Sea-Level Variability

Sea-level rise and the associated erosion and inundation problems are currently extremely important issues for many of the US affiliated islands.  There are three factors that affect the impacts of sea-level rise on islands: the natural amount of sinking and rising of the individual islands (due to plate tectonics); the rate and extent of global sea-level rise; and the occurrence of periodic events, such as extreme lunar tides, ENSO related changes, and storm-related wave conditions. 

Many of the projected consequences of long-term sea-level rise, such as salt water intrusion into freshwater lenses and coastal erosion are already problems in some if not most island jurisdictions.  Climate-related changes of these conditions are seen, therefore, as magnifying existing problems rather than as problems in isolation from other stresses.  Island communities must deal with these problems today and, in so doing, can develop important insights into how they might most effectively respond to climate-related changes in sea level over both the short- and long-term. 

Historic and projected changes in sea level (inches above baseline) based on the Canadian and Hadley model scenarios.  The Canadian model projection includes only the effects of thermal expansion of warming ocean waters (F. Zwiers, personal communication). The Hadley model simulation adds on the sea level increment of melting of mountain glaciers (Gregory and Oerlemans, 1998). Neither model includes consideration of possible changes of sea level (upward or downward) due to melting or accumulation of snow on Greenland and Antarctica.

Environmental Impacts

Rising sea levels over the past century have already resulted in salt water entering into island freshwater lenses, overflowing coastal vegetation (such as taro, pulaka, and yams), and coastal erosion.  Coastal erosion is of concern because a typical beach erosion rate can be 150 times the amount of sea-level rise.  According to the US Geological Survey for example, about 25 percent of the sand beaches on Oahu, Hawaii have been lost or severely degraded during the last 60 years. 

The Gulf and Caribbean region experienced on average a sea-level rise compared to land-levels of nearly 4 inches (10 cm) during the 20^th century.  In the Pacific region, it is a bit more complicated.  As around the globe, actual sea level is also rising in the Pacific.  However, different islands are experiencing different rates of sea-level rise (see table).  This occurs because a number of Pacific Islands are themselves rising due to geologic uplift -- plate tectonic movements.  As a result of this uplift, it can look like sea level is not changing or even falling in some locations.  Therefore it is difficult to establish an average relative sea-level rise rate for the Pacific and the global rise rate can even be hidden. 

Long-term global rates of sea-level rise are projected to be 2 to 5 times faster in the 21^st century than during the 20^th century.  In addition to considering only the consequences of a gradual, long-term rise in sea level, island communities will continue to face short-term sea level changes as well.  In some locations in the Pacific, temporary rises in sea level from storms, lunar tides, and ENSO events raise the sea level even higher than is projected for the next century.  Future sea-level rise, both global and periodic (because increasing global sea level will also raise the level from which temporary events occur), will increasingly contribute to negative consequences for populations and ecosystems. 

Societal and Economic Impacts

Risk of flooding, inundation, and coastal erosion on particular islands depends on both physical properties of the island - elevation, rock and soil-type, location - and on biological properties -- whether the island has protection by such as coral reefs or mangroves.  The most at-risk island types are low-lying coral atolls because they are both low and porous (allows seawater infiltration).  An area is considered vulnerable if it has: areas of value (valuable areas could include: natural resources, transportation and social infrastructure, or businesses, for example); and human populations.  Islands will be particularly vulnerable to sea level changes and resulting impacts if they are low-lying and have limited resources for protecting their coastline.  In the Pacific, potentially vulnerable island groups include the atolls of the Republic of the Marshall Islands and the Federated States of Micronesia.  In the Caribbean, much of the metropolitan area of San Juan in Puerto Rico is already close to sea level.  Islands that are high volcanic or limestone, mountainous, naturally rising, or lack resources and populations in coastal areas will be less vulnerable. 

Strategies to Address Potential Sea level Variability

Coping with sea level change and the resulting impacts (e.g., flooding, inundation of freshwater and agricultural systems, erosion, destruction of transportation and other built infrastructure) will require a variety of strategies.  Because the communities will act to mitigate the increased climate change problems periodically, they will not be adjusting to 100 years of climate change/sea level effects at one time, but rather to smaller effects more frequently during the century.  This allows for staggered adaptation.  Some of the options that are likely to be needed as communities cope could include: 

• Protecting coastal infrastructure (both social and transportation), water systems, agriculture, and communities by, for example, building sea walls;

• Integrated coastal zone management;

• Crop diversification and the use of salt-resistant crops; and 

• Moving away from risk-prone, low-lying areas is also likely to be necessary in some cases, but will be complicated due to land ownership, and could have significant consequences for social and cultural identity.