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Page 1
Executive Summary
The evidence of natural variations in the climate
system—which was once assumed to be relatively
stable—clearly reveals that climate has changed, is changing,
and will continue to do so with or without anthropogenic
influences. Such variability influences society through a multitude
of impacts while operating over a continuum of time scales, from
seasonal through centennial and longer. Variability in climate (the
time-averaged weather) manifests itself in a variety of forms, such
as trends, cycles, and complex regional interactions.
Recent progress in the documentation of climate variability, and
in our ability to predict certain climatic events and their
regional impacts, has had a significant beneficial effect on
society. Climate prediction, typically a season to a year or so in
advance, has been successful to some extent for certain regions and
climatic phenomena—for example, El Niño and its remote
effects, tropical rainfall in Africa and Brazil, and precipitation
in northwestern Europe and western North America. Some climatic
properties, such as temperature or pressure, tend to preserve their
spatial structure through time, or assume a limited number of
related shapes, while their amplitude, phase, and sometimes
geographic position change. Some of these patterns have also begun
to be recognized and documented. The apparent persistence of such
patterns, even allowing for their slow evolution, indicates that it
may be possible to exploit these ''signals'' to help us understand
and predict future climate variability and change.
The success of the short-term climate predictions mentioned
above, together with the growing documentation of coherent climate
patterns, provides some confidence that improved understanding
might lead to broader and longer-range forecasting skills, with
commensurate benefits to society. The extension of forecasts beyond
seasonal-to-interannual time scales is not, however, a simple
undertaking. This difficulty reflects the complexity of the Earth's
climate system, which involves the atmosphere, ocean, land,
biomass, and cryosphere, together with their various interactions.
The complexity of this system is further enhanced by anthropogenic
and natural changes in radiative forcing, as well as the
anthropogenic changes that result from increased production of
greenhouse gases. Furthermore, the slow evolution of climate
associated with variability on decade-to-century scales may lead to
gradual, but compounded changes in short-time-scale phenomena.
Therefore, simply maintaining our present prediction capabilities
may require continued investment in developing our understanding of
the interaction of climate phenomena operating on different time
and space scales.
The international scientific community and policymakers
recognize the importance of the current successes and the potential
for building on them, as well as the complexity of the problem.
They have undertaken to organize global efforts to identify the
most important problems yet to be resolved, and to target key
research areas and regions. The present report is motivated by this
international effort, as well as by our national interest in most
efficiently using limited research dollars to improve the
understanding of climate change, establish the means for detecting
climate change, and ultimately realize broader and longer climate
predictions.
Decade-to-Century-Scale Climate
Variability
This report focuses on decade-to-century-scale (dec-cen) climate
variability and change. A separate report by the NRC's Global
Ocean-Atmosphere-Land System Panel (GOALS) deals with
seasonal-to-interannual prediction (NRC, 1998a). Certainly the
physics of climate change extend across all these time scales.
There are, however, fundamental differences in the nature of
dec-cen and seasonal-to-interannual change: the dominant processes
driving the change, the manner in which they affect society, and
the manner in which society might use the predictive information.
For example, on longer time scales, more of the slower components
of the climate system can typically be expected
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Page 1
Executive Summary
The evidence of natural variations in the climate
system—which was once assumed to be relatively
stable—clearly reveals that climate has changed, is changing,
and will continue to do so with or without anthropogenic
influences. Such variability influences society through a multitude
of impacts while operating over a continuum of time scales, from
seasonal through centennial and longer. Variability in climate (the
time-averaged weather) manifests itself in a variety of forms, such
as trends, cycles, and complex regional interactions.
