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1
Introduction
Society and a Varying Climate
System
As the glaciers of the last ice age receded and temperatures
rose, humans moved into new territories and began to raise crops
rather than seek them. The establishment of agriculture contributed
to the gathering together of people in stable communities, and to
the creation of early cities. In many regions, these agricultural
communities were sensitive to the stability of the climate. While
they might survive a singularly bad year or two, they were often
vulnerable to prolonged or abrupt anomalies. Indeed, there is much
circumstantial evidence to suggest that prolonged climatic
variations contributed to the collapse of several well-established
civilizations at certain times in the past (Weiss et al., 1993).
For example, shifts in precipitation patterns in the early part of
this millennium led to the demise of irrigation-based agriculture
in Central America and the Peruvian highlands, causing starvation,
population dispersal, and the end of once-prosperous
civilizations.
Today we tend to think of society as well insulated from such
catastrophes, yet with agriculture increasingly focused in certain
regions of the globe, populations concentrated in large urban
agglomerations, and world economic markets more responsive and
competitive, our society's well-being and stability may be even
more susceptible to global-scale climate change than were the
societies of earlier civilizations. With the world's population
nearing six billion, and continuing to increase at unprecedented
rates, the security provided by a stable climate, and our potential
vulnerability to its change, is becoming increasingly
recognized.
With this increasing awareness comes greater evidence that
climate has varied significantly in the past, and will continue to
vary over time scales of decades to centuries. In 1992 and 1993,
ice cores approximately 3 km long were extracted from the heart of
the Greenland ice sheet, revealing changes in the Earth's climate
system over the last 150,000 years or so (White et al., 1997a). One
of the most remarkable revelations of these cores was the fact that
the climate in the Holocene (the last 10,000 years)—a period
that we might consider representative of our modem climate
conditions—has undergone considerable natural variation. For
instance, evidence from the tropics shows that large hydrologic
changes have characterized much of the Holocene, with major impacts
on biota and human societies. Prior to the Holocene, as the Earth
warmed from the last glacial maximum (approximately 18,000 years
ago), the climate system underwent large swings or cycles, and,
even more surprisingly, abrupt temperature changes in decades or
even shorter periods.
The long-held, implicit assumption that we live in a relatively
stable climate system is thus no longer tenable. Furthermore,
compounding the inevitable hazard of natural climate variations is
the potential for long-term anthropogenic climatic alteration. The
likelihood of changes arising from human influence adds another
element of doubt to the possibility of predicting future climatic
states and stability on these longer time scales; moreover, the
uncertainty associated with the natural variability of the climate
system precludes our ability to clearly assess human-induced
climate change. Together, the evidence of natural variations and
the potential for anthropogenic change have altered our way of
viewing the climate system: Climate has changed and will continue
to do so with or without anthropogenic influences, and a society
that has been built around the perception of a stable climate
system can only benefit by improving the understanding, assessment,
prediction, and early detection of such change—both the
natural variability and any possible anthropogenic changes.
Better understanding and prediction are particularly important
for climate variability over long time scales, since such change
has the potential for surpassing the significant social, economic,
and political impacts of shorter-scale variations, which are often
addressed through disaster relief. Over decades to centuries, the
impacts of climate change can be considerable, and adaptation and
mitigation (of both the forcing and the response) are likely to
involve policy decisions
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and investments in infrastructure. Changes in frequency and
intensity of extreme weather events may accompany such changes in
climate (Karl et al., 1996), such as the devastating Midwestern
floods that struck the United States in 1993 and again in 1997. The
remarkable change in the flood frequency of the American River
above Sacramento, California, is the subject of a current NRC study
in the Water Science and Technology Board. The Folsom Dam was built
in 1945 to provide flood protection for Sacramento. Eight floods
greater than the largest flood in the 1905-1945 period have
occurred since 1945. A similar situation exists for several of the
other Sierra Nevada rivers in California. These high floods have
led people to question the level of flood protection actually
provided by the dam, and, more important, how flood risk should be
analyzed.
