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Updated 12 October, 2003
Ecosystem Changes for the United States Under a Climate Warming
USGCRP Seminar, 18 March 1999
Dr. Margot Anderson
Dr. Ronald P. Neilson
The potential for future global warming has focused on two fundamental questions regarding the role of the terrestrial biosphere. Will the biosphere exacerbate a general climate warming or will the biosphere exert a cooling influence under a general climate warming? What might be the ecological, environmental and socio-economic impacts of climate change? The former question commands much attention and addresses issues of water, energy, and carbon exchange between the atmosphere and the biosphere. The latter question is the focus of an ongoing, federally sponsored assessment activity for the U.S., and will be the primary focus of this seminar.
This analysis of ecosystem changes in the U.S., under a general climate warming at a doubling of CO2, employs the MAPSS (a vegetation distribution and hydrology model) and DGVM (Dynamic Global Vegetation Model) models. These models are integrated using climate output from six different General Circulation Models (GCMs) in order to examine the potential biotic responses of terrestrial ecosystems and water resources over the conterminous U.S. Because of natural variations and because of uncertainties in projections of climate change, GCMs produce a wide variety of possible future climates on the regional scale, creating a range of uncertainty and confidence regarding possible impacts. By considering this range of scenarios, however, one can gain insight into the changes that are most likely. These six scenarios address the following questions:
Significant insights into likely ecosystem changes can be gained by isolating or highlighting regions that display consistent, parallel responses in all six model scenarios, in contrast to regions where responses are significantly different from model to model. However, analyses of results in those regions of the U.S. for which there are less consistent model results from model to model, can show complexities that may emerge along the complex future trajectory of change.
Model results to date suggest that the prevalence of forests of mixed conifers and hardwoods of the northeastern U.S. could shift into Canada and decrease in area in the U.S. The occurrence of cool maple-beech-birch forests also decreases in most scenarios, but increases in area in a few scenarios. Oak-hickory forests decline in area under all but one scenario. Some of the model-based declines in forest area are induced by drought-stress with some increase in fire disturbance. In the southeastern U.S., mixed pines and hardwoods increase in area under most scenarios, but largely due to a shift northward, displacing current oak-hickory forests. In the scenarios with the largest changes, models suggest the possibility of significant drought-induced forest dieback in the southeastern U.S., with conversion of land cover to savanna and grassland. However, under the scenarios with only small amounts of warming, the southeastern forests could see increased growth.
In the west, many high-elevation forests shift off the top of the mountains, with possible species losses. In the northwest, the distribution and movement of Douglas fir is somewhat unclear, exhibiting an increase under a moderate warming, and a decrease under still warmer climate conditions. As temperatures increase, the coastal area now occupied by spruce-hemlock-redwood could extend into regions now occupied by Douglas fir, while the fir trees shift elsewhere. The southwestern U.S. is a hotspot of diversity, much of which could 'invade' the Great Basin. Saguaro cactus, for example, could shift dramatically north through the Great Basin, possibly as far as eastern Washington. Such a vegetation shift in the Western interior region could have significant implications with regard to existing plant communities and wildlife habitats, which could be compressed in area or displaced upslope.
Fire frequency could increase
over large areas of the country, in some cases due to increased drought
stress (e.g., in the Great Lakes region and forested areas of the Southeast
and Northwest), and in other cases due to increased fuel loads from
excess moisture. Coupled with occasional drying from El Nino-La Nina
oscillations, fire potential in the interior West and the California
woodlands and forests may increase. Although there could be a significant
trend toward mid-continent soil drying with reduced average runoff,
year-to-year climate oscillations could bring both floods and droughts
to the region. Much of the West, especially the Northwest, could experience
large increases in annual runoff, possibly resulting in increased winter/spring
flooding and landslides.
Productivity of Vegetation
Overall productivity could increase over large areas of the U.S. due to longer growing seasons, CO2 fertilization, and in some cases, more favorable water conditions. This may be the case over much of the west and, paradoxically, much of the Great Plains. It might be feasible under some scenarios, to grow more trees in the Great Plains, allowing more sequestration of carbon. However, the potential for carbon sequestration must be considered in the context of available water resources.
