Future Climate Variability and Ecosystem Response

There continues to be concern about the possible effects on global climate change related to increased greenhouse gases in the atmosphere. If such climate change does occur, it is difficult to conceive of a potentially more important example of climate variability and ecosystem response. Future climate change will have complex, cascading, and, in some cases, detrimental effects on the ecosystems of the PNW.

Global General Circulation Models (GCMs) of the atmosphere and ocean are being used to investigate the question of what possible climate change might occur. Many caveats accompany the use of these models to estimate the potential changes of climate that might occur in a particular region. Apart from model deficiency, one of the most important caveats relates to the uncertainty in the rate of greenhouse gas emissions over the next 100 years. Also the range of temperature and precipitation projected by different models is quite large. For example, projected monthly temperatures vary by about 4°C (7°F) for different models. However, the models have been able to reproduce the increase of global temperatures in the twentieth century and the possibly anthropogenic part of that increase since 1970 (JISAO CIG 1999).

Output from the Canadian Centre for Climate Modeling and Analysis (CCC) GCM was employed by the JISAO group to suggest possible changes of climate in the PNW in the twenty-first century. This model suggests an increase in the cool season (Oct.-Mar.) temperature of about 5.5°C (10°F) and about 4°C (7°F) in the warm season (Apr.-Sept.) by the year 2100. A decrease of cool season temperature of about 5.5°C might eliminate subfreezing temperatures in some parts of the PNW. This might greatly decrease the snowpack and lead to a nonlinear response. The CCC model also suggested an increase in precipitation of about 330 mm (13 in.) in the cool season and 25 mm (1 in.) in the warm season. Despite the projected increase in precipitation, the rise may not be beneficial to forest environments because the large addition in the cool season will increasingly fall as rain, as opposed to snow, because of the higher temperatures. Much of this precipitation will run off and not add to the winter snowpack, if it still exists, for later release as snowmelt. The consensus of opinion of the University of Washington Climate Impacts Group, based on the output of seven GCMs, was that "the models are generally in agreement that winters will be warmer and wetter, but are divided about whether summers will be wetter or drier" (JISAO CIG 1999, p. 20). If some of these scenarios come to pass, the effect on PNW forests might not be favorable. The higher temperatures and possibly decreased amount of warm season soil moisture might increase the possibility of forest fires. Directly, these changes will lead to changes in the rates of growth, seed production, and seedling mortality. Indirectly, they will influence the disturbance regimes of fire, insect infestation, landslides, and disease (Franklin et al. 1992). The fossil record suggests that climate change coupled with disturbance will lead to disequilibrium between vegetation and climate as species adjust to new conditions and competitive interactions change.

Modeling studies (Urban et al. 1993), using an increase in temperature of 2.0-5.0°C, showed some altitudinal zonal and plant composition changes in Cascade ecosystems, but these studies used models that were set to run for 1000 years. Other model studies of biome and hydrologic response currently take an equilibrium approach, so they do not provide information on how, when, or even whether the vegetation/hydrosphere can respond to climate changes of a 2.0-5.0°C magnitude in the next century. Nonetheless, the equilibrium changes in hydrology and vegetation in the West are dramatic. An assessment by Thompson et al. (1998) suggests that it is unlikely that biotic adjustments can be accomplished in the next century for several reasons. First, vegetation responds more slowly than the projected climate change, especially long-lived species such as those in the PNW. The best paleoecological estimates for plant migration rates in the past are 40 times slower than those needed to keep pace with a doubled CO2-related climate change in the twenty-first century. The plant species that predate humans did not have to contend with human land-use alteration that set up impediments to migration and dispersal. Second, species may not be able to migrate without assistance across a landscape fragmented by past land use. Third, the models only describe what potential, as op posed to actual, vegetation could occupy the climate space. Nonclimatic factors, including competition, will slow the migration process. Disturbance will probably be the catalyst of vegetation change, and the increase in severe fires during the last decade may already be the harbinger of effects of climate conditions to come.

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