Global Change and Coastal Environments
Global climate change over the remainder of the twenty-first century will have major impacts on coastal environments. The changes include increases in seasurface temperature and sea level, decreases in sea-ice cover, and changes in salinity, wave climate, and ocean circulation.
What changes have already occurred? According to recent reports of the Intergovernmental Panel on Climate Change, sea level has risen about 15 cm since 1870. Most of this rise is due to thermal expansion of sea water in response to global warming of about 0.75°C (1.35°F) since 1906, but some is also due to the melting of glaciers and ice caps. Arctic sea-ice cover is decreasing at a rate of about 3 percent per decade, with summer ice cover decreasing by about 7 percent per decade. Extreme weather events are increasing, including the number of heat waves, number of precipitation events leading to flooding, the extent of drought-affected regions, and the intensity and duration of tropical storms.
What changes are in store? Between 1990 and 2100, global average surface temperature will increase between 1.7 and 6.4°C (3.1 and 10.5°F), depending on the scenario for future economic growth and usage of fossil fuels. Sea level will rise from present levels by about 21 cm to 47 cm (8.3 to 18.5 in.), again depending on the scenario. Snow and ice cover will continue to decrease, and mountain glaciers and ice caps will continue their retreat of the twentieth century. Tropical cyclone peak wind and peak precipitation intensities will increase, and El Nino extremes of flood and drought will be exaggerated.
These changes are bad news for coastal environments. Let's begin with coastal erosion. Global warming will increase the frequency of high winds and heavy precipitation events, amplifying the effects of severe coastal storms. More frequent and longer El Ninos will increase the severity and frequency of Pacific storms, leading to increased sea-cliff erosion along Southern California's south- and southwestfacing coastlines. During La Nina events, Atlantic hurricane frequency and intensity will increase with increased risk of damage to structures and coastal populations.
How will sea-level rise impact coastlines? Over the past 100 years or so, about 70 percent of sandy shorelines have retreated and 10 percent have advanced. The long-term effect of sea level is to push beaches, salt marshes, and estuaries landward. Beaches disappear and are replaced by sea walls. Salt marshes are drained to reduce inland flooding. Estuaries become shallower and more saline. In this way, the most productive areas of the coast are squeezed between a rising ocean and a water's edge that is increasingly defended.
Sea-level rise will not be uniform. Modifying factors of waves, currents, tides, and offshore topography can act to magnify the rise, depending on the location. Some models predict doubled rates of sea-level rise for portions of the eastern United States, North American Pacific coast, and the western North American arctic shoreline. Land subsidence is a contributing factor to the impact of sea-level rise. Many coastlines are fed by rivers that have now been dammed, often many times, which reduces the amount of fine sediment brought to the coast. Without new sediment, coastal wetlands slowly sink as the older sediment that supports them compacts. This subsidence increases the effects of sea-level rise.
Delta coasts are especially sensitive to sediment starvation and subsidence. Here, rates of subsidence can reach 2 cm/yr (0.8 in./yr). The Mississippi has lost about half of its natural sediment load, and sediment transport by such rivers as the Nile and Indus has been reduced by 95 percent.
According to recent estimates, sea-level rise and subsidence could cause the loss of more than 22 percent of the world's coastal wetlands by the year 2100. Coupled with losses directly related to human activity, coastal wetlands could decrease by 30 percent or more, with major impacts on commercially important fish and shellfish populations.
Coral reefs, like coastal wetlands, perform important ecological functions. They are highly biodiverse. They also serve as protective barriers to coastlines against storm waves and surges. However, more than half of the total area of living coral reefs is thought to be threatened by human activities ranging from water pollution to coral mining. When stressed by a rise in temperature, many corals respond by “bleaching”. In this process, they expel the algae that live symbiotically inside their structures, leaving the coral without color. The bleaching may be temporary if the stress subsides, but if permanent, the corals die. Major episodes of coral bleaching have been associated with increased water temperatures during strong El Ninos.
Many pristine stretches of arctic shoreline are threatened by global warming. As global temperatures rise, the shoreline is less protected by sea ice, frozen ground, and ground ice. Greater expanses of open sea allow larger waves to attack the coast. Moreover, global warming will be especially severe at high latitudes. Rapid coastal recession has already been reported along the Beaufort Sea.
It is apparent that global climate change will have major impacts on coastal environments, with very broad implications for ecosystems and natural resources. It will take careful management of our coastlines to reduce those impacts on both human and natural systems.