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The Need for Better Scientific Understanding of Sea-Level Change

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Coastal regions are dynamic, yet sensitive, environments composed of complex geologic features that are influenced by a mix of physical and biological processes operating across a wide range of spatial and temporal scales. Understanding these processes is crucial because all the States bordering the oceans and the Great Lakes, as well as the United States' Caribbean and Pacific island territories, are undergoing widespread, long-term, and increasingly severe coastal erosion and property damage due to various complex geologic processes.

In 2000, the H. John Heinz III Center for Science, Economics and the Environment, a nonprofit institution in Washington, DC, reported that an explosive population shift and associated increase in development in coastal zones during the past 50 years has resulted in greatly increased risk to 160 million Americans and more than $3 trillion in coastal development. Continuing urbanization of the coastal zone in a time of rising sea level increases the risks from such hazards as major storms. Loss of wetlands, coral-reef ecosystems, and sandy beaches due to manmade alterations; saltwater intrusion into coastal ground-water aquifers; and acceleration of global sea-level rise are also predicted to increase significantly during the 21st century. These coastal hazards and the issues confronting the United States are common to most other coastal regions of the world as well. A compelling need therefore exists for coastal scientists to be able to reliably predict coastal change on spatial and temporal scales and with confidence limits that are meaningful to society in planning for the next 50 to 100 years.

Coastal environments owe their dynamic nature to physical processes, such as storm waves and currents and winds, that act on the widely varying geologic landforms composing coasts. Biological processes affect sediment behavior and modify the influence of geologic processes on wetlands and estuaries. Sea level relative to the land surface is controlled by several interacting processes: isostatic and tectonic crustal movements, thermal expansion and contraction of the oceans, grounded-ice-sheet volumes, and short-term ocean-atmosphere interactions, such as the El Nio-Southern Oscillation (ENSO) in the Pacific Basin and the North Atlantic Oscillation (NAO) in the Atlantic Basin. Although sea-level change has little capacity to actually erode and transport sediment, sea-level rise can be a major factor in effecting coastal change by

  1. inundating low-lying regions with low sedimentation rates, such as southern Louisiana and eastern Chesapeake Bay;
  2. expanding estuaries and bays, such as Galveston Bay; and
  3. raising the base at which storm waves and currents erode coastal landforms and transport sediment.

As such, sea-level change plays a critical role, at least on scales of centuries and longer, in determining shoreline position and coastal stability.

The scientific challenge is to isolate and quantify the processes and agents that drive coastal change and to understand the role and importance of feedbacks and thresholds of change. Empirical approaches that monitor morphological change and measure contemporary processes are necessary, but linking cause and effect is difficult, owing to the nonlinear and highly complex behavior of coastal systems. Difficulties in predicting the long-term evolution of the coastal zone are further compounded by the fact that the net morphological change resulting from sea-level rise is typically several orders of magnitude smaller than the gross morphological change resulting from depositional and erosional events acting over days, months, and years.

The geologic record shows that global sea level has risen more than 100 m since the end of the latest glacial maximum about 20,000 years ago. Although the overall picture is generally understood, important details on the rate and timing of marine transgressions, regressions, and stillstands are largely unknown.

Analyses of tide-gauge records show that global sea level has risen 20 to 30 cm during the past century. Tide-gauge records around the United States show significant variations in relative sea-level rise:

  • 2.2 mm/yr for Key West, FL,
  • 3.1 mm/yr for Baltimore, MD,
  • 10 mm/yr for Grand Isle, LA, and
  • 1.5 mm/yr for San Francisco, CA

This regional variation is due to geologic subsidence or uplift superimposed on global sea-level rise. A report released in 2001 by the Intergovernmental Panel on Climate Change (IPCC) projects that global sea level will rise 48 cm by 2100, which is about double the rate of the past century. Thus, sea-level change has the potential to be a major factor for society in planning to manage and protect coastal resources for the future. However, our scientific understanding of recent sea-level history and the relationships between sea-level change and coastal evolution is grossly incomplete. An improved understanding of sea-level history and how the coasts have evolved during the recent past (the past 10,000 years) is likely to be our most reliable means of predicting the future effects of sea-level change on coastal systems. A longer-term view is provided by the geologic record, which spans millions of years; it records the net result of coastal processes operating over long time periods and thus the systematic, rather than short-term, components of coastal evolution. Placing recent and predicted sea-level trends within a longer-term geologic context is critical to improve our scientific understanding of the links between climate change, sea-level change, coastal erosion and accretion, and changes in wetland and coral-reef ecosystems.

Related Sound Waves Stories
Sea-Level Change—a Workshop to Define Science Needs and Future USGS Research Directions
November 2002
USGS Briefs Massachusetts Coastal Managers on Sea-Level Rise and Other Coastal Hazards
May 2002
Forum on Sea-Level Rise and Coastal Disasters
Dec. 2001 / Jan. 2002

Related Web Sites
H. John Heinz III Center for Science, Economics and the Environment
non-profit institution
Intergovernmental Panel on Climate Change (IPCC)
World Meteorological Organization (WMO) / United Nations Environment Programme (UNEP)

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in this issue: Fieldwork cover story:
Seagrass Restoration in Tampa Bay

Tracking Pintail-Duck Population Decline

Remote Sensing of Coral Reefs at Biscayne National Park

Exploring the Puerto Rico Trench

Research Assateague Island Restoration

Outreach Dedication of New Lake Mead Research Vessel

Meetings Sea-Level Change Workshop

The Need for Better Scientific Understanding of Sea-Level Change

Remote-Sensing at Cape Cod National Seashore

Familiar Faces at Fall Meetings

Giving Interns a View of Science Career Paths

Staff & Center News Visiting Engineer Brings Modeling Expertise

Parsons Succeeds Lee as Acting Chief Scientist for WRCMG Team

Publications November Publications List

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