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This Overview section

More detailed chapter from  "Foundation" report.

The National Assessment Overview and Foundation Reports were produced  by the National Assessment Synthesis Team, an advisory committee chartered under the Federal Advisory Committee Act, and were not subjected to OSTP's Information Quality Act Guidelines. The National Assessment was forwarded to the President and Congress in November 2000 for their consideration.

For background information on scenarios used in this assessment, see About Scenarios and Uncertainty and  Tools for Assessing Climate Change Impacts

Atlantic and Gulf Coast shorelines are especially vulnerable to long term sea-level rise as well as any increase in the frequency of storm surges or hurricanes. Most erosion events on these coasts are the result of storms, and the slope of these areas is so gentle that a small rise in sea level produces a large inland shift of the shoreline.

Winter temperatures are projected to continue to increase more than summer temperatures, resulting in a narrowing of the annual water temperature range of many estuaries. This is likely to cause species' ranges to shift and increase the vulnerability of some estuaries to non-native invasive species.

 There has also been an upsurge in the variety, incidence, and virulence of coral diseases in recent years, with major die-offs in Florida and much of the Caribbean region. In addition, increasing atmospheric CO2 concentrations could possibly decrease the calcification rates of the reef-building corals, resulting in weaker skeletons, reduced growth rates and increased vulnerability to erosion.

With few exceptions, the potential consequences of climate change are not yet being considered in coastal management. It is especially urgent to begin adaptation now with regard to development of land in the coastal zone.

KEY ISSUES • Shoreline Erosion and Human Communities • Threats to Estuarine Health • Coastal Wetland Survival • Coral Reef Die-offs • Stresses on Marine Fisheries

The US has over 95,000 miles of coastline and over 3.4 million square miles of ocean within its territorial waters. These areas provide a wide range of essential goods and services to society. Some 53% of the total US population live on the 17% of land in the coastal zone, and these areas become more crowded every year. Because of this growth, as well as increased wealth and affluence, demands on coastal and marine resources for both aesthetic enjoyment and economic benefits are rapidly increasing.

Coastal and marine environments are intrinsically linked to climate in many ways. The ocean is an important distributor of the planet's heat, with major ocean currents moving heat toward the poles from the equator. There is some chance that this distribution of heat through the ocean's "conveyor belt" circulation would be strongly influenced by the changes projected in many global climate models. Sea-level rise is another climate-related phenomenon with a major influence on coastlines. Global sea level has already risen by 4 to 8 inches (10-20 cm) in the past century and models suggest this rise is very likely to accelerate. The best estimate is that sea level will rise by an additional 19 inches (48 cm) by 2100, with an uncertainty range of 5 to 37 inches (13-95 cm). Geological forces (such as subsidence, in which the land falls relative to sea level) play a prominent role in regional sea-level change. Accelerated global sea-level rise is expected to have dramatic impacts in those regions where subsidence and erosion problems already exist.

Shoreline Erosion and Human Communities

Coastal erosion is already a widespread problem in much of the country and has significant impacts on undeveloped shorelines as well as on coastal development and infrastructure. Along the Pacific Coast, cycles of beach and cliff erosion have been linked to El Niño events that raise average sea levels over the short term and alter storm tracks that affect the coastline. For example, during the 1982-83 El Niño and the 1997-98 El Niño, erosion damage was widespread along the Pacific Coastline. If increases in the frequency or intensity of El Niño events occur, they would likely combine with long-term sea-level rise to exacerbate these impacts.

Atlantic and Gulf Coast shorelines are especially vulnerable to long term sea-level rise as well as any increase in the frequency of storm surges or hurricanes. Most erosion events on these coasts are the result of storms, and the slope of these areas is so gentle that a small rise in sea level produces a large inland shift of the shoreline. When buildings, roads, and seawalls block this natural shift, the beaches and shorelines erode, especially during storm events. This increases the threats to coastal development, transportation infrastructure, tourism, freshwater aquifers, and fisheries (which are already stressed by human activities). Coastal cities and towns, especially those in storm-prone regions such as the Southeast, are particularly vulnerable to extreme events. Intensive residential and commercial development in such regions puts life and property at risk.

The preliminary results shown on this map illustrate the relative vulnerability to sea-level rise along the New York and New Jersey coastline.

These bars show the square miles of coastal land at risk from a 20-inch rise in sea level, for seven areas of the US. Coastal wetlands projected to be inundated are shown in yellow while drylands projected to be inundated are shown in blue.

Threats to Estuarine Health

Estuaries are extremely productive ecosystems that are affected in numerous ways by climate. Winter temperatures are projected to continue to increase more than summer temperatures, resulting in a narrowing of the annual water temperature range of many estuaries. This is likely to cause species' ranges to shift and increase the vulnerability of some estuaries to non-native invasive species. Either increases or decreases in runoff would very likely create impacts to estuaries. Increased runoff would likely deliver increased amounts of nutrients such as nitrogen and phosphorous to estuaries, while simultaneously increasing the stratification between freshwater runoff and marine waters. Both nutrient additions and increased stratification would increase the potential for blooms of algae that deplete the water of oxygen, increasing stresses on sea grasses, fish, shellfish, and other living things on the bottom of lakes, streams, and oceans. Decreased runoff would likely reduce flushing, decrease the size of estuarine nursery zones, and allow predators and pathogens of shellfish to penetrate further into the estuary.

