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The U.S. National Climate Change Assessment: Do the Climate Models Project a Useful Picture of Regional Climate?
Subcommittee on Oversight and Investigations
Dr. Anthony C. Janetos
I am pleased to have the
opportunity to address the US House of Representatives Committee on Energy and
Commerce, Subcommittee on Oversight and Investigations on the topic of "The
US National Climate Change Assessment: Do the Climate Models Project a Useful
Picture of Climate Change?" The
US National Assessment, Climate Change
Impacts on the United States: the Potential Consequences of Climate
Variability and Change was released in November of 2000, following an
extensive series of peer reviews and public comment.
This first document, the Overview, was followed about a month later by
the release of the Foundation, a much more extensive, fully documented
background document that lays out all of the analytical detail and data that
were used in the National Assessment. The
National Assessment is an extensive synthesis of the best available scientific
information on this important topic. There are three questions about climate change that dominate discussions of this important topic. How much climate change is going to occur? What will happen as a result? What can countries do about it? There are obviously heated political opinions about each of these, but the issues are real, and it is critical to understand the underlying scientific knowledge about each if sound decisions are to be made. The assessment report focuses on the second of these questions. A
national assessment of the potential impacts of climate change was called for
in the 1990 legislation that established the US Global Change Research Program
(USGCRP). For several years, the
research program focused on developing the basic scientific knowledge that the
international scientific assessment process overseen by the Intergovernmental
Panel on Climate Change (IPCC ) depends on. The IPCC was jointly established
by the World Meteorological Organization and the United Nations Environmental
Programme in 1988. As
scientific research has provided compelling evidence that climate change is in
fact occurring, it has become increasingly clear that there is a need to
understand what is at stake for natural resources and human well-being in the
US. In response to this
need, in 1998, Dr. John H. Gibbons, then Science Advisor to the President,
requested the USGCRP to undertake
A variety of efforts emerged in response to Dr. Gibbons' charge. Over
twenty workshops were held around the country, involving academics,
business-people representing a range of industries including manufacturing,
power generation and tourism, and people who work closely with land and water
ecosystems including resource managers, ranchers, farmers, foresters and
fishermen. Each workshop
identified a range of issues of concern to stakeholders in those regions, many
of them quite unrelated to climate change, per se. Most workshops
were followed by the initiation of scientific, university-led regional
studies. In addition to these kind of "bottom-up" efforts, it was decided that it was also necessary to create a national-level synthesis of what is known about the potential for climate impacts for the US as a whole, addressing the issues identified in the regional workshops and national studies. This synthesis obviously needed to build on the work that had begun to emerge from the subsequent regional and national studies, but also to draw on the existing scientific literature and analyses done with the most up-to-date ecological and hydrological models and data that could be obtained. The National Assessment Synthesis Team (NAST) was established by the National Science Foundation as an independent committee under the Federal Advisory Committee Act (FACA) specifically in order to carry out this second step. This committee was made up of experts from academia, industry, government laboratories, and non-governmental organizations (NGO's) (membership list is Attachment 1). In order to ensure openness and independence, all meetings of the NAST were open to the public, all documents discussed in its meetings are available through the National Science Foundation, as are all the review comments already received and responses to them. This is perhaps out of the ordinary for a scientific study; but most scientific studies do not focus on issues of such broad and deep implications for American society, and about which there is such heated rhetoric. The
NAST's first action was to publish a plan for the conduct of the national
synthesis. In addition, five issues (agriculture, water, forests, health, and
coastal and marine systems), out of the many identified, were selected to be
topics for national studies. Carrying
out this plan was a major undertaking. The
end result has been the production of a comprehensive two-volume national
assessment report. The
"Foundation" volume is more than 600 pages long, with more than 200 figures
and tables, with analyses of the five national sectors, and 9 regions that
together cover the entire US. It is
extensively referenced, and a commitment was made that all sources used in its
preparation were to be open and publicly available.
The "Overview" volume is about 150 pages long, written in a style
that is more accessible to the lay public, and summarizes the Foundation in a
way that is understandable and informative, and which we are confident is
scientifically sound. Both
documents have already been through extensive review.
At the end of 1999, two rounds of technical peer review were undertaken,
and during the spring of 2000, an additional review by about 20 experts outside
the assessment process was undertaken. Over
300 sets of comments have been received from scientists in universities,
industry, NGO's, and government laboratories. The responses to all external
comments have been described in comprehensive review memorandums.
The final stage of the process, a 60 day public comment period
specifically requested by Congress, after which final revisions was then
completed, and the report was submitted to the President so that it could be
transmitted to Congress, as called for in the original legislation.
