What is the purpose of this Assessment?
The Assessment's purpose is to synthesize, evaluate, and report on
what we presently know about the potential consequences of climate
variability and change for the US in the 21st century. It has sought to
identify key climatic vulnerabilities of particular regions and sectors,
in the context of other changes in the nation's environment, resources,
and economy. It has also sought to identify potential measures to adapt to
climate variability and change. Finally, because present knowledge is
limited, the Assessment has sought to identify the highest priority
uncertainties about which we must know more to understand climate impacts,
vulnerabilities, and our ability to adapt.
How did the process involve both stakeholders and scientists in
This first National Assessment involved both stakeholders and
scientific experts. Stakeholders included, for example, public and private
decision-makers, resource and environmental managers, and the general
public. The stakeholders from different regions and sectors began the
Assessment by articulating their concerns in a series of workshops about
climate change impacts in the context of the other major issues they face.
In the workshops and subsequent consultations, stakeholders identified
priority regional and sector concerns, mobilized specialized expertise,
identified potential adaptation options, and provided useful information
for decision-makers. The Assessment also involved many scientific experts
using advanced methods, models, and results. Further, it has stimulated
new scientific research in many areas and identified priority needs for
What is the breadth of this Assessment?
Although global change embraces many interrelated issues, this first
National Assessment has examined only climate change and variability, with
a primary focus on specific regions and sectors. In some cases, regional
and sector analyses intersect and complement each other. For example, the Forest
sector and the Pacific Northwest have
both provided insights into climate impacts on Northwest forests.
The regions cover the nation. Impacts outside the US are considered
only briefly, with particular emphasis on potential linkages to the US.
Sector teams examined Water, Agriculture,Human Health, Forests,
and Coastal Areas and Marine Resources.
This first Assessment could not attempt to be comprehensive: the choice of
these five sectors reflected an expectation that they were likely to be
both important and particularly informative, and that relevant data and
analytic tools were available -- not a conclusion that they are the only
important domains of climate impact. Among the sectors considered, there
was a continuum in the amount of information available to support the
Assessment, with some being at far earlier stages of development. Future
assessments should consider other potentially important issues, such as
Energy, Transportation, Urban Areas, and Wildlife.
Each regional and sector team is publishing a separate report of its
own analyses, some of which are still continuing. The Overview
and Foundation reports
consequently represent a snapshot of our understanding at the present
After identifying potential impacts of climate change, what kinds
of societal responses does this report explore?
Responses to climate change can be of two broad types. One type
involves adaptation measures to reduce the harms and risks, and maximize
the benefits and opportunities, of climate change, whatever its cause. The
other type involves mitigation measures to reduce human contributions to
climate change. After identifying potential impacts, this Assessment
sought to identify potential adaptation measures for each region and
sector studied. While this was an important first step, it was not
possible at this stage to evaluate the practicality, effectiveness, or
costs of the potential adaptation measures. Both mitigation and adaptation
measures are necessary elements of a coherent and integrated response to
climate change. Mitigation measures were not included in this Assessment,
but are being assessed in other bodies such as the United Nations Intergovernmental
Panel on Climate Change (IPCC).
Does the fact that this report excludes mitigation mean that
nothing can be done to reduce climate change?
No. An integrated climate policy will combine mitigation and adaptation
measures as appropriate. If future world emissions of greenhouse gases are
lower than currently projected, for whatever reason, including intentional
mitigation, then the rate of climate change, the associated impacts, and
the cost and difficulty of adapting will all be reduced. If emissions are
higher than expected, then the rate of change, the impacts, and the
difficulty of adapting will be increased. But no matter how aggressively
emissions are reduced, the world will still experience at least a century
of climate change. This will happen because the elevated concentrations of
greenhouse gases already in the atmosphere will remain for many decades,
and because the climate system responds to changes in human inputs only
very slowly. Consequently, even if the world takes mitigation measures, we
must still adapt to a changing climate. Similarly, even if we take
adaptation measures, future emissions will have to be curbed to stabilize
climate. Neither type of response can completely supplant the other.
How are computer models used in this Assessment?
State-of-the-science climate models have been used to generate climate
change scenarios. Computer models of ecological systems, hydrological
systems, and various socioeconomic systems have also been used in the
Assessment, to study responses of these systems to the scenarios generated
by climate models.
What additional tools, besides models, were used to evaluate
potential climate change impacts?
In addition to models, the Assessment has used two other ways to think
about potential future climate. First, the Assessment has used historical
climate records to evaluate sensitivities of regions and sectors to
climate variability and extremes that have occurred in the 20th century.
Looking at real historical climate events, their impacts, and how people
have adapted, gives valuable insights into potential future impacts that
complement those provided by model projections. In addition, the
Assessment has used sensitivity analyses, which ask how, and how much, the
climate would have to change to bring major impacts on particular regions
or sectors. For example, how much would temperature have to increase in
the South before agricultural crops such as soybeans would be negatively
affected? What would be the result for forest productivity of continued
increases in temperature and leveling off of the CO2 fertilization effect?
Has this report been peer reviewed?
This Overview and the underlying Foundation document have been
extensively reviewed. More than 300 scientific and technical experts have
provided detailed comments on part or all of the report in two separate
technical reviews. The report was reviewed at each stage for technical
accuracy by the agencies of the US Global Change Research Program. The
public also provided hundreds of helpful suggestions for clarification and
modification during a 60-day public comment period. A panel of
distinguished experts convened by the President's
Committee of Advisors on Science and Technology has provided broad
oversight, and monitored the authors response to all reviews.