Better information on likely climate change and the associated
regional patterns—for example, the probability that such
floods may occur in clusters, say six or seven times over a 20-year
period—would not only permit the mitigation of negative
impacts but afford the opportunity to exploit positive impacts.
Governments and individuals alike would benefit from advance
knowledge of any climate changes that would have a major impact on
agriculture, energy production and utilization, water resources and
quality, air quality, health, fisheries, forestry, insurance,
recreation, and transportation—all fundamental to society's
well-being, all vulnerable to any prolonged change or abrupt shift
in our climate system. Not only would society benefit from
increased climate-prediction skill by being better prepared to ward
off adverse climatic consequences, but advance knowledge of climate
variations would also enable society to capitalize on
opportunities, such as increased geographical ranges for certain
crops.
Unfortunately, the subtlety of slow changes over long time
scales (relative to diurnal, seasonal, and interannual variations)
tends to disguise their potential long-term severity, and thus
limits society's willingness to address them in advance; this lack
of urgency is exacerbated by the uncertainty in scientists' ability
to forecast such change. Given the requisite understanding of
climate variability, we hope to ultimately forecast and detect
alterations in climate change (distinguishing natural variability
from anthropogenic change), providing a rational basis for future
policy and infrastructure-management decisions.
The limitations of the instrumental data on which our current
state of understanding is based are readily exposed by evaluating
their ability to help answer some of our most fundamental questions
involving decadal or centennial change. For example, questions such
as "Is the planet getting warmer? Is the hydrologic cycle changing?
Are the atmospheric and oceanic circulations changing? Are the
weather and climate becoming more extreme or variable? Is the
radiative forcing of climate changing?" cannot yet be answered
definitively. Each one of these apparently simple questions is
actually quite complex, both because of its multivariate aspects
and because global spatial and temporal sampling is required to
address it adequately. The global observing systems needed to
provide the answers are either inadequate or non-existent. For
science to provide society with the information it needs, better
data are essential. The models that will yield predictions require
these data to improve our understanding of decade-to-century-scale
climate change, its rate and range of variability, its likelihood
and distribution of occurrence, and the sensitivity of the climate
to changes in the forcing (natural and anthropogenic).
A U.S. Dec-Cen Science Strategy
The fundamental need to develop a good scientific understanding
of climate variability and change over decade-to-century time
scales, the inadequacy of our current understanding, and the
limited resources available to increase this understanding all
point to the need for a nationally recognized dec-cen science plan.
The present report articulates the primary scientific issues that
must be addressed in order to advance most efficiently toward the
necessary understanding. In developing this plan, the members of
the Dec-Cen panel have taken special care to recognize that
research directed toward decade-to-century-scale change and
variability will differ in two remarkable respects from research
directed at shorter-time-scale variability.
First, research on these intermediate time scales is relatively
new. As noted above, only recently have we obtained sufficiently
long high-resolution paleoclimate records to allow the examination
of past change on dec-cen time scales, and acquired faster
computers and improved models that can perform the long simulations
needed for studying such change. Consequently, we are on the steep
slope of the learning curve, with new results and dramatic insights
arising at an impressive rate. The fundamental scientific issues
requiring our primary attention are evolving rapidly. Flexibility
and adaptability in response to new opportunities and promising
directions will be imperative if we are to optimally advance our
understanding of medium- and long-range climate change and
variability.
