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September 12, 2016
Sea-Level Rise and U.S. Coasts
Policymakers are interested in sea-level rise because of the
Figure 1. RSL Trends for the United States (2014)
risk to U.S. coastal populations and infrastructure and
changes to coastal ecosystems. Some coastal states and U.S.
territories have a considerable share of their assets, people,
economies, and water supplies vulnerable to sea-level rise.
Policy choices related to sea-level rise have the potential to
shape the future development and resiliency of U.S. coasts.
What Is Sea-Level Rise?
Scientists commonly use two descriptions of sea level:
global sea level (GSL) and relative sea level (RSL). GSL is
the average height of the Earth’s oceans. RSL refers to the
elevation of the sea level relative to the land surface from
which it is measured. From 1901 to 2010, GSL rose an
estimated 187 millimeters (mm), or 7.4 inches, averaging a
1.7 mm (0.07 inches) rise annually, according to several
studies using tide gage data. Satellite measurements,
Source: National Oceanic and Atmospheric Administration, Tides &
available since 1992, indicate an increase in the annual rate
Currents, “U.S. Sea Level Trend Map.” Figure modified by
of GSL rise to 3.2 mm (0.13 inches) through 2010. The
Congressional Research Service (CRS).
drivers for rising GSL since 1900 are predominantly
Note: Trends represent a snapshot in time for 2014 in mm per year
thermal expansion of the oceans due to warming ocean
(and calculated as feet per century using the 2014 rate).
water and melting glaciers and ice sheets. The oceans have
warmed and glaciers and ice sheets have melted due to a
Many scientists conclude that GSL will continue to rise for
combination of natural variability and the influence of
centuries even if GHG concentrations in the atmosphere are
greenhouse gas (GHG) emissions on atmospheric
stabilized. Global sea levels may rise by 0.2 meters (8
temperatures.
inches) by 2100, or they could rise 10 times that amount, or
even more, depending on the behavior of the Antarctic and
The trend and direction of RSL change varies by location.
Greenland ice sheets, according to many scientists.
RSL is rising at a rate of 9 mm-12 mm per year along
Estimates of the Antarctic ice sheet contributions to GSL
Louisiana’s Mississippi Delta region near New Orleans
rise between now and 2100 vary widely. A better scientific
(i.e., the land is sinking and sea level is rising). In contrast,
understanding of how the two large ice sheets could
RSL values are dropping along parts of the Pacific
contribute to future sea-level rise would assist coastal
Northwest coastline and southern Alaska (i.e., the land is
communities with their coastal planning and investments.
rising faster than sea level is rising). (See Figure 1.)
U.S. Coasts and Their Development
The same drivers that influence GSL change also influence
In 2010, roughly 100 million people lived in coastal
RSL change, but in some cases regional or local factors are
shoreline counties of U.S. states and 2.9 million people
more significant than global factors. These factors can be
resided in coastal shoreline counties of U.S. territories. Of
entirely natural, such as the land rebounding upward after
the people who reside in coastal state shoreline counties,
continental ice sheets melted at the end of the last ice age.
nearly 8 million live in the 1% coastal flood zone (i.e., area
They also can be due to human activities that may cause
in which the annual probability of coastal flooding is 1 in
coastal lands to subside, such as groundwater pumping, oil
100 or greater). Some coastal areas have high property
and gas extraction, sediment compaction, and land
values, whereas others have populations with more limited
management practices.
economic means. Most decisions about coastal land
development and protection are made by states, localities,
Uncertainties in Future Sea-Levels
and other stakeholders. Future growth in coastal areas may
Policymakers may contend with a pattern of sea-level rise
be shaped by the perceived risk from coastal hazards, such
that is very different in the 21st century compared to the
as sea-level rise and coastal storms, and by the efficacy of
past, as the longer-term trends accumulate over time and
private and public responses to mitigate that risk.
regional trends change. A number of studies indicate that
the rate of sea-level rise in the 21st century is very likely to
Effects of Sea-Level Rise on U.S. Coasts
exceed the rate of sea-level rise in the 20th century. Future
Some of the effects of sea-level rise on U.S. coasts can be
rates and levels of sea-level rise will be determined by a
broadly categorized as permanent or episodic flooding of
complex mix of phenomena and human activities.
low-lying lands, increased erosion and shoreline change,
increased damages from coastal storms, and saltwater
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Sea-Level Rise and U.S. Coasts
intrusion of coastal freshwaters. These effects and examples
of sea-level rise for current and future generations? How
of their societal impacts are shown in Figure 2.
does the suite of federal programs harm or bolster coastal
resilience?
