Rising seas and changing currents

Posted: 10 October 2000

Author: John Pernetta

Author Info: John C. Pernetta, Senior Programme Officer International Waters, at the UNEP/GEF Co-ordination office, United Nations Environment Programme, PO Box 30552, Nairobi, Kenya.

The opinions expressed in this article are those of the author and do not necessarily represent those of either UNEP or the Global Environment Facility.

Despite recent re-estimates of expected sea level rise, small island nations and deltas are still under threat. John Pernetta reports on the latest evidence.

During the mid-1980s when estimates of global mean sea level rise as a result of climate change were in excess of one metre by the middle of this century, great concern was expressed by leaders of the small island states and low lying coastal countries concerning the potentially devastating impacts that might result. As scientists have refined those estimates, the magnitude of the anticipated sea level rise has declined, and so apparently has international interest and concern.

The present estimates of a rise in global mean sea level of around 5 mm per year nevertheless represent a significant threat to low-lying states composed of atoll islands such as the Maldives, the Marshall Islands, Kiribati and others in the Caribbean, Pacific and Indian Oceans. It has been estimated for example that of the total land area of the Maldives (398 square kilometres) some 65 per cent is less than one metre above present sea level. The rate at which is such islands can accrete vertically through natural production and accumulation of coral sand is unknown and thus the ultimate fate of these islands cannot be predicted.

Part of the Maldives archipelago - under threat from sea rise© D. Sansoni/Panos PicturesWhat is clear however is that islands such as Male the capital of the Maldives, which have undergone extensive development, including the construction of sea walls and armouring of the shore-line cannot grow upwards naturally. Such islands lie on top of porous limestones, thus as the sea rises outside the walls it also rises inside, restricting the depth of the freshwater lens that represents the major source of potable freshwater for the human population. As sea level rises seawalls will need to be raised and pumping installed to maintain a dry land surface. The recurrent costs of pumping, using imported fossil fuels will be beyond the capacity of the nation to support, and there is a need therefore to examine the feasibility of using alternative and renewable energy sources such as solar, wind and biogass.

While many atolls rest on solid geological foundations and the rates of geological subsidence are low, low-lying coastal areas, particularly those occurring in the major delta's of the world, face added problems. The relative rate of sea level rise in these areas is much greater than the rate of global mean sea level rise due to the added problems of a sinking land surface. The land may be sinking naturally due to subsidence of the underlying strata and compaction of the softer sediments above, but natural subsidence is greatly increased in areas such as Bangkok where groundwater is extracted for human use. In addition the major rivers that under natural conditions provided annual inputs of sediments to the delta, thus raising the land surface, have been dammed or their channels canalised so that additions of silt to the land surface no longer occur, or occur only in greatly reduced quantities.

Such problems are not insurmountable as shown by the Netherlands where much of the land surface lies below present sea-level and a dry land surface is maintained by means of protective dikes and seawalls and continuous pumping out of excess water. But the costs of such a response to rising sea level can be met only in wealthier nations. For poorer developing countries such as Bangladesh the costs would be prohibitive.

The time lags between increasing greenhouse gas concentrations in the atmosphere, rising temperatures and expansion of the sea surface waters causing increasing mean sea levels are long. Even if greenhouse gas concentrations in the atmosphere are stabilised, the problems of sea level rise will continue for many decades, and continued rapid development of coastal areas will enhance their vulnerability in the longer term.

More sustainable patterns of coastal and island development are needed if the economic costs of the responses are to be kept within the bounds of the possible.

Ocean Conveyor

The future impact of global warming on ocean currents provides another conundrum for scientists and policy-makers.

Due to its great capacity for storing energy and its relatively slow response to rising global atmospheric temperatures the ocean has been described as the flywheel of the Earth's climate system. Transporting heat energy from the tropics and releasing it to the atmosphere at higher latitudes the ocean moderates the climate of continental areas at higher latitudes. Changes in density of the sea surface waters resulting from heating and cooling, freshwater inputs and evaporation that alter the salt concentration result in cold, dense water sinking in the North Atlantic and flowing as deep water southwards.

The Great Ocean Conveyor belt moves water between the ocean basins over thousands of years and while this pattern of circulation is unlikely to be changed by rising atmospheric temperatures it could be changed if the freshwater run-off from the land surface changes markedly with climate change. In contrast to the deeper ocean waters the circulation and movement of surface waters responds more rapidly to changes in atmospheric temperature and circulation.

And, as we know, alterations in the strength and direction of flow within such surface systems can affect nutrient upwelling, plankton production and fish stocks are well illustrated in the El Niño Southern Oscillation.

During strong El Niño events the upwelling of nutrient rich deep ocean water off the Pacific coast of Latin America is suppressed with resultant reduction in phytoplankton productivity and collapse of the pelagic (open sea) anchovetta fishery. Not only does this have severe economic impacts but other species dependent on this productive pelagic system suffer starvation and high mortality rates.

During the 1972-73 and 1982-83 El Niño events the catch of anchovetta fell to one six and one six hundredth of their normal levels. Following the 1972-73 event the populations of cormorants, boobies and brown pelicans fell to around 6 million birds from approximately 30 million in 1950 and following the 1982 event a further decline reduced their numbers to around 300,000.

Similar inter-annual variations in current patterns in the Northwest Pacific also result in inter annual variability in the Japanese catch of sardine. Not only would a permanent change in surface ocean currents impact fish stocks in this way but changes in water temperatures and currents will affect the distribution and stocks of Demerol in-shore fish and other species. During the 1982-83 El Niño, for example, water temperatures around 1.5oC higher than normal bleached and killed large areas of coral around the Gorgona Islands of Panama. Elsewhere, off Peru, the warmer conditions led to a ten-fold increase in lorna fish.

While the overall productivity of the oceans and the global total harvest of marine species are unlikely to change under conditions of climate change and changing current patterns, the distribution of the stocks and the relative importance of different species is likely to alter, with consequent economic impacts on the fisheries and countries that depend upon these resources.

Enhancing our capacity to understand, model and predict inter-annual variability in oceanic conditions and its consequences for marine biodiversity and fisheries production thus has a strong economic justification.