Second, the paradigm developed for the study of climate change
on seasonal-to-interannual time scales cannot be applied to the
study of climate problems on longer time scales. We have recently
achieved considerable success in studying short-time-scale climate
problems by generating hypotheses and models that are quickly
evaluated and improved through analysis of the existing and rapidly
expanding instrumental records. For longer-time-scale problems, the
existing paleoclimate records are still too sparse and the
historical records too short; as for future records, multiple
decades will be required before even a nominal comparison with
model predictions becomes possible. Furthermore, the change in
atmospheric composition as a consequence of human actions
represents a forcing whose future trends can be estimated
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only with considerable uncertainty. Making progress in dec-cen
climate prediction will require heavy reliance on improved and
faster models, an expanded paleoclimate database, and assumed
anthropogenic and natural forcing scenarios. The inherent slowness
of obtaining new dec-cen time-scale climate observations
necessitates the use of additional climate-data sources (e.g.,
paleoclimate proxy data) to most efficiently validate and improve
the models used to assess dec-cen climate variability and change.
Considerable effort is required to use such alternative means,
because of the steps that must be taken to understand the limits
and implications of the proxy indicators constituting the
paleoclimate records. Considerable effort is also needed to monitor
actual rates of anthropogenic emissions, as well as natural
concentrations of radiatively active atmospheric constituents that
force climate on dec-cen time scales. We can only begin collection
of those data that will ultimately aid future generations of
scientists in understanding decade-to-century-scale climate
variability and change.
This Dec-Cen report identifies the fundamental science issues
that must be addressed in order to realize the following ultimate
goals:
• Characterize and assess natural climate
variability. Achieving this objective will require a solid
statistical grasp of natural variability that will serve as a
baseline for gauging anthropogenic change. This will help to reduce
a vast, complex system to manageable components that encapsulate
its key aspects and allow us to evaluate its mechanisms and
determine the likelihood of future changes. Meeting this goal will
depend on the availability of greatly expanded paleoclimate and
historical databases, and on believable simulations by
comprehensive climate models.
• Design a comprehensive system to forecast change in
the climatic mean and in climate variability. Developing such a
predictive capability demands a good understanding of the climate
system, tested through controlled hindcasting experiments. A
forecasting system is required in order to assess the likely
response to changes in the forcing, which will then permit us to
address important questions regarding adaptation versus mitigation
measures, especially for anthropogenic climate change. Some
reliable indication of future change can be realized in the interim
through existing models or statistical formulations.
• Develop a strategy for detecting climate change.
This strategy will provide the basis for testing and refining our
ultimate predictive capabilities, while the relevant observations
will provide the ground truth for such predictions. Reaching this
goal will require identification of the sensitive components of the
climate system that must be monitored to evaluate both natural and
anthropogenic climate change. Understanding and characterization of
the natural variability of the climate system on dec-cen time
scales are crucial if the anthropogenic "signal" is to be
distinguished from the natural climatic "noise." All statements
about detection of anthropogenic climate change imply knowledge of
the background variability, so we must achieve greater certainty
about the latter.
• Provide the physico-biogeochemical parameters or
parameterizations required by social scientists for socioeconomic
and environmental impact assessments and basic human-dimensions
studies. The societal consequences of climate variability on
dec-cen scales—those of the human lifetime—are likely to
be quite different from those of both shorter and longer time
scales. Human-dimensions studies specific to the dec-cen time scale
need to be performed, and scientists must be able to provide the
necessary climate-related information.
Predicting and assessing the consequences of climate change and
climate variability over dec-cen time scales will involve
considerable scientific breadth: observing past, present, and
future climate; understanding the processes of natural and
anthropogenic change and variability; and modeling variability and
change through a hierarchy of approaches. Potential consequences
can be properly addressed only within the holistic perspective
afforded by such breadth. This science strategy attempts to provide
that perspective. Our strategy for achieving it is to include
components that have already received considerable and widespread
attention (e.g., those aspects of anthropogenic climate change
highlighted in the recent Intergovernmental Panel on Climate Change
document (IPCC, 1996a), while fleshing out the relevant issues of
components that have received less institutional consideration
(e.g., natural variability, and the interactions between natural
and anthropogenic influences). Thus, the bulk of this report
describes the latter, while including overviews of the former at
the level needed to confer the necessary holistic dec-cen
perspective.
Representative terms from entire chapter:
natural variability