Results from the US National Assessment

The most extensive investigation into the underlying vulnerability of ecosystems, natural resources, health, agriculture and coastal systems in the US was the US National Assessment of the Potential Consequences of Climate Variability and Change (NAST, 2000). This national process investigated, through a collection of sectoral and regional studies, the current sensitivities and potential changes to these sectors in the US, as they might occur as a result of several climate scenarios.

Some of the most important vulnerabilities occur in unmanaged ecosystems. Figure 12.5 (Plate 17) shows results from the MAPSS ecosystem model for two different climate scenarios generated through application of GCM models to

Had ley Model
Figure 12.5 Distribution of ecosystems across the US at present and for two climate change scenarios based on projections to 2100 from results prepared for the US National Assessment (see Plate 17 for color version)

Note: Hadley = warm-moist; Canadian = hot-dry. Source: NAST, 2000.

existing patterns of interannual and interdecadal variability. In these analyses, the warm-wet scenario generated by the Hadley second generation model resulted in a general northward migration of potential natural vegetation, but also led to significant break-up of the vegetation types of the arid Southwest. Alternatively, the hot-dry scenario generated from the results of the Canadian Climate Model not only showed similar patterns for the Southwest, but also showed the typical forest vegetation of the Southeast breaking up as a result of extensive fire disturbances, and being replaced by savanna and grassland, because forests would no longer be able to regenerate in the very different climate conditions associated with this scenario.

Such potential impacts are seen not only at the ecosystem level, but also at the individual species level. Figure 12.6 (Plate 18) shows projections for changes in the distribution of sugar maple in the US, using scenarios generated by five different GCMs. In each case, the climatic conditions associated with a doubling of the atmospheric CO2 concentration would result in sugar maple trees no longer being able to regenerate within its original range in the US. While the dynamics of such a change are not yet adequately modeled, the general pattern for ecosystem change is very robust across many species and many ecosystem types.

For the range of results described in the US National Assessment, regional sensitivities of ecosystems for each region in the US are summarized in Figure

Figure 12.6 Present and projected distribution of sugar maple trees based on the results of five general circulation climate models assuming a doubling of the atmospheric CO2 concentration (see Plate 18 for color version)

Source: Prasad and Iverson, 2007.

Figure 12.7 Types of impacts associated with various regions of the US based on studies done for the US National Assessment

Source: NAST, 2000.

Note: NE = Northeast; SE = Southeast; MW = Midwest; GP = Great Plains; WE = Western US; PNW = Pacific Northwest; AK = Alaska; IS = Islands.

12.7. Significant sensitivities for many ecosystems were noted for each region in the US.

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