	Introduction

Although the extent of future sea-level rise remains uncertain, sea-level rise generally is anticipated to have a range of economic, social, and environmental effects on U.S. coasts. From 1901 to 2010, global sea levels rose an estimated 187 millimeters (mm; 7.4 inches), averaging a 1.7 mm rise annually; estimates are that from 1992 to 2010, the rate increased to 3.2 mm annually. Policy Considerations and Questions .") Sea-Level Rise Issues for Federal Policymakers

In 2010, roughly 100 million people lived in U.S. coastal shoreline counties (exclusive of the Great Lakes) and 2.9 million people resided in coastal shoreline counties of the U.S. territories. Higher sea levels increase permanent or temporary coastal land inundation, change shoreline dynamics and coastal erosion, and increase saltwater intrusion and hydrodynamic changes to coastal freshwater aquifers. If sea-level rise contributes to flooding and associated damages, the federal government can become involved through disaster assistance and the National Flood Insurance Program (NFIP) for homeowners and businesses. Relative Sea Level

Relative sea level (RSL) refers to the elevation of sea level relative to the land surface from which it is measured. 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 ( Table 2 ). The oceans have warmed due to a combination of natural variability and the influence of greenhouse gas (GHG) emissions on atmospheric temperatures. Similarly, glaciers and polar ice sheets have melted since 1900 due to a combination of natural variability and GHG-induced climate change and deposition of pollutants. These regional or local factors can be natural, such as the land rebounding upward after continental ice sheets melted at the end of the last ice age, or they can be due to human activities, such as groundwater pumping, oil and gas extraction, sediment compaction, land management practices, or other factors. In its assessment of future GSL rise, the U.S. National Climate Assessment expressed very high confidence that GSL will rise at least 0.2 meters but no more than 2.0 meters by 2100 (i.e., at least 8 inches to as much as 6.6 feet). Coasts

After a brief introduction to the effects of sea-level rise on U.S. coasts, Part II of the report is divided into the following three sections:
effects on shorelines and ecosystems, which discusses how coastal processes and topography influence how U.S. shorelines and ecosystems may change with sea-level rise; effects on development and society, which describes U.S. coastal development and its coastal storm and nuisance flooding risks; and actions addressing the impacts of sea-level rise, which describes the activities of various federal agencies, existing federal coastal management statutes, and the role of public and private actions. A public policy challenge related to sea-level rise is the intergenerational transfer of risk. Current government programs, policies, and funding would continue. Policies could promote actions that reduce the consequences of the effects of sea-level rise. For all of the policy options, there are the underlying questions of the policies' costs and benefits and of who will bear the costs of not pursuing or pursuing the policies. Considerations for Congress related to the causes of sea-level rise include the following: 
How well understood is the current and projected rate of sea-level rise? How may factors that affect sea level change in the future? A challenge for federal lawmakers and other policymakers 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, state, and private roles in shaping coastal development and ecosystem health. Among the policy questions associated with sea-level rise facing federal policymakers are the following:
In the U.S. federalist system of shared responsibilities, who is responsible for the costs associated with adjusting to sea-level rise? Are local, regional, and state land-use and development decisions and building requirements contributing to or eroding resilience to sea-level rise and coastal hazards, and what are the implications for the federal role in addressing sea-level rise? Some stakeholders are concerned that insufficient attention is paid to the risk posed by sea-level rise, as well as to the existing risk associated with coastal hazards. Other stakeholders are concerned that overestimating the risk of sea-level rise could result in overinvesting and overdesigning protections and mitigations to sea-level rise.
Policymakers are interested in sea-level rise because of the risk to coastal populations and infrastructure and the consequences for coastal species and ecosystems. From 1901 to 2010, global sea levels rose an estimated 187 millimeters (mm; 7.4 inches), averaging a 1.7 mm (0.07 inch) rise annually. Estimates are that the annual rate rose to 3.2 mm (0.13 inches) from 1992 to 2010. Although the extent of future sea-level rise remains uncertain, sea-level rise is anticipated to have a range of effects on U.S. coasts. It is anticipated to contribute to flood and erosion hazards, permanent or temporary land inundation, saltwater intrusion into coastal freshwaters, and changes in coastal terrestrial and estuarine ecosystems.
Some states, such as Florida and Louisiana, and U.S. territories have a considerable share of their assets, people, economies, and water supplies vulnerable to sea-level rise. In 2010, roughly 100 million people lived in U.S. coastal shoreline counties. Increased flood risk associated with sea-level rise may increase demand for federal disaster assistance and challenge the National Flood Insurance Program. Federal programs support local and state infrastructure investments that may be damaged or impaired, such as roads, bridges, and municipal water facilities. Sea-level rise also is anticipated to affect numerous federal facilities.
Global and Relative Sea Levels. Sea levels are expressed in terms of global sea levels, which is the average value of sea surface heights around the globe, and relative sea levels, which is the sea level relative to the land surface. Since 1900, expanding oceans due to warming ocean water and melting glaciers and ice sheets have been the main drivers of global sea-level rise. Oceans have warmed due to a combination of natural variability and the influence of greenhouse gas emissions on atmospheric temperatures. Similarly, glaciers and ice sheets since 1900 have been melting due to both natural variability and greenhouse gas emissions. In 2012, the U.S. National Climate Assessment expressed very high confidence in global sea levels rising at least 0.2 meters (8 inches) but no more than 2.0 meters (6.6 feet) by 2100.
There are regional and local variations in the rate of sea-level rise. Regional or local factors can be natural, such as the land rebounding upward after continental ice sheets melted at the end of the last ice age, or they may be due to human activities, such as groundwater pumping, oil and gas extraction, sediment compaction, and land management practices, among others. With few exceptions, sea levels are rising relative to the coastlines of the contiguous United States, as well as parts of the Alaskan and Hawaiian coastlines.
Policy Considerations. Policy choices related to sea-level rise have the potential to shape the future development and resiliency of U.S. coasts. Policy options include a continuation of current government programs and policies, actions that address the forces contributing to sea-level rise globally or locally, and actions that reduce the vulnerability to and consequences of sea-level rise on U.S. coasts. For all the policy options, there are underlying questions of costs and benefits and who bears the costs of pursuing or not pursuing the policies. 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. Related policy questions include the following: To what extent do federal programs, regulations, and funding influence how coasts develop and redevelop? Who is responsible for the costs associated with adjusting to sea-level rise? Who will bear the risks associated with vulnerable coastal development and infrastructure? Some stakeholders are concerned that governments at all levels are paying insufficient attention to the risks posed by sea-level rise; others are concerned that overestimating the risk of sea-level rise could result in foregoing current uses of coastal areas and promoting overinvestment and overdesign of sea-level rise mitigation and adaptation.
CRS In Focus IF10468, Sea-Level Rise and U.S. Coasts, also provides a brief overview of sea-level rise science and policy options.