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The oceans have significant adaptive capacity to store heat and are the largest reservoir of water vapor and CO2, although the storage capacity for CO2 in the Southern Ocean recently has been questioned (Caldeira and Duffy, 2000). In the oceans, climate change will induce temperature changes and associated adjustments in ocean circulation, ice coverage, and sea level. Changes in the frequency of extreme events also may be expected. These changes, in turn, will affect marine ecosystem structure and functioning, with feedback to global biogeochemical cycles and the climate system.
Recent investigations have shown that there has been a general warming of a large part of the world ocean during the past 50 years (Levitus et al., 2000). Analysis of historical SST data by Cane et al. (1997) shows an overall increase associated with land-based global temperature trends. Regional differences exist such that over the past century a cooling was observed in the eastern equatorial Pacific, combined with a strengthening of the zonal SST gradient.
Global mean sea-level has risen by about 0.1-0.2 mm yr-1 over the past 3,000 years and by 1-2 mm yr-1 since 1900, with a central value of 1.5 mm yr-1. TAR WGI Chapter 11 projects that for the full range of the six illustrative scenarios in the IPCC's Special Report on Emissions Scenarios, sea level will rise by 0.09-0.88 m between 1990 and 2100. This range is similar to the total range of projections given in the SAR of 0.13-0.94 m. Higher mean sea level will increase the frequency of existing extreme levels associated with storm waves and surges.
The El Niño-Southern Oscillation is a natural part of the Earth's climate. A major issue is whether the intensity or frequency of ENSO events might change as a result of global warming. Timmermann et al. (1999) suggested an increased frequency of El Niño-like conditions under future greenhouse warming and stronger "cold events" in the tropical Pacific Ocean. Cooling has been observed in the eastern equatorial Pacific, not reproduced in most GCMs, and has been explained by an increase in upwelling from the strengthening of trade winds because of a uniform warming of the atmosphere (Cane et al., 1997). If temperature differences between the tropics and polar regions are reduced, however, a weakening of the atmospheric circulation patterns that cause upwelling could be expected.
In recent years there have been several studies of global ocean wind and wave climates (e.g., Young, 1999), but analyses of changes over the past few decades have been limited to a few regions. In the past 30 years there has been an increase in wave height over the whole of the North Atlantic, although scientists are not certain that global change is the cause of this phenomenon (Guley and Hasse, 1999). Similarly, analyses of wave buoy data along the entire west coast of North America demonstrate that the heights of storm-generated waves have increased significantly during the past 3 decades (Komar et al., 2000). On the U.S. east coast, analyses have shown that there has been no discernible long-term trend in the number and intensity of coastal storms during the past century, although there has been considerable interdecadal variation (Zhang et al., 2000). The sensitivity of storm waves to a hypothetical sea-level rise and increase in wind strength recently has been modeled for Uruguay (Lorenzo and Teixeira, 1997).
Projected changes in tropical cyclone frequency and intensity remain inconclusive, although some studies have suggested that the maximum intensity of tropical cyclones may rise by 10-20% (Henderson-Sellars et al., 1998; Knutson et al., 1998). Walsh and Pittock (1998) and Walsh and Katzfey (2000) also suggest that once formed, tropical cyclone-like vortices might travel to higher latitudes and persist for longer as a result of increased SST.
Increased precipitation intensity in extreme events is suggested by climate models under doubled CO2 for Europe (Jones et al., 1997) and the United States (Mearns et al., 1995), and there is firm evidence that moisture in the atmosphere is increasing over China, the Caribbean region, and the western Pacific (Trenberth, 1999). Heavy rainfall increased during the 20th century in the United States (Karl and Knight, 1998), and there is evidence for increased precipitation rates in Japan and Australia (Iwashima and Yamamoto, 1993; Suppiah and Hennessy, 1998). Changes in the probability of heavy precipitation also are regarded as important indicators of climate change.
Globally, oceanic thermohaline circulation plays an important role in controlling the distribution of heat and greenhouse gases. This circulation is driven by differences in seawater temperature and salinity. There is some evidence that the global thermohaline circulation will weaken as a result of climate change, although views on this issue are still evolving.
Sea ice covers about 11% of the ocean, depending on the season. It affects albedo, salinity, and ocean-atmosphere thermal exchange. The latter determines the intensity of convection in the ocean and, consequently, the mean time scale of deep-ocean processes affecting CO2 uptake and storage. Projected changes in climate should produce large reductions in the extent, thickness, and duration of sea ice. Major areas that are now ice-bound throughout the year are likely to have lengthy periods during which waters are open and navigable. Observations in the northern hemisphere already have shown a significant decrease in spring and summer sea-ice extent by about 10-15% since the 1950s. It also has been suggested that the decline in ice volume is underestimated because of significant thinning of sea ice in the Arctic (Rothrock et al., 1999). Evidence from whaling records implies a decline in Antarctic ice extent by as much as 25% between the mid-1950s and the early 1970s (de la Mare, 1997).
The foregoing physical responses in the ocean-climate system have implications for habitat and ecology in the oceans and coastal seas. Projected climate changes have the potential to become a major factor affecting marine living resources over the next few decades. The degree of the impact is likely to vary within a wide range, depending on the species and community characteristics and the regional specific conditions. Smith et al. (1999) review the sensitivity of marine ecosystems to climate change.
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