Martin G. Raphael, Ecosystems Processes, Pacific Northwest Research Station
The current version of this paper (2013) is available at www.fs.usda.gov/ccrc/topics/wildlife/birds
Climate change impact assessments seek to characterize, diagnose, and project risks or impacts of environmental change on people, communities, economic activities, infrastructure, ecosystems, or valued natural resources (1). Impact assessments have been requested at the national scale by US Congress (2) through the Global Change Research Act of 1990 which requires the National Science and Technology Council to analyze effects of global change on the natural environment, agriculture, energy, land and water resources, transportation, human health and welfare, human social systems, and biological diversity. Assessments have also been requested at more local scales by city officials such as Aspen, Colorado (3). Non-governmental organizations have also identified the value of impact assessments (4).
Impact assessments are motivated by the users' information needs and can be developed in multiple ways. An analysis often begins by examining changes to temperature, precipitation, and other climatic variables under multiple scenarios of greenhouse gas emissions (5). The analysis then considers the potential impacts on a geographic area, economic sector, community, or resource in order to provide information to managers, decision makers or policymakers. These assessments can focus on the synthesis of scientific information and may also include extensive quantitative analysis using models to explore the effects of a number of potential future climates. The development of an assessment often will involve a dialogue between those conducting the assessment and stakeholders. The assessments can be periodic (e.g., 6) or a one-time assessment (e.g., 3). A technical review by a set of independent experts will ensure that the summary statements and conclusions are well-supported in the scientific research and well-documented in the report.
During the last 2 billion years, the Earth has gone through several extended periods of cooling and warming, which have resulted in dramatic changes in vegetation and species composition (Graham and Grimm 1990, MacDonald et al. 2008). Effects of the current trend in global climate change include extensive warming, changing patterns of precipitation, changes in season lengths, declining snowpack, and increasing frequency of severe weather events.
How might these changes affect birds? Here is an example: using the National Audubon Society's Christmas Bird Count data, Price and Root (2001) have shown that, in North America, the northern limits of many bird species are strongly associated with various climatic variables (e.g., winter temperature). Further analyses revealed that both the ranges and the abundances of birds shift, on an annual basis, in concert with temperature. Indeed, studies have shown that a significant number of migrating birds are arriving up to 3 weeks earlier now than they did in 1960. Apparently, many bird species can and do respond to changing climatic conditions. Because their ranges are limited by vegetation, however, these birds will probably not be able to shift their ranges with the changing climate- at least not until the vegetation itself shifts. Consequently, natural communities of birds may change dramatically in the future as changes in climate and vegetation favor some species and harm others. It is difficult to predict how these changes will influence community structure or function.
Long-distance migrants may be more vulnerable to global warming than other species. As winter temperatures increased between 1980 and 1992 at Lake Constance in Central Europe, the proportion of long-distance migrant bird species decreased, and the number and proportion of residents and short-distance migrants increased. In North America, many songbirds are long-distance migrants. Species such as Baltimore oriole (Icterus galbula), barn swallow (Hirundo rustica), wood thrush (Hylocichla mustelina), and scarlet tanager (Piranga olivacea) could well be driven from the places where we expect to find them, and more ominously, from the habitats to which they are best suited.
A 90-percent decline in sooty shearwaters (Puffinus griseus) off the California coast in just 7 years (1987-1994) has been associated with warming of the California Current, which flows from southern British Columbia to Baja California.