Breakout Session 1-- Climate Change Scenarios

What are likely scenarios for climate change in the North Pacific and how would they influence the ecosystem?

Discussion Leaders: Nick Bond and Robert DeLong
Participants: Richard Beamish, Steve Hare, Steve Ignell, Evelyn Lessard, Allen Macklin, Brenda Norcross, Jim Overland, William Peterson, Alan Springer, Ted Strub, Ron Thom, Cynthia Tynan.

This breakout session was devoted to discussions of potential impacts of regime shifts possibly caused by increased CO2 from anthropogenic activities and subsequent warming (the greenhouse effect). Case studies were presented for possible impacts of global warming in the Gulf of Alaska and Subarctic Pacific (Table 1), and Bering Sea (Table 2). These case studies are discussed in detail in Appendix 1. These case studies were tentative and served to initiate discussions for this breakout group.

Summary of Discussions:

Although the group discussed some likely scenarios for climate change in the North Pacific, the group decided to defer to climatologists who will generate detailed predictions of climate changes that are likely to occur in the areas of interest: the Subarctic North Pacific, Gulf of Alaska, and the Bering Sea. The ecosystem of the North Pacific Ocean will be sensitive to the geographical distribution of the changes in the atmosphere-ocean climate system. These distributions are not yet reliably predicted by climate models. But U.S. GLOBEC need not wait until these models are improved and verified. Studies of how physical forcing couples to the ecosystem now will help to anticipate changes in the latter, no matter how the climate evolves. Short-term (seasonal) to long-term (decadal) climate variations appear to significantly impact the biological environment (see collection of papers in Beamish (1995)). The interdependence of these climate fluctuations and the nature of the biological responses can be used as a proxy for at least some aspects of future long-term changes. A combination of retrospective, monitoring, and process studies should be used to document and better understand current linkages between changes in the physical and biological environments. This working group has identified four key aspects of the physical environment for which long-term changes would have an especially profound effect on the biology.

1) Mixed Layer Depth

Time variation in late spring/summer mixed layer depth is the physical oceanographic measurement which may correlate most highly with primary and secondary productivity in the coastal Gulf of Alaska and Bering Sea shelf. The spring phytoplankton bloom depends on stratification of the water column to retain cells above the depth at which light levels allow positive net production. After the bloom has begun and when nutrient depletion is imminent, mixing can prolong production by large phytoplankton cells by providing new nitrogen (sensu Dugdale and Goering, 1967) from below the nitricline. Variability in summertime mixed layer depth would generally be expected to indicate the active mixing and introduction of new nitrogen.

In the open Subarctic Pacific, predicting the influence of climate change on the mixed layer depth and the response of phytoplankton to this change is more complex. There, iron, a micro-nutrient, limits phytoplankton growth; its suspected source is dust carried with the winds from Mongolia/Siberia (Miller et al. 1993). Variation in mixed layer depth may have little affect on annual production in this region. On the other hand, higher mean irradiance in the mixed layer may increase primary production in the Subarctic Pacific. This increase in primary production may increase the recycling time of iron and the available iron may sustain populations longer.

The Bering Sea Shelf has three distinct hydrographic and biological regimes (Coachman, 1986; Cooney and Coyle, 1982). Hydrographic structure and mixing is determined by the balance and depth of influence of wind versus tidal mixing. Climate change is not expected to strongly affect the net mixing over the shelf, although there are likely to be changes in the annual cycle of stratification over the shelf and near the shelf break, due to changes in sea ice melt, insolation, wind forcing, and the mean currents. It is possible that the waters will be well mixed during a larger portion of the year which may increase productivity if there is sufficient light available for positive net production. On the other hand, if nutrients are limiting, increased mixing would only enhance production where a nutrient reservoir beneath the pycnocline exists. Yet another scenario would predict that increased cloudiness would decreased the amount of light and production might decrease.

2) Changes in Marine Habitat (Volume and Location)

Oceanographic warming is expected to have density dependent effects on the growth rate, size, and survival of salmon. Increases in salmon habitat in the open subarctic due to warming could increase growth or survival resulting in larger or more or fish in the future than there are today (similar responses might occur in other species). The species mix within the salmon guild may be somewhat different than it is today because warmer marine temperatures may favor one species over another.

The response of marine communities to climate change may differ in the Bering Sea and Gulf of Alaska because of the location of the species within its zoogeographic range (Bailey and Incze 1985). In the Gulf of Alaska, where some species are at the southern edge of their range, warm ocean conditions may cause poor survival. In contrast, in the Bering Sea, some species are at the northern edge of their range, and warmer ocean conditions may favor survival and expansion into new habitats.

3) Sea Ice

Under most global warming scenarios, sea ice distribution is expected to decrease both in time and space. An ice free Bering Sea shelf might increase spawning habitat for demersal species such as pollock. The ice free environment would delay the spring bloom because of a delay in the establishment of water column stability and lack of seed populations of ice algae. The spring bloom over the shelf has been shown to be delayed by about two weeks south of the ice covered regions. The retreat of the ice to a higher latitude of the Bering Sea or Bering Straits would dramatically alter the distribution of five species of ice dwelling pinnipeds which use the ice edge for rookery habitat and change levels of predation on fish and invertebrates of the Bering Sea shelf. Other marine mammal and seabird species would likely occupy the open waters of the Bering Sea during times of the year when they are now excluded by ice coverage.

4) River Flows (Precipitation)

With global warming, there may be significant changes in precipitation patterns and in river flows feeding into the Bering Sea, Gulf of Alaska, Southeast Alaska and Canada. High latitude Alaskan rivers may remain ice free for longer periods of the year increasing their production capacity for salmonids whereas rivers in southern British Columbia may have lower snowpack thereby decreasing their capacity for freshwater production of salmon. Both spawning success and juvenile survival can be expected to change in response to shifts in river flow characteristics which will be mediated by climatic warming.

Coastal currents can be expected to be enhanced during much of the year due to increased rainfall, but perhaps be less variable in that less of the volume of water will originate from snow and glacial melt. Stronger currents may be expected to cause increases in advection of plankton and some nekton as well as transport of nutrients.