Vegetation dynamics are tightly coupled with hydrologic processes. From the perspective of the six different climate scenarios used, average annual runoff could decrease over most of the continental interior, including the Great Plains and the southeastern U.S. However, there is a broad range of possible outcomes among the six model scenarios, creating significant uncertainty in the risks that may arise. Runoff could increase in the vicinity of the Great Lakes, New England and the mid-Atlantic regions due to a reduction of vegetation (drought and fire-induced). The Ohio and Tennessee valleys lie between regions of decreased moisture in the southeastern U.S., and regions of increased moisture in the northern U.S. and, therefore, the prospects are uncertain. The entire Mississippi drainage could see a decrease in annual runoff by as much as 18%, averaged across all six model scenarios (with a range of +2% to -40%). Implications for shipping, irrigation, and domestic water uses would be profound. The Northwest, California, and the Great Basin could see large increases in runoff, primarily in winter, with the possibility of serious flooding. In areas with considerable summer rainfall, that is, east of the Rockies and in the Southwest, changes in vegetation and runoff tend to be opposite (i.e., decreased vegetation is associated with increased runoff, and increased vegetation is associated with decreased runoff). Summer rains provide considerable input to both streams and vegetation. If there is less vegetation, there is more runoff and vice versa. In areas with high winter rainfall, but dry summers, the runoff-vegetation relationship is quite complex. In addition, both vegetation and runoff can increase in the West. Runoff in the West is largely snowmelt dominated, and under a global warming scenario, generally increases in the winter. Under a moderate warming, there is still sufficient soil moisture recharge such that, with a longer growing season, forest growth may be enhanced. However, with an even more pronounced climate warming, runoff is still likely to increase, but forests may likely experience drought-induced dieback due to the stress of summertime warming.
Timing: Getting from Here to There
The trajectory of change from the present through the 21st Century could be very complex, especially in regions of apparently high uncertainty (i.e., regions dominated by complex interactions and feedbacks). At least one hypothesis emerges when examining impacts from all six of the possible future climate scenarios. Early in any future global warming, while temperature increases are still relatively modest, forests may be more productive and their uptake and storage of carbon may increase (due in part, to CO2 fertilization). However, as temperatures continue to increase, the CO2 effect may be overwhelmed by exponential increases in evapotranspiration. In this latter case, there could be a threshold response resulting in a shift from increased productivity to a rapid, drought-induced dieback, resulting in a release of carbon back to the atmosphere, with climate implications. Areas potentially susceptible to this are the Pacific Northwest and the Southeast.
Possible Coping Strategies and Opportunities
Dr. Ronald P. Neilson is a bioclimatologist with the USDA Forest Service and an Adjunct Professor with Oregon State University. His research for the past 25 years has focused on understanding the mechanisms that govern the distribution and function of vegetation as they relate to climate at scales ranging from local to global. He is the designer/creator of the MAPSS vegetation distribution model.
Dr. Neilson and his colleagues have conducted a number of assessments of the impacts of climate change. He was a chief scientist involved in coordinating a report to Congress from the EPA on the impacts of climate change (1990). In addition, the MAPSS model has been used in various IPCC (Intergovernmental Panel on Climate Change) assessments. Dr. Neilson was nominated as the lead author for the section on the forests of North America for a special IPCC report, "The Regional Impacts of Climate Change." He was also the convening lead author of an Annex to the above report that described possible vegetation changes for the entire world. Dr. Neilson is currently on the forest sector assessment team of the National Assessment being conducted by the USGCRP.
Dr. Neilson has published numerous, peer-reviewed articles in a number of scientific journals, and he is a member of seven professional scientific societies. He is a recipient of the Ecological Society of America's W.S. Cooper Award for excellence in Physiographic Ecology.
Dr. Neilson received his Ph.D. from the University of Utah in 1981