Executive Summary

Ecosystems & Global Climate Change:  A Review of Potential Impacts on U.S. Terrestrial Ecosystems and Biodiversity

Climate is the single most important factor determining the geographic distributions of species and major vegetation types. It also influences the properties of ecosystems and the flows of energy and materials through them.

Global warming of the magnitude anticipated — a 1ºC to 4ºC (1.8ºF to 7.2ºF) increase in global mean temperatures over this century — will cause major changes in ecosystem distributions in the United States. In the eastern United States, these changes will result in a general northward shift in vegetation types. Results are more complex in the western United States due to local topography variation and small-scale climatic variations that result in complex, small-scale changes rather than broad northward shifts. The potential exists for significant reductions in the geographic extent of some ecosystems, especially those occurring in colder locales.

These shifts in major vegetation types due to global warming parallel the responses of the individual species that comprise these ecosystems. Thus, with global warming, shifts in the distributions of individual species are expected — in particular, a general poleward movement of distributions. Species have shifted their distributions in the past in response to changing climates; however, estimates of the rate of warming suggest that it may occur relatively quickly, some 10 times faster than the warming at the end of the recent glacial maximum, for example. It is not known whether species will be able to keep up with the rapidly shifting climatic zones. It is likely that some species will be unable to move at these high rates and hence may gradually die out as climatic conditions become increasingly unsuitable. The more rapid the rate of climate change, the greater the potential for this filtering effect. With higher temperatures, less of the earth will experience the cold conditions required by arctic and alpine species. As warming proceeds, these habitats are expected to decrease in size, leading to populations that are more isolated and to higher probabilities of extinction over time.

Climate change will also influence the functioning of ecosystems — the characteristic ways in which energy and chemicals flow through the plants, herbivores, carnivores, and soil organisms that comprise the living components of ecosystems. Models of overall changes in plant productivity indicate a wide range of possible changes across the lower 48 states, from slight declines (averaging 0.7 percent) to large increases (39 percent). Part of the uncertainly reflects poor understanding of how changes in temperature, moisture, and concentrations of carbon dioxide interact in influencing plant growth. Regional changes in productivity are not homogeneous, however, with some areas in the United States experiencing gains and others declines. For example, some scenarios show increases in plant productivity in the southeastern United States, whereas others showed large decreases under the influence of drier conditions. At the same time that increasing temperatures may lead to higher plant growth, they may also lead to higher decomposition rates and hence to increases in the rate at which carbon dioxide is being added to the atmosphere. It may be possible to increase the amount of carbon stored in ecosystems, and hence temporarily slow the rate of accumulation of carbon in the atmosphere (which comes primarily from the burning of fossil fuels) by planting forests on lands that currently do not support forests and by maintaining or increasing areas of mature and old growth forest.

Research on ongoing ecosystem change for several ecosystem types suggests that the effects of global warming on terrestrial ecosystems may already be altering ecosystems properties and species distributions. Nonetheless, there are substantial uncertainties as to how climate change will affect ecosystems and biodiversity in the United States. These uncertainties stem from not knowing the exact pattern of regional climate change as well as questions about how these patterns will affect the complex interactions and feedbacks among species and climatic conditions that characterize ecosystems. The effects of climate change on ecosystems and species are likely to be exacerbated in ecosystems that already are under pressure from human activities, including air and water pollution, habitat destruction and fragmentation, and the introduction of invasive species.

The effects of climate change on ecosystems threaten to jeopardize the numerous economically valuable goods and services that ecosystems provide to human societies, including services often undervalued in traditional economic analyses. In some cases, climate change will directly influence economic returns by affecting harvest levels; for example, warming-induced loss of salmon habitat from the United States would have a direct economic impact. Less easily measured are the potential effects of reduced species diversity on the ability of ecosystems to maintain local environmental quality; for example, removing pollutants from air and water and controlling soil erosion. Ultimately, the value of ecosystems must also be considered in a broad context, including the moral, cultural, and aesthetic values of ecosystems and species.

Efforts to lessen the detrimental effects on species and ecosystems from climate change should focus on maintaining habitats as well as on maintaining overall ecosystem structure and species composition. Thus, adaptation to climate change may benefit from existing strategies to conserve biodiversity, such as reducing fragmentation and degradation of habitats, increasing connectivity among habitat blocks and fragments, and reducing external anthropogenic environmental stresses. However, the ability to actively manage ecosystems to ameliorate the effects of climate change by, for example, actively assisting plant species to migrate, is constrained by lack of understanding and by the complexity of the underlying ecological systems. Even the seemingly simple task of reintroducing plants into former parts of their range has met with little success so far.