8.3.6.2. Oceans
An early review (Wright et al., 1986, summarized by Mann, 1993) projected that
for the northern Atlantic, some of the consequences of global warming could
include:
- A rise in the average sea surface temperature, causing an increase in evaporation
and a more vigorous hydrological cycle of precipitation, runoff, and so forth
- The greatest increase in evaporation in mid-latitudes, leading to increased
precipitation in northern regions, increased river runoff, increased stability
of the water column, and increased strength of buoyancy-driven currents such
as the Labrador current
- An increase in the north-south gradient in salinity
- A decrease in the thickness and extent of ice cover
- A reduction of the north-south temperature gradient and possibly a reduction
in average wind stress over the whole of the north Atlantic, which could lead
to a decrease in the strength of wind-driven currents such as the Gulf Stream.
In their summary of the Symposium on Climate Change and Northern Fish Populations,
Sinclair and Frank (1995) described the variability of the northern Pacific
in circulation and mixing and linked that variability in part to shifts in atmospheric
circulation-specifically, the changes in the location and level of the Aleutian
low-pressure system. Existing models have not been able to shed light on the
most probable responses of the northern Pacific to a doubling of atmospheric
CO2.
Mann (1993) briefly considered various sources of data for the wind-driven
coastal upwelling system off California. He suggested that available data could
be used to support the hypothesis that coastal upwelling increases during global
cooling but decreases during global warming.
8.3.6.3. Impacts, Adaptations, and Vulnerabilities of Ocean Fisheries Resources
Overall, there likely will be relatively small economic and food supply
consequences at the regional/national level; however, impacts are expected to
be more pronounced at the subregional level.
Natural climate variability-for example, changes in ocean temperatures and
circulation patterns associated with the El Ni�o phenomenon and with the northern
Pacific gyre-affects the distribution and composition of fisheries. Because
interannual and decadal-scale natural variability is so great relative to global
change and the time horizon on capital replacement (ships and plants) is so
short, impacts on fisheries can be easily overstated; there likely will be relatively
small economic and food supply consequences in the United States and Canada
at the national level. At the state or regional level, impacts (positive and
negative) will be more pronounced, particularly when a center of production
shifts sufficiently to make one fishing port closer to a resource while a traditional
port becomes more distant. Over time, fishing vessels and their support structure
will relocate, followed by processors and eventually families as well. Community
impacts can be significant.
Changes in primary production levels in the ocean as a result of climate change
may affect fish stock productivity. As a first step in assessing the role of
changes in primary production on fish productivity, global primary production
in the ocean has been estimated by Longhurst et al. (1995) using satellite measurements
of near-surface chlorophyll fields. Annual global primary production was estimated
at 45-50 Gt carbon (C)/year. This annual global primary production is the sum
of the annual primary production in 57 biogeochemical provinces covering the
world ocean. More than 10 such provinces border North America. For example,
the total primary production is estimated at 0.37 Gt C/year in the "California
Upwelling Coastal" province and 1.08 Gt C/year in the "NorthWest Atlantic Continental
Shelf" province.
Exactly how climate-induced changes in primary production would affect the
next trophic link, zooplankton, remains a matter of debate (e.g., Banse, 1995).
However, changes in zooplankton biomass are known to affect fish stock productivity.
Brodeur and Ware (1995) identified a twofold increase in salmonid biomass in
the eastern subarctic Pacific since the 1950s, coincident with a large-scale
doubling of the summer zooplankton biomass in the same region. Beamish and Bouillon
(1995) examined trends in marine fish production off the Pacific coast of Canada
and the United States. They concluded that the carrying capacity for fish in
the northern North Pacific Ocean and the Bering Sea fluctuates in response to
long-term trends in climate.
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