that climate change could cause the extinction of some species and the expansions of others.

Over two-thirds of fish caught for human consumption, as well as many birds and terrestrial animals, depend on coastal marshes and swamps for part of their life cycle. Coastal wetlands can keep pace with a slow rate of sea level rise. This ability has enabled the area of intertidal wetlands to increase severalfold, with the slow rate of rise over the last few thousand years. However, if sea level rises too rapidly, the natural succession of the coastal ecology could be affected, leading to disruption in the life cycles of many species.

ADAPTIVE MEASURES

The reactions to a changing environment may have as much impact on users of aquatic resources as the change itself. Several measures are currently being taken to reduce vulnerability to climate changes.

• Determine the relationship of aquatic organisms to their environment and how the relationship changes with a changing climate. This work includes movement towards adoption of large marine ecosystems as the focus of research on biota in the oceans. Work in this area is being integrated domestically among various federal agencies and internationally through several scientific organizations.

• Strengthen fisheries management research, policies, and institutions. In fisheries management, large marine ecosystems increasingly are being considered as the management unit for the conservation and management of living marine resources (Sherman and Gold 1990). In addition, the Magnuson Fisheries Conservation and Management Act allows U.S. fisheries managers considerable flexibility in dealing with shifting fisheries production. To further improve its fisheries management, the United States is educating its fisheries managers and its fishing industry about the possible consequences of a changing climate. Finally, with respect to the impacts of international policies on fisheries and fisheries management, the United States has advocated the view that international treaties and agreements on fisheries and marine mammal conservation should consider the implications of the different population structures and distributions that would result from a changing climate.

Source: ERC 1991

Improve methods to assess socioeconomic impacts of climate changes. Resource managers are conducting workshops on this subject to bring experts together to share information.

• Consider habitat needs for aquatic resources in planning coastal protection measures and water resource actions. For example, the need for wetlands to be able to migrate inland and the implications of salinity impacts due to sea level rise are addressed in the Delaware River Basin Commission dam operation/construction plan. • As habitats change, restock them with ecologically sound species. Research is under way in various U.S. institutions to learn more about the physiological needs and the predator/prey relationships of both aquacul

ture and wild fish species. If and when stocking becomes advisable, there will be a strong scientific basis for making proper selections and avoiding harmful effects on native populations. Opportunities also exist for aquaculture to mitigate some impacts on species that are already severely threatened, such as the Headstart programs in which turtles are raised for a suitable period in facilities before being released into the wild.

• Consider using aquatic species as indicators of climate change to validate climate models and to show whether climate change is already occurring. Various researchers are working to determine if apparent changes in populations are early indicators of a changing environment.

As described in earlier parts of this chapter, climate change and the attendant fluctuations in temperature and moisture could have a significant impact on terrestrial ecosystems.

With regard to climate-related variables, past state-ofthe-art modeling studies of the effects of climate change on U.S. ecosystems have examined the sensitivities of certain species to particular variables. They haven't examined how a full, internally consistent set of changes in climate variables would affect ecosystems as a whole (Smith and Tirpak 1990). This is partly due to the lack of reliable climate change scenarios, as well as an insufficient understanding of the biophysical and ecological processes under new atmospheric and climate regimes (IPCC 1990b; Goklany 1992).

Due to temperature change alone, productivity in the higher latitudes would increase (because of a longer growing season). However, in the interior mid-latitudes, potentially drier conditions coupled with higher temperatures could reduce productivity. The direct effects

A number of field experiments on the fertilization effects of enhanced CO, levels in the atmosphere (i.e., without consideration of other changes in climate conditions) on U.S. ecosystems show mixed results. An experiment on an Alaskan tussock tundra system suggests this phenomenon may only have short-term benefits for trees and natural vegetation, possibly because of acclimation of carbon metabolism (Tissue and Oechel 1987). However, two other longer-term experiments, both on different wetland ecosystems in Chesapeake Bay, showed significant increases in carbon accumulation for the first four years, both above and below

ground. These investigations note that there is no evidence in those data that carbon accumulation would reach saturation or decline due to elevated CO, (Arp and Drake 1991; Drake and Leadley 1991; Idso and Kimball 1991; Ziska et al. 1991).

