Key National Findings
the U.S. National Assessment
Increased warming is projected for the 21st century -- Assuming
continued growth in world greenhouse gas emissions, the primary climate
models drawn upon for the analyses carried out in the U.S. National
Assessment projected that temperatures in the contiguous United States
will rise 3 -- 5°C (5 -- 9°F) on average during the 21st century. A wider range
of outcomes, including a smaller warming, is also possible.
Impacts will differ across regions -- Climate change and its
potential impacts are likely to vary widely across the country.
Temperature increases are likely to vary somewhat among regions. Heavy
precipitation events are projected to become more frequent, yet some
regions are likely to become drier.
Ecosystems are especially vulnerable -- Many ecosystems are highly
sensitive to the projected rate and magnitude of climate change, although
more efficient water use will help some ecosystems. A few ecosystems, such
as alpine meadows in the Rocky Mountains and some barrier islands, are
likely to disappear entirely in some areas. Other ecosystems, such as
southeastern forests, are likely to experience major species shifts or
break up into a mosaic of grasslands, woodlands, and forests. Some of the
goods and services lost through the disappearance or fragmentation of
natural ecosystems are likely to be costly or impossible to replace.
Widespread water concerns arise -- Water is an issue in every
region, but the nature of the vulnerabilities varies. Drought is an
important concern virtually everywhere. Floods and water quality are
concerns in many regions. Snowpack changes are likely to be especially
important in the West, Pacific Northwest, and Alaska.
Food supply is secure -- At the national level, the agriculture
sector is likely to be able to adapt to climate change. Mainly because of
the beneficial effects of the rising carbon dioxide levels on crops,
overall U.S. crop productivity, relative to what is projected in the
absence of climate change, is very likely to increase over the next few
decades. However, the gains are not likely to be uniform across the
nation. Falling prices are likely to cause difficulty for some farmers,
while benefiting consumers.
Near-term forest growth increases -- Forest productivity is likely
to increase over the next several decades in some areas as trees respond
to higher carbon dioxide levels by increasing water-use efficiency. Such
changes could result in ecological benefits and additional storage of
carbon. Over the longer term, changes in larger-scale processes, such as
fire, insects, droughts, and disease, could decrease forest productivity.
In addition, climate change is likely to cause long-term shifts in forest
species, such as sugar maples moving north out of the country.
Increased damage occurs in coastal and permafrost areas -- Climate
change and the resulting rise in sea level are likely to exacerbate
threats to buildings, roads, power lines, and other infrastructure in
climate-sensitive areas. For example, infrastructure damage is expected to
result from permafrost melting in Alaska and from sea level rise and storm
surges in low-lying coastal areas.
Adaptation determines health outcomes -- A range of negative health
impacts is possible from climate change. However, as in the past,
adaptation is likely to help protect much of the U.S. population.
Maintaining our nation's public health and community infrastructure, from
water treatment systems to emergency shelters, will be important for
minimizing the impacts of water-borne diseases, heat stress, air
pollution, extreme weather events, and diseases transmitted by insects,
ticks, and rodents.
Other stresses are magnified by climate change -- Climate change is
very likely to modify the cumulative impacts of other stresses. While it
may magnify the impacts of some stresses, such as air and water pollution
and conversion of habitat due to human development patterns, it may
increase agricultural and forest productivity in some areas. For coral
reefs, the combined effects of increased CO2 concentration, climate
change, and other stresses are very likely to exceed a critical threshold,
causing large, possibly irreversible impacts.
Uncertainties remain and surprises are expected -- Significant
uncertainties remain in the science underlying regional changes in climate
and their impacts. Further research would improve understanding and
capabilities for projecting societal and ecosystem impacts. Increased
knowledge would also provide the public with additional useful information
about options for adaptation. However, it is likely that some aspects and
impacts of climate change, both positive and negative, will be totally
unanticipated as complex systems respond to ongoing climate change in
Sources: NAST 2000, 2001.