|The Regional Impacts of Climate Change|
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6.3.3. Oceans, Saltwater Fisheries, and Coastal Zones
The oceans function as regulators of the Earth's climate and sustain planetary biogeochemical cycles. Natural fluxes of CO2 among the atmosphere, oceans, and land biota are much larger than anthropogenic perturbations (IPCC 1996, WG I, Section 2.1). The oceans are estimated to have absorbed about 30% of CO2 emissions arising from fossil fuel use and tropical deforestation between 1980 and 1989 (Siegenthaler and Sarmiento, 1993), thus slowing the rate of greenhouse warming. The oceans also play a major role in the global hydrological cycle. Cycling of the oceanic freshwater fraction through advection, evaporation, precipitation and-in higher latitudes and elevations-the solid-ice phase is affected by changes in wind systems and oceanic current systems, as is the case with the Latin American climate (see Section 6.2). A changing pattern of rainfall over the oceans would cause changes in the rainfall pattern over land-which would, in turn, have a considerable effect on the salinity of marginal seas.
Living marine resources include fish, crustaceans and shellfish, marine mammals, and seaweed. In 1990, the world's fish catch was 97 million tons (14 Mt from inland sources and 83 Mt from marine sources) (FAO, 1992a). Latin America, which is located between two large oceans, has a recognized wealth of marine biotic resources, sustaining large schools of fish and enabling the development of some of the most important saltwater fisheries in the world. The average annual catch by Latin American countries during the period 1985-87 was about 13 Mt, or about 17% of the world's catch (WRI, 1992). Although these figures do not include catches made by other nations plying the region's seas, they provide a clear idea of the economic importance of the oceans in Latin America. Furthermore, krill-a pelagic member of the crustacean suborder Euphausiacea-is of great importance in the trophic chain of the oceans (as food for various fish, birds, and whales, particularly blue whales and finback whales). From January to April, swarms of krill (Euphausiacea superba) appear in the Antarctic Ocean and, because of the predominant sea currents, move northward into the Atlantic and Pacific Oceans surrounding South America. They have a large impact on fisheries along the eastern and western coasts of the subcontinent. It should be stressed that, because of their vast numbers and nutritive qualities, krill have been regarded by ecologists as a potential food source for humans. During the Southern Hemisphere summer, the open waters of the Antarctic Ocean may contain as much as 20 kg/m3 of these animals.
Currently, the annual rate of growth in the world marine catch is declining (FAO, 1992b) as a consequence of uncontrolled growth in fishing activities and overfishing of important stocks in the Atlantic Ocean, including the waters off the South American coasts. A combination of human activities (e.g., overfishing; pollution of estuaries and coastal oceans; and the destruction of habitat, especially wetlands and seagrasses) currently exerts a far more powerful effect on marine fisheries than declines expected from climate change (IPCC 1996, WG II, Section 8.2.2).
The oceans also are climate regulators: The combined effect of atmospheric and oceanic circulation defines particular weather and climate conditions throughout the region. The effect of sea currents is more evident during ENSO events, when resulting climatic variabilities have strong socioeconomic implications (see Section 6.2.3). The catastrophic effects of storms and storm surges are well-known, particularly for their role in exacerbating flooding in coastal areas and in erosion and restructuring of coastal formations (IPCC 1996, WG II, Section 8.4).
It is very probable that climate change would have significant negative impacts on human uses of the oceans-the most important resulting from impacts on biotic resources. It also is very likely that increased precipitation, river runoff, and atmospheric deposition from land-based activities would lead to increased loading of pollutants in coastal waters, with an adverse impact on fisheries, coral production, and tourism (IPCC 1996, WG II, Section 8.4).
Global warming may lead to poleward shifts in species ranges and migration patterns within the seas along the South American coast; this shift could lead to increased survival of economically valuable species and increased yields in marine fisheries. Such cases have been observed as a result of the large and intense 1983 El Niņo event (Wooster and Fluharty, 1985), which may be considered a test for future climate scenarios. However, the impacts of El Niņo are not the same for different regions of the Pacific Ocean. In fact, in areas where productivity depends on upwelling (e.g., the Pacific coast of Peru and Chile), the El Niņo phenomenon leads to a decrease in ocean productivity (Jordan Sotelo, 1986; Budyko and Izrael, 1987; Lapenis et al., 1990; IPCC 1996, WG II, Box 16.6).
Under IPCC scenarios, saltwater fisheries production should remain about the same-or undergo a significant increase, if management deficiencies are corrected. These conclusions depend on the assumption that interannual and decadal natural climate variability and the structure and strength of winds and ocean currents will remain about the same. Changes in any of these variables are expected to have significant effects on the distribution of major fish stocks, although not on overall production. Even without major changes in atmospheric and oceanic circulation, local shifts in centers of production and mixes of species in marine and fresh waters are expected to occur as ecosystems are displaced geographically and change internally.
Ecosystems and the services that they provide are sensitive to the rate and extent of changes in climate. The distribution of the world's major biomes, for example, is correlated with mean annual temperature and mean annual precipitation. The composition, dominances, and geographic distribution of many ecosystems will likely shift as individual species respond to environmental change driven by changes in climate: There will likely be changes in biological diversity and in the goods and services that ecosystems provide. Some ecological systems may not reach a new equilibrium for several centuries after the climate achieves a new balance.
Changes in the extent and duration of sea ice, combined with changes in the characteristics of sea currents, may affect the distribution, abundance, and harvesting of krill-an important link in the ocean fauna in the southern oceans. If there is a rapid retreat of sea ice in Antarctica or if the sea ice is reduced in extent, the krill fishery could become more attractive to nations involved in krill fishing (IPCC 1996, WG II, Section 188.8.131.52). Such a possibility could critically harm fishing activities in South America.
In tropical latitudes, most migratory organisms are expected to be able to tolerate climate warming, but the fate of sedentary species will be highly dependent on local climate changes. For example, corals are sensitive to changes in temperature; coral bleaching occurred during El Niņo events in 1983 and 1987 (Glynn, 1989; Brown and Odgen, 1993), when the ocean temperatures were higher than normal. Therefore, under warmer environmental conditions resulting from climate change, the expectation is that corals and other sedentary species would be affected. Coral mortality is positively correlated with the intensity and length of warming episodes (Glynn, 1989; Glynn and Cruz, 1990); recent paleoclimatic investigations of ENSO phenomena show that coral records can be read as a proxy for increased sea-surface temperarures (SSTs), potentially over several thousand years (Cole et al., 1992; Shen, 1993). Researchers recognize, however, that other environmental stresses-such as pollution, sedimentation, or nutrient influx-also would affect corals and other sedentary species (Maul, 1993; Milliman, 1993; IPCC 1996, WG II, Section 9.4).
Regarding the impacts of climate change on navigation in the southern oceans, there is no clear consensus-despite recent high rates of iceberg calving (Doake and Vaughan, 1991; Skvarca, 1993, 1994)-on whether the abundance of icebergs and their danger to shipping would change with global warming (IPCC 1996, WG II, Section 7.5.3).
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