Page 18 - Climate Change Impacts in the United States

7: FORESTS
TRaceable accounTs

fectively detect the location and intensity of disturbances (http:// more frequently. 16,23,25 This is based on the strong relationships
17
www.ntsg.umt.edu/project/mgdi). Moderate severity disturbance between climate and forest response, shown observationally and
22
is mapped in orange and represents a 65%-100% divergence of experimentally. Expected responses will increase substantially
the current-year MODIS Global Disturbance Index value from the to warming and also in conjunction with other changes such as
range of natural variability, High severity disturbance (in red) sig- an increase in the frequency and/or severity of drought and am-
nals a divergence of over 100%. 7 plification of pest and pathogen impacts. Eastern forests are less
likely to experience immediate increases in wildfire unless/until a
New information and remaining uncertainties point is reached at which warmer temperatures, concurrent with
Forest disturbances have large ecosystem effects, but high interan- seasonal dry periods or more protracted drought, trigger wildfires.
nual variability in regional fire and insect activity makes detection
of trends more difficult than for changes in mean conditions. 20,21,70 Key message #2 Traceable accounT
Therefore, there is generally less confidence in assessment of fu- U.S. forests and associated wood products cur-
ture projections of disturbance events than for mean conditions rently absorb and store the equivalent of about
21
(for example, growth under slightly warmer conditions). 16% of all carbon dioxide (CO2) emitted by fossil
fuel burning in the U.S. each year. Climate change,
There are insufficient data on trends in windthrow, ice storms, combined with current societal trends in land use
hurricanes, and landslide-inducing storms to infer that these types and forest management, is projected to reduce this
of disturbance events are changing. rate of forest CO2 uptake.

Factors affecting tree death, such as drought, warmer tempera- Description of evidence base
tures, and/or pests and pathogens are often interrelated, which The key message and supporting text summarizes extensive evi-
means that isolating a single cause of mortality is rare. 11,12,13,17,22,68 dence documented in the TIR, “Effects of Climatic Variability and
Change on Forest Ecosystems: A Comprehensive Science Synthe-
1
Assessment of confidence based on evidence sis for the U.S. Forest Sector.” Technical input reports (58) on a
Very High. There is very high confidence that under projected wide range of topics were also received and reviewed as part of the
climate changes there is high risk (high risk = high probability Federal Register Notice solicitation for public input.
and high consequence) that western forests in the United States
3
will be affected increasingly by large and intense fires that occur A recent study has shown that forests are a big sink of CO2 na-
tionally. However, the permanence of this carbon sink is contin-
Confidence Level gent on forest disturbance rates, which are changing, and on eco-
56
nomic conditions that may accelerate harvest of forest biomass.
Very High Market response can cause changes in the carbon source/sink
Strong evidence (established dynamics through shifts in forest age, 39,40 land-use changes and
41
42
theory, multiple sources, con- urbanization that reduce forested areas, forest type changes,
sistent results, well documented and bioenergy development changing forest management. 41,43,44,45
and accepted methods, etc.),
high consensus Additionally, publications have reported that fires can convert a
25
forest into a shrubland or meadow, with frequent fires perma-
High 49
nently reducing the carbon stock.
Moderate evidence (several
sources, some consistency, New information and remaining uncertainties
methods vary and/or documen-
tation limited, etc.), medium That economic factors and societal choices will affect future carbon
consensus cycle of forests is known with certainty; the major uncertainties
Medium come from the future economic picture, accelerating disturbance
rates, and societal responses to those dynamics.
Suggestive evidence (a few
sources, limited consistency,
models incomplete, methods Assessment of confidence based on evidence
emerging, etc.), competing Based on the evidence and uncertainties, confidence is high that
schools of thought climate change, combined with current societal trends regarding
Low land use and forest management, is projected to reduce forest
Inconclusive evidence (lim- CO2 uptake in the U.S. The U.S. has already seen large-scale
ited sources, extrapolations, shifts in forest cover due to interactions between forestland use
inconsistent findings, poor docu- and agriculture (for example, between the onset of European
mentation and/or methods not settlement to the present). There are competing demands for how
tested, etc.), disagreement or
lack of opinions among experts forestland is used today. The future role of U.S. forests in the

192 CLIMATE CHANGE IMPACTS IN THE UNITED STATES