Hundreds of additional comments were received during the public comment
period, each of which was responded to. In
order to ensure that the NAST carried out its charge well, an oversight panel
was also established through the offices of the President's Council of
Advisors on Science and Technology (membership list is Attachment 2).
The oversight panel was chaired by Dr. Peter Raven, Director of the
Missouri Botanical Garden and former Home Secretary of the National Academy of
Sciences, and Dr. Mario Molina, Professor of Atmospheric Chemistry at MIT, and
recent Nobel-prize winner for his research on stratospheric ozone depletion.
Its membership, like the NAST's, was also drawn from academia,
industry, and NGO's. It reviewed
and approved the plans for the assessment, reviewed each draft of the report,
and reviewed the response of the NAST to all comments. What
have been the results of this extraordinarily open process?
What assumptions drive the analysis?
What conclusions have been reached? It
is important to realize that the national assessment does not attempt to predict
exactly what the future will hold for the US.
It examined the potential implications of two primary climate scenarios,
each based on the same assumptions about future "business as usual" global
emissions of greenhouse gases that the IPCC has used for many of its analyses.
The two climate scenarios were based on output from two different global climate
models used in the IPCC assessment. They
are clearly within the range of global annual average temperature changes shown
by many such models, one near the low and one near the high end of the range.
Both exhibit warming trends for the US that are larger than the global
average. This is not surprising.
For many years, one of the most robust results of global climate models
has been that greater warming is expected in more northerly latitudes, and that
land surfaces are expected to warm more than the global average.
We have used assumptions that are entirely consistent with those used by
the IPCC. In addition to the two
primary models from the Canadian Climate Centre and the Hadley Centre, results
from climate models developed at the National Center for Atmospheric Research,
NOAA's Geophysical Fluid Dynamics Laboratory, NASA's Goddard Institute for
Space Studies, and the Max Planck Institute were also used in various aspects of
the Assessment. The
NAST was aware of the scientific issues surrounding the use of regional results
from any general circulation models. In
the analyses done with the climate models' regional outputs, simulations from
the models were used to adjust historically observed data in order using methods
that had already been peer-reviewed in other studies, in order to depict
scenarios that had sufficient regional richness for analysis.
In
addition to models, the Assessment used two other ways to think about potential
future climate. First, it 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 used sensitivity analyses, which ask how, and how much, the climate
would have to change to bring major impacts on particular regions and sectors. These
climate scenarios describe significantly different futures that are
scientifically plausible, given our current understanding of how the climate
system operates. As importantly,
they describe separate baselines for analysis of how natural ecosystems,
agriculture, water supplies, etc. might change as a result.
In order to investigate such changes, i.e. the potential impacts of
climate changes, the report relies on up-to-date models, on empirical
observations from the literature, on investigations of how these systems have
responded to climate variability that has been observed over the past century in
the US, and on the accumulated scientific knowledge that is available about the
sensitivities of resources to climate, and about how the regions of the US have
and potentially could respond. One
additional important point about the scenarios should be mentioned.
The report does not average the results of models that disagree; it
explicitly avoids doing so. The
best example of this is in the analysis of potential changes in precipitation,
where the two models used to create the scenarios give quite different results
for some areas of the US. We have
chosen to highlight these differences and explain that regional-scale
precipitation projections are much more uncertain compared with temperature,
rather than attempting to merge the results or guess which is more likely.
The knowledge that the direction of precipitation change in some areas is
quite uncertain is valuable for planning purposes, and clearly represents and
important research challenge. There
is however, consistency among models and observations on other aspects of
precipitation changes. For example,
both models and observations show an increase in the proportion of precipitation
derived from heavy and extreme events as the climate warms. So, both types of information are pertinent to help with the
identification of potential coping actions.
In this respect, the report follows the procedure that the IPCC itself
uses for its global impacts reports, each of which examines the potential
impacts for entire continents. The
US national assessment presents the results for each scenario clearly, and then
takes the important additional step of explicitly describing the NAST's
scientific judgment about the uncertainty inherent in each result.
Those results that are viewed to be robust are described in more certain
terms; those viewed to be the result of poorly understood or unreconciled
differences between models are described in more circumspect language.
The lexicon of terms used to denote the NAST's greater or lesser
confidence is explicitly described in the beginning of the Overview report.
This helps ensure that the report does not mask important results by
thoughtlessly merging
models, or overstating the scientific capability for assessing potential
impacts. Finally, the report begins
to identify possible options for adaptation to this changing world.
