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