In the past decade there has been considerable improvement in our knowledge of the impacts of climate change on coastal zones and marine ecosystems. This improvement has been far from uniform, either thematically or in regional coverage, and there are still substantial gaps in our understanding.

The First and Second Assessment Reports on ocean systems (Tsyban et al., 1990; Ittekkot et al., 1996) conclude that global warming will affect the oceans through changes in sea-surface temperature (SST), sea level, ice cover, ocean circulation, and wave climate. A review of the global ocean thermohaline circulation system—for which the term "ocean conveyor belt" has been coined (Broecker, 1994, 1997)—emphasizes the role of the global ocean as a climate regulator. Ittekkot et al. (1996) notes that the oceans also function as a major heat sink and form the largest reservoir of the two most important greenhouse gases [water vapor and carbon dioxide (CO₂)], as well as sustaining global biogeochemical cycles.

Climate-change impacts on the ocean system that were projected with confidence by Ittekkot et al. (1996) include SST-induced shifts in the geographic distribution of marine biota and changes in biodiversity, particularly in high latitudes; future improvement of navigation conditions in presently ice-infested waters; and sea-level changes resulting from thermal expansion and changes in terrestrial ice volume. Regional variations caused by dynamic processes in the atmosphere and ocean also were identified with some confidence. Less confident predictions include changes in the efficiency of carbon uptake through circulation and mixing effects on nutrient availability and primary productivity; changes in ocean uptake and storage capacity for greenhouse gases; and potential instability in the climate system caused by freshwater influx to the oceans and resultant weakening of the thermohaline circulation.

The SAR includes a comprehensive review of climate-change impacts on fisheries (Everett et al., 1996). Principal impacts are believed to be compounded by overcapacity of fishing fleets, overfishing, and deterioration of aquatic habitats. The authors also note that the impacts of natural climate variability on the dynamics of fish stocks is being considered as an important component of stock management, although the nature and magnitude of that variability are not clear.

In the present report, we identify new information about the impacts of climate change that has accumulated since the SAR. We include assessments of impacts on fisheries, marine mammals, sea birds, aquaculture, and marine diseases. We show that more recent information identifies natural multi-year climate-ocean trends as an essential consideration in fisheries management and stewardship of marine ecosystems. We point out that separating the impacts of natural climate variability and regime shifts from those associated with long-term climate change will be important, although distinguishing between the two will be a difficult task.

The potential impacts of sea-level rise on coastal systems have been emphasized in recent years. Much less attention has been given to the effects of increases in air and sea-surface temperatures; and changes in wave climate, storminess, and tidal regimes. There are at least two reasons for this lack of attention. First, low-lying coastal areas such as deltas, coastal plains, and atoll islands are regarded as particularly vulnerable to small shifts in sea level. Second, global sea-level rise is regarded as one of the more certain outcomes of global warming and already is taking place. Over the past 100 years, global sea level has risen by an average of 1-2 mm yr^-1, and scientists anticipate that this rate will accelerate during the next few decades and into the 22nd century.

The FAR (Tsyban et al., 1990) regards sea-level rise as the most important aspect of climate change at the coast and identifies seven key impacts:

1. Lowland inundation and wetland displacement
2. Shoreline erosion
   
3. More severe storm-surge flooding
   
4. Saltwater intrusion into estuaries and freshwater aquifers
   
5. Altered tidal range in rivers and bays
   
6. Changes in sedimentation patterns
   
7. Decreased light penetration to benthic organisms.

In the SAR, Bijlsma et al. (1996) acknowledge the importance of these impacts and further conclude, with high confidence, that natural coastal systems will respond dynamically to sea-level rise; responses will vary according to local conditions and climate; and salt marshes and mangroves may survive where vertical accretion equals sea-level rise—but built infrastructure limits the potential for landward migration of coastal habitats. That report also provides summaries of national and global vulnerability assessments, focusing on numbers of people, land areas, and assets at risk. Several coastal adaptation strategies are identified. The importance of resilience in coastal systems is hinted at and subsequently has become an important consideration in vulnerability analysis of sectors and geographical regions.

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