In the absence of reasonably accurate regional climate change scenarios, experimental studies—like modeling studies-cannot replicate future conditions. Because of these shortcomings, the change in the net primary productivity of either portions of or the entire United States cannot be estimated with confidence. However, some investigators have also noted that CO2 fertilization could explain the "missing" CO, terrestrial sink suggested by some studies (Tans et al. 1990). CO2 fertilization would also explain the increasing amplitude of the seasonal swings in atmospheric CO2 concentrations (Idso 1991).

The combined effect of changes in CO2 concentrations and climate variables would be that ecosystems, which are constantly evolving, would evolve along a different path. Each species would react differently, forming new assemblies and relationships (Patterson and Flint 1990). There is also general agreement that the magnitude of impact will depend upon the rate of change in critical climate factors; that species at the edges of their range would be most affected, some negatively and others positively; and that the smaller the population or range, the greater the effect (IPCC 1990b). Some fear too rapid a change could cause the dieback or extinction of some species. On the other hand, other species may thrive. In particular, grassland and forest ecosystems and migratory species may be affected as follows:

• Grassland Ecosystems. As climate changes, grasses, forbs, and shrubs best suited to the changed climate will flourish, and those no longer suited will die back. Range managers will focus on how to sustain the ecosystem to meet the continually shifting set of economic, social, and cultural demands placed on it. In addition, managers would need to guard against pathogens and diseases that also could migrate as climate changes. Animal communities. would also move in response to any changes in the abundance and distribution of their food sources and habitat. Plants will also colonize territory that has recently become suitable.

• Unmanaged Forest Ecosystems. Most unmanaged forest ecosystems (old-growth forests and abandoned areas) are highly diverse genetically and are thus capable of selecting variants to accommodate some climate change. As with range ecosystems, it is fully expected that the organization, structure, and composition of any particular habitat will change as the

external climate variables change. Because most trees live for decades, if not centuries, once they are established, global climate change may be reflected by the vigor and health of existing forest communities, as well as by patterns of reproduction (or lack of reproduction).

• Migratory Species. Migratory species depend on the quantity and quality of food sources and habitat in more than one area. They may be adversely affected if the food and habitat in some of these areas are no longer suitable because of changes in climate (e.g., sea level rise). These effects would be mitigated if migratory species are able to locate suitable new habitat, or if their current habitat expands or becomes more productive.

Adaptive Measures

Because of the uncertainties regarding the vulnerability of ecosystems to climate change, and given other inevitable agents of global change as well as the potential magnitude of their effects, it is prudent to approach adaptation along the lines recommended by the IPCC (1990d) specified earlier in this chapter.

The majority of the measures that fit the IPCC definition are anticipatory in that, by reducing the vulnerability to other agents of change, they would increase the future resiliency of ecosystems and species and their ability to deal with the impacts of climate change, if and when they are manifested. Other actions would maintain institutional, legal, and economic flexibility so that site-specific "reactive" measures could be taken in response to specific effects, once their need becomes apparent. Specific adaptation measures that the United States is undertaking (or has undertaken) to adapt to any future climate change follow.

IMPROVING THE KNOWLEDGE BASE

The U.S. government spends over one billion dollars a year on research on environmental biology so that reasoned judgments can be made on the sustainable use and management of ecosystems. More than $430 million of that total is from federal land management agencies and is devoted to research efforts undertaken to assist the rational use and management of the biological resources for which they are responsible (CEES 1992). These funds also are used to develop inventories, data bases, and systems to monitor the current state of natural resources, improve methodological tools for assessing impacts, and estimate the sensitivity and adaptability of natural resources to different global change scenarios.