It does not do a complete analysis of the costs, benefits, or feasibility
of these options however, which is a necessary next step for developing policies
to address these issues. The
report's key findings present important observations for all Americans: 1.
Increased warming. Assuming
continued growth in world greenhouse gas emissions, the climate models used in
this Assessment project that temperatures in the US will rise 5-10oF
(3-5oC) on average in the next 100 years. 2.
Differing regional impacts. Climate
change will vary widely across the US. Temperature increases will vary somewhat
from one region to the next. Heavy and extreme precipitation events are likely
to become more frequent, yet some regions will get drier. The potential impacts
of climate change will also vary widely across the nation. 3.
Vulnerable ecosystems. Many
ecosystems are highly vulnerable to the projected rate and magnitude of climate
change. A few, such as alpine meadows in the Rocky Mountains and some barrier
islands, are likely to disappear entirely in some areas.
Others, such as forests of the Southeast, are likely to experience major
species shifts or break up. The goods and services lost through the
disappearance or fragmentation of certain ecosystems are likely to be costly or
impossible to replace. 4.
Widespread water concerns. Water
is an issue in every region, but the nature of the vulnerabilities varies, with
different nuances in each. Drought is an important concern in every region.
Floods and water quality are concerns in many regions. Snow-pack changes are
especially important in the West, Pacific Northwest, and Alaska. 5.
Secure food supply. At
the national level, the agriculture sector is likely to be able to adapt to
climate change. Overall, US crop productivity is very likely to increase over
the next few decades, but the gains will not be uniform across the nation.
Falling prices and competitive pressures are very likely to stress some farmers,
while benefiting consumers. 6.
Near-term increase in forest growth. Forest
productivity is likely to increase over the next several decades in some areas
as trees respond to higher carbon dioxide levels. Over the longer term, changes
in larger-scale processes such as fire, insects, droughts, and disease will
possibly 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 US. 7.
Increased damage 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 climatically
sensitive places. For example,
infrastructure damage is related to permafrost melting in Alaska, and to
sea-level rise and storm surge in low-lying coastal areas. 8.
Adaptation determines health outcomes. A
range of negative health impacts is possible from climate change, but adaptation
is likely to help protect much of the US 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. 9.
Other stresses magnified by climate change.
Climate
change will very likely magnify the cumulative impacts of other stresses, such
as air and water pollution and habitat destruction due to human development
patterns. For some systems, such as
coral reefs, the combined effects of climate change and other stresses are very
likely to exceed a critical threshold, bringing large, possibly irreversible
impacts. 10.
Uncertainties remain and surprises are expected. Significant
uncertainties remain in the science underlying regional climate changes and
their impacts. Further research
would improve understanding and our ability to project societal and ecosystem
impacts, and provide the public with additional useful information about options
for adaptation. However, it is
likely that some aspects and impacts of climate change will be totally
unanticipated as complex systems respond to ongoing climate change in
unforeseeable ways. Given
these findings it is clear that climate impacts will vary widely across the
Nation, as one would expect for a country as large and ecologically diverse as
the US. Natural ecosystems appear
to be highly vulnerable to climate changes of the magnitude and rate which
appear to be likely; some ecosystems surprisingly so.
The potential impacts on water resources are an important issue in every
region examined, although the nature of the concern is very different for the
mountainous West than for the East. The
potential for drought is a concern across the country.
The nation's food supply appears secure, but there are very likely to
be regional gains and losses for farmers, leading to a more complex picture on a
region-by-region basis. Forests are
likely to grow more rapidly for a few decades because of increasing carbon
dioxide concentrations in the atmosphere, but it is unclear whether those trends
will be maintained as the climate system itself changes, leading to other
disturbances such as fire and pest outbreaks. However, the climate change itself will, over time, lead to
shifts in the tree species in each region of the country, some of them
potentially quite profound. Coastal
areas in many parts of the US and the permafrost regions of Alaska are already
experiencing disruptions from sea-level rise and recent regional warming; these
trends are likely to accelerate. Climate
change will very likely magnify the cumulative impacts of other environmental
stresses about which people are already concerned, such as air and water
pollution, and habitat destruction due to development patterns.
There are clearly links between human health, current climate, and air
pollution. The future vulnerability
of the US population to the health impacts of climate change depends on our
capacity to adapt to potential adverse changes.
Many of these adaptive responses are desirable from a public health
perspective irrespective of climate change.
Future assessments need to consider climate change in the context of the
suite of environmental stresses that we all face.
Perhaps most importantly, the report acknowledges very clearly that
scientific uncertainties remain, and that we can expect surprises as this
uncontrolled experiment with the Earth's geochemistry plays out over the
coming decades.
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