Water resource scenarios have been developed at different time and space scales. For example, projections to 2025 on the basis of national water resource monitoring data have been reported by Shiklomanov (1998). Model-based projections of water use and availability to 2025 at the river basin scale have been made by Alcamo et al. (2000), assuming a BAU scenario and two alternative, normative scenarios that focus on water conservation. Some results of these scenario exercises are shown in Table 3-3. Among the most developed scenarios of water quality are model-based scenarios of acidification of freshwaters in Europe (e.g., NIVA, 1998). More general normative scenarios describing rural and urban access to safe drinking water by 2025 and 2050 are presented by Raskin et al. (1998). Scenarios of water availability have been applied in several climate change impact studies. Most of these are in the water resources sector and are reported in Chapter 4. However, they are increasingly being applied in multi-sectoral and integrated assessments (e.g., Strzepek et al., 1995).
Marine pollution is the major large-scale environmental factor that has influenced the state of the world oceans in recent decades. Nutrients, oxygen-demanding wastes, toxic chemicals (such as heavy metals, chlorinated hydrocarbons, potential endocrine-disrupting chemicals, and environmental estrogens), pathogens, sediments (silt), petroleum hydrocarbons, and litter are among the most important contaminants leading to degradation of marine ecosystems (Izrael and Tsyban, 1989; GESAMP, 1990; Tsyban, 1997). The following ranges of concentrations of heavy metals are characteristic of open ocean waters: mercury (0.3-7 ng l-1), cadmium (10-200 ng l-1), and lead (5-50 ng l-1); levels of chlorinated hydrocarbons are a few ng l-1. Chemical contaminants and litter are found everywhere in the open ocean, from the poles to the tropics and from beaches to abyssal depths. Nonetheless, the open ocean still remains fairly clean relative to coastal zones, where water pollution and the variability of contaminant concentrations are much higher (often by one to two orders of magnitude; specific values depend on the pattern of discharge and local conditions).
Data characterizing the state of the marine environment have been obtained through national as well as international monitoring programs in recent decades, and analysis of tendencies may serve as an initial basis for developing environmental scenarios. At present, expert judgment appears to be the most promising method of scenario development because modeling methods are insufficiently developed to facilitate prediction.
In qualitative terms, trends in marine pollution during the 21st century could include enhanced eutrophication in many regions, enhancement of exotic algal blooms, expanded distribution and increased concentration of estrogens, invasion of nonindigenous organisms, microbiological contamination, accumulation of pathogens in marine ecosystems and seafood, and increases of chemical toxicants (Izrael and Tsyban, 1989; Goldberg, 1995).
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