March 18th Global Change Seminar: "Projected Ecosystem Changes for the United States Under a Climate Warming"
tsocci at usgcrp.gov
Mon Mar 15 16:35:18 EST 1999
U.S. Global Change Research Program Seminar Series
Projected Ecosystem Changes for the United States Under a Climate Warming
What are the projected changes in ecosystems within the United States
resulting from a climate warming resulting from a doubling of the
concentration of CO-2 in the atmosphere? Which ecosystems appear to prosper
and which ecosystems appear to be negatively impacted under such a climate
warming? Which regions of the United States are more likely to witness
dramatic ecosystem changes, with significant social and economic
consequences? How much uncertainty/confidence is there in the various
ecosystem model results?
Thursday, March 18, 1999, 3:15-4:45 PM
Dirksen Senate Office Bldg., Room G-11
Dr. Margot Anderson, Head of the Global Change Program Office, U.S.
Department of Agriculture, Washington, DC
Dr. Ronald P. Neilson, US Department of Agriculture Forest Service,
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 CO-2, 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:
Are there any consistent responses at regional scales within the
U.S. across all six model scenarios?
Are there clues to possible impacts on biological diversity and
Are there important interactions between vegetation responses
and available water resources within different regions?
Might there be changes in regional disturbance patterns, notably
droughts, floods, and wildfires?
Are there possibilities for enhanced carbon sequestration within
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.
Biodiversity and the Distribution of Vegetation
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, CO-2 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 CO-2 fertilization). However, as temperatures
continue to increase, the CO-2 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
Seasonal dynamics of water and vegetation, as well as the total
abundance of water and vegetation, will clearly change under a
global warming. Even if the amount of carbon stored
remains about the same, or increases over the nation as a whole,
the regional distribution of vegetation/ecosystems will change.
Some areas will see an increase in vegetation while others will
will see losses. These changes could result in localized
pressures on the economy and the environment, and will likely
require significant and strategic management. For example, if
forests over large areas begin to undergo drought stress, it might
be prudent to 'trim the wick,' that is, to exert some manner of
density control in order to save trees and to conserve water in
Over large areas of the country the nation's need for water
resources may be in sharp competition with the water
requirements of ecosystems. Planting more trees
may result in less available water for irrigation, commerce, and
domestic uses. A possible alternative is to use the landscape as a
'carbon pump,' moving wood from rapidly growing forests, over
short rotations, into long-lived forest products.
Ultimately, given the current model uncertainties, development of
a set of contingency plans might be one of many reasonable
approaches to resource management in the future. Monitoring
stations might be deployed and designed to serve as an early
warning system of ecosystem changes, while research and
modeling might assist in exploring and evaluating alternative
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.
Acknowledgments: The work presented in this seminar is derived in part,
from papers published in "Global Change Biology", "Global
Biogeochemical Cycles" and "Northwest Science", with Raymond J.
Drapek, James M. Lenihan, Dominique Bachelet, Christopher Daly and
The Next Seminar is scheduled for Tuesday, April 20, 1999
Tentative Topic: Arctic Sea-Ice Thinning: Observations, Possible Causes,
For more information please contact:
Anthony D. Socci, Ph.D., U.S. Global Change Research Program Office, 400
Virginia Ave. SW, Suite 750, Washington, DC 20024; Telephone: (202)
314-2235; Fax: (202) 488-8681 E-Mail: TSOCCI at USGCRP.GOV.
Additional information on the U.S. Global Change Research Program (USGCRP)
and this Seminar Series is available on the USGCRP Home Page at:
http://www.usgcrp.gov. A complete archive of seminar summaries can also be
found at this site. Normally these seminars are held on the second Monday
of each month.
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