Figure 2. Effects of Rising Sea Levels on Coasts
General categories of policy options related to sea-level rise
include the following:
Maintaining the Status Quo. Current government
programs, policies, and funding would continue.
Reducing Rise in Global Sea Level. Policies for
addressing the human activities influencing sea-level
rise could include pursuing domestic and international
GHG mitigation efforts.
Reducing Rise in Relative Sea Level. Policies to
address the local or regional drivers of sea-level rise
could focus on activities that contribute to land
subsidence.
Reducing Vulnerabilities to Sea-Level Rise. Policies
could target reducing vulnerability to the effects of sea-
Source: CRS.
level rise (e.g., coastal flood risk reduction projects
using dunes or storm-surge gates). Policies also could
Federal Government Effects and Actions
attempt to foster environmental and social resilience;
The federal government has an interest in how coasts are
these could include protection of certain coastal habitats,
developing and adjusting to sea-level rise. In a 2016 report
including those that contribute to natural coastal flood
titled Potential Increases in Hurricane Damage in the
defenses.
United States: Implications for the Federal Budget, the
Congressional Budget Office estimated current annual
Reducing Consequences of Sea-Level Rise. Policies
federal spending associated with hurricanes at $18 billion
could promote actions that reduce the consequences of
on average. In addition to the demand for federal assistance
the effects of sea-level rise. These actions could include
associated with coastal disasters, sea-level rise could affect
various hazard-mitigation measures, such as
the federal government in other ways. Sea-level rise could
development restrictions, building codes, flood-proofing
affect federal facilities and lands (e.g., military
of structures, buyouts of vulnerable properties, and
installations) and federal projects (e.g., navigation
improved evacuation routes.
improvements, coastal flood risk reduction projects).
Federal programs support local and state infrastructure
For all of the policy options, there are the underlying
investments such as roads, bridges, and municipal water
questions of their costs and benefits and who will bear the
facilities, which may experience damage or impaired
costs of not pursuing or pursuing the policies. For many of
operations. Increased coastal flood risk associated with sea-
the above policy options, a challenge for federal lawmakers
level rise may increase demands on the National Flood
is how to deal with the tension between federal efforts to
Insurance Program. Federal agencies also are directly
manage national and federal government risks (e.g., federal
involved in sea-level rise science and research, coastal
disaster costs, coastal ecosystem shifts) related to sea-level
regulatory activities, and protection and restoration efforts
rise and the local and state roles in shaping coastal
in coastal areas. Federal activities under the Federal Coastal
development and ecosystem health. Therefore, in the U.S.
Zone Management Act (P.L. 92-583) also assist state
federalist system of shared responsibilities, who is
programs for coastal zone management.
responsible for the costs associated with adjusting to sea-
level rise and the risks associated with vulnerable coastal
In recent years, the federal government has become
development and infrastructure is a significant question.
increasingly involved in guiding, shaping, and informing
how communities and individuals prepare for and respond
For a more detailed discussion of sea-level rise and policy
to sea-level rise. These activities include efforts to reduce
issues, see CRS Report R44632, Sea-Level Rise and U.S.
the impact of flooding on federally funded structures and
Coasts: Science and Policy Considerations, by Peter Folger
facilities, provide more technical assistance and information
and Nicole T. Carter.
on sea-level rise, and improve coastal hazard mapping.
Nicole T. Carter, Specialist in Natural Resources Policy
Policy Considerations & Questions
Peter Folger, Specialist in Energy and Natural Resources
Policymakers face several policy considerations and
Policy
questions surrounding sea-level rise, including the
following: What are the guiding principles for the federal
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role in coastal projects and activities? What are the
consequences of overestimating or underestimating the risk
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Sea-Level Rise and U.S. Coasts
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