CIESIN Reproduced, with permission, from: United Nations Environment Programme. 1990. Climate change and sea-level. United Nations Environment Programme Information Unit for Climate Change Fact Sheet 102. Nairobi, Kenya: United Nations Environment Programme (UNEP) Information Unit for Climate Change (IUCC).

Fact sheet 102

Climate change and sea-level

The global mean sea-level may have already risen by around 15 centimetres during the past century. According to a number of studies, the sea has been rising at the rate of 1-2 millimetres per year over the past 100 years. Measuring past and current changes in sea-level, however, is extremely difficult. There are many potential sources of error and systematic bias, such as the uneven geographic distribution of measuring sites and the effect of the land itself as it rises and subsides.

Climate change is expected to cause a further rise of about 20 centimetres by the year 2030. Forecasts of a rising sea-level are based on climate model results, which indicate that the earth's average surface temperature may increase by 1.5-4.50C over the next 100 years. This warming would cause the sea to rise in two ways: through thermal expansion of ocean water, and through the shrinking of ice caps and mountain glaciers. According to the Intergovernmental Panel on Climate Change (IPCC -- see fact sheet 208), if no specific measures are taken to abate greenhouse gas emissions, these two factors are likely to cause the sea to rise by around 65 cm from current levels by the year 2100. This expected rate of change (an average of 6 cm per decade, with an uncertainty range of 3-10 cm) is significantly faster than that experienced over the last 100 years.

Forecasting sea-level rise involves many uncertainties. While most scientists believe that man-made greenhouse gas emissions are changing the climate, they are less sure about the details, and particularly the speed, of this change. Global warming is the main potential impact of greenhouse gas emissions, but other aspects of the climate besides temperature may also change. For example, some studies suggest that changes in precipitation will increase snow accumulation in Antarctica, which may help to moderate the net sea-level rise. Another complication is that the sea-level would not rise by the same amount all over the globe due to the effects of the earth s rotation, local coastline variations, changes in major ocean currents, regional land subsidence and emergence, and differences in tidal patterns and sea-water density.

Higher sea-levels would threaten low-lying coastal areas and small islands. The sea-level rise figures given by the IPCC may appear modest. However, the forecasted rise would put millions of people and millions of square kilometres of land at risk. The most vulnerable land would be unprotected, densely populated, and economically productive coastal regions of countries with poor financial and technological resources for responding to sea-level rise. Clearly, a rise in sea-level would create irreversible problems for low-lying island nations such as the Maldives and the Pacific atolls (fact sheet 203). Elsewhere, tourist beaches, cultural and historical sites, fishing centres, and other areas of special value would be at risk. The costs of protecting this land from the sea and preventing constant erosion would be enormous. Additional investments would also be needed to adapt sewage systems and other coastal infrastructure. On the other hand, some localities, such as shallow ports, would benefit from a higher sea-level.

Groundwater in some coastal regions would become more saline. Rising seas would threaten the viability of freshwater aquifers and other sources of fresh-groundwater. Communities may have to pump out less water to prevent aquifers from being ref illed with sea-water. Coastal farming would face the triple threat of inundation, freshwater shortages, and salt

damage. In Indonesia, for example, agricultural settlements in marshy areas close to the coast would be highly sensitive to small shifts in o cean

levels (fact sheet 104).

The flows of estuaries, coastal rivers, and low-lying irrigation systems would be affected, and tidal wetlands and mangrove forests would face erosion and increased salinity. Wetlands not only help to control floods, but they are critical to biodiversity and to the life-cycles of many species. While many marshlands would be able to migrate inland as the sea rose, some species would suffer serious losses during the transition. Flat river deltas, which are often agriculturally productive, would also be at risk. Among the most vulnerable are the Amazon, Ganges, Indus, Mekon, Mississippi, Niger, Nile, Po, and Yangtze.

The damage caused by floods, storms, and tropical cyclones might worsen. Major harbour areas would experience more frequent flooding during extreme high tides and, in particular, during storm surges. Countries already prone to devastating flood s, such as low-lying Bangladesh, would be the most affected. Warmer water and a resulting increase in humidity over the oceans might even encourage tropical cyclones, and changing wave patterns could produce more swells and tidal waves in certain regions.

******************************************************************** Last revised August 1992 by the Information Unit on Climate Change, UNEP, Palais des Nations, CH-1211 Geneva 10, Switzerland.

tel: (+41) 22 789-4062

fax: (+41) 22 789-4073