deciding which ones are actually useful for climate analyses, are presented by way of examples of particular water level series. Methodology is an important issue, because researchers have obtained very different results for global sea level rise during the last several decades, in spite of their use of a common data base of sea levels. This chapter also examines the issue of whether an acceleration of sea level has occurred in the last 100 years and provides a list of tide gauge records and trends suitable for determinations of the 20th-century rate of global sea level rise.
The fourth chapter of the book considers in some detail the extended geophysical response of the earth to the melting of the great ice sheets that began at the time of the last glacial maximum about 21,000 calendar years ago. The time after 4000 years ago, when glacial melting was completed, is especially interesting and important for investigations of modern sea level rise. This is so because relative sea levels continued to change in response to removal of the ice load, a process ongoing to this day at rates significant compared to measured trends of sea level. The new results in Chapter 4 yield corrections for glacial isostatic adjustment (GIA) for present-day observed sea level trends, enabling computation of an accurate estimate of 20th century global sea level rise. A convincing case is made that the 20th century rate of rise must be in the range 1.8-1.9 mm/yr, rather than at the much lower values suggested by some other investigators.
Chapter 5 covers a topic that seems implausible at first thought—the effect on the rate of sea level rise due to impoundment of water in large and small reservoirs and groundwater mining. This topic has many facets (some of which have a fair degree of uncertainty), but it appears possible that a significant increase in the rate of global sea level rise during the past 50 years has been masked by anthropogenic alterations to the hydrological cycle.
Chapter 6 presents an exciting new technology: the direct measurement of sea level by an Earth-orbiting satellite. It sounds improbable that a satellite-borne instrument could detect variations of global average sea level to an accuracy on the order of a millimeter per year, but the results obtained thus far are persuasive. Satellite altimetry has provided a breakthrough technology for observing seasonal-interannual El Niño events and may well achieve the same result for global sea level change. It will be an important milestone for climate science to have an estimate of the rate of sea level rise from altimetric satellites for the decades immediately before and after the change of the millennium for comparison to the rate for the entire 20th century.
The seventh chapter is concerned with explaining the prominent seasonal-to-interdecadal variations of sea level. These large fluctuations are highly variable regionally and are large enough to obscure an underlying trend of sea level for at least 50 years or more. Results indicate that it is possible to calculate the amplitude and phase of these variations from meteorological data to a high degree of accuracy for some sites. This creates the possibility of obtaining much "cleaner" records of long-term sea level change and earlier detection of an acceleration of the rate that can be used to confirm satellite altimetric estimates.
Chapter 8 deals with the impact of sea level rise on coastal habitability. Land is lost directly by inundation and over the long term by coastal erosion that takes place at a rate several orders of magnitude greater than the rate of sea level rise. Sea level rise over an extended period also exacerbates the damage caused by severe storms since the energetic storm-driven waves reach farther inland. We present the evidence for sea level rise as the fundamental driving force for long-term beach erosion and wetlands loss, and consider the consequences for highly developed coastal regions and island nations.
Included with this book is a CD-ROM containing monthly mean tide gauge data from the Permanent Service for Mean Sea Level. Corrections for GIA calculated by W. R. Peltier are given for each site to facilitate correcting sea level trends for this important effect. Also included is an animated presentation of global sea surface temperature anomalies during the years 1982-1999 which vividly illustrates the two largest El Niño events of the 20th century. The material on the CD-ROM is in the form of either text files or Web sites that can be opened by widely available Web browsers.
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