Coastal Wetland Survival

Coastal wetlands (marshes and mangroves) are highly productive ecosystems that are strongly linked to fisheries productivity. They provide important nursery and habitat functions to many commercially important fish and shellfish populations. Dramatic losses of coastal wetlands have already occurred on the Gulf Coast due to subsidence, changes caused by dams and levees that alter flow and reduce sediment supply, dredge and fill activities, and sea-level rise. Louisiana alone has been losing land at rates between 24 and 40 square miles per year during the last 40 years, accounting for as much as 80% of the total US coastal wetland loss.

In general, coastal wetlands will survive if soil buildup equals the rate of relative sea-level rise or if the wetland is able to migrate inland. However, if soil accumulation is unable to keep pace with high rates of sea-level rise, or if wetland migration is blocked by bluffs, coastal development, or shoreline protective structures (such as dikes, sea walls, and jetties), the wetland will be excessively inundated and eventually lost. The projected increase in the current rate of sea-level rise will very likely exacerbate coastal wetland losses nationwide, although the extent of impacts will vary among regions.

The rate of sea-level rise is projected to accelerate 2-5 fold over the next 100 years.  The delivery of sediments to coastal wetlands is extremely important in determining the potential of these systems to maintain themselves in the face of current and future sea-level changes.

Coral Reef Die-offs

Coral reefs play a major role in the environment and economies of two states (Florida and Hawaii) as well as most US territories in the Caribbean and Pacific. Coral reefs are valuable economic resources for fisheries, recreation, tourism, and coastal protection. In addition, reefs are one of the largest global storehouses of marine biodiversity, with untapped genetic resources. Some estimates of the global cost of losing coral reefs run in the hundreds of billions of dollars each year. The demise or continued deterioration of reefs could have profound implications for the US.

The last few years have seen unprecedented declines in the health of coral reefs. The 1998 El Niño was associated with record sea-surface temperatures and associated coral bleaching (when coral expel the algae that live within them and are necessary to their survival); in some regions, as much as 70% of the coral may have died in a single season. There has also been an upsurge in the variety, incidence, and virulence of coral diseases in recent years, with major die-offs in Florida and much of the Caribbean region. In addition, increasing atmospheric CO2 concentrations could possibly decrease the calcification rates of the reef-building corals, resulting in weaker skeletons, reduced growth rates and increased vulnerability to erosion. Model results suggest that these effects would likely be most severe at the current margins of coral reef distribution.

Stresses on Marine Fisheries

In the US, the total economic contribution of recreational and commercial fishing has been estimated at approximately $40 billion per year, with total marine landings averaging about 4.5 million metric tons over the last decade. Climate change is very likely to substantially alter the distribution and abundance of major fish stocks, and have important implications for marine populations and ecosystems. Changes over the long term are likely to include poleward shifts in distribution of marine populations. With changing ocean temperatures and conditions, shifts in the distribution of commercially important species are likely. For example, models suggest that several species of Pacific salmon are likely to have reduced distribution and productivity, while species that thrive in warmer waters, such as Pacific sardine and Atlantic menhaden, are likely to have increased distribution.

Along the Pacific Coast, impacts to fisheries related to the El Niño/Southern Oscillation illustrate how climate directly affects marine fisheries on short time scales. For example, elevated sea-surface temperatures associated with the 1997-98 El Niño had a tremendous impact on the distribution and abundance of market squid, California's largest fishery by volume. Landings fell to less than 1,000 metric tons in the 1997-98 season, down from a record-breaking 110,000 tons in the 1996-97 season. Many other unusual events occurred during this same El Niño as a result of elevated sea-surface temperatures. Examples include widespread sea lion pup deaths in California, catches of warm-water marlin in the usually frigid waters off Washington State, and poor salmon returns in Bristol Bay, Alaska.

Adaptation Strategies

It is difficult to assess the potential effects of climate change over the next few decades on coastal and marine resources, especially as climate variability is a dominant factor in shaping coastal and marine systems. The effects of future change will vary greatly in the diverse coastal regions of the nation. Additionally, human-induced disturbances also influence coastal and marine systems, often reducing the ability of systems to adapt, so that systems that might ordinarily be capable of responding to variability and change are less able to do so. In this context, climate change is likely to add to the cumulative impact of both natural and human-caused stresses on ecological systems and resources. This makes devising adaptation strategies particularly challenging.

With few exceptions, the potential consequences of climate change are not yet being considered in coastal management. It is especially urgent to begin adaptation now with regard to development of land in the coastal zone. In areas where beaches or wetlands must migrate inland to survive, it has been shown that implementing protection or retreat strategies for coastal developments can substantially reduce the economic impacts of inundation and shoreline movement. For example, coastal management programs in Maine, Rhode Island, South Carolina, and Massachusetts have implemented various forms of "rolling easement" policies to ensure that wetlands and beaches can migrate inland as sea level rises, and coastal landowners and conservation agencies can purchase the required easements. However, some regulatory programs continue to permit structures that may block the inland shift of wetlands and beaches. Additionally, allowing for such shoreline movement is only feasible in some locations due to the high degree of development on many coastlines.