Recent progress in the documentation of climate variability, and
in our ability to predict certain climatic events and their
regional impacts, has had a significant beneficial effect on
society. Climate prediction, typically a season to a year or so in
advance, has been successful to some extent for certain regions and
climatic phenomena—for example, El Niño and its remote
effects, tropical rainfall in Africa and Brazil, and precipitation
in northwestern Europe and western North America. Some climatic
properties, such as temperature or pressure, tend to preserve their
spatial structure through time, or assume a limited number of
related shapes, while their amplitude, phase, and sometimes
geographic position change. Some of these patterns have also begun
to be recognized and documented. The apparent persistence of such
patterns, even allowing for their slow evolution, indicates that it
may be possible to exploit these ''signals'' to help us understand
and predict future climate variability and change.
The success of the short-term climate predictions mentioned
above, together with the growing documentation of coherent climate
patterns, provides some confidence that improved understanding
might lead to broader and longer-range forecasting skills, with
commensurate benefits to society. The extension of forecasts beyond
seasonal-to-interannual time scales is not, however, a simple
undertaking. This difficulty reflects the complexity of the Earth's
climate system, which involves the atmosphere, ocean, land,
biomass, and cryosphere, together with their various interactions.
The complexity of this system is further enhanced by anthropogenic
and natural changes in radiative forcing, as well as the
anthropogenic changes that result from increased production of
greenhouse gases. Furthermore, the slow evolution of climate
associated with variability on decade-to-century scales may lead to
gradual, but compounded changes in short-time-scale phenomena.
Therefore, simply maintaining our present prediction capabilities
may require continued investment in developing our understanding of
the interaction of climate phenomena operating on different time
and space scales.
The international scientific community and policymakers
recognize the importance of the current successes and the potential
for building on them, as well as the complexity of the problem.
They have undertaken to organize global efforts to identify the
most important problems yet to be resolved, and to target key
research areas and regions. The present report is motivated by this
international effort, as well as by our national interest in most
efficiently using limited research dollars to improve the
understanding of climate change, establish the means for detecting
climate change, and ultimately realize broader and longer climate
predictions.
Decade-to-Century-Scale Climate
Variability
This report focuses on decade-to-century-scale (dec-cen) climate
variability and change. A separate report by the NRC's Global
Ocean-Atmosphere-Land System Panel (GOALS) deals with
seasonal-to-interannual prediction (NRC, 1998a). Certainly the
physics of climate change extend across all these time scales.
There are, however, fundamental differences in the nature of
dec-cen and seasonal-to-interannual change: the dominant processes
driving the change, the manner in which they affect society, and
the manner in which society might use the predictive information.
For example, on longer time scales, more of the slower components
of the climate system can typically be expected
OCR for page 2
Page 2
to play a role in climate change. On short time scales, while
the properties of the upper layer of the ocean play an important
role in climate variations, as demonstrated by El Niño
predictions, the deep ocean currents have little direct influence,
given their slow rate of movement and change. However, on longer
and longer time scales, these slow, vast deep-ocean currents may
play a more important role, perhaps even a dominant one.
Similarly, the anthropogenic increase of greenhouse gases is not
likely to produce a perceptible difference in climate from one year
to the next, so it should have little impact on seasonal or
interannual predictions for the next few years. Over longer
periods, however, the effects can accumulate to produce a
significant change. Likewise, climate processes that introduce very
small alterations on a year-to-year basis, such as subtle yearly
changes in the thickness of sea ice in polar regions (affecting the
surface temperature of the ice and its albedo), might show
long-term accumulative influences extending well beyond their
immediate region. Therefore, although such processes might be
ignored for climate prediction on seasonal-to-interannual time
scales, they must be considered for longer-term climate prediction,
so that the resulting "drift" of climate away from its previous
state is taken into account.
In addition to emphasizing different physical components, short-
and long-term climate predictions have different implications for
society. Predictions involving short-time-scale events such as El
Niño can be used to formulate short-term mitigating actions
for any negative impacts, such as reinforcing sea walls, preparing
for excessive rain, or increasing local disaster-relief funds.
Anticipated variations in conditions from one season or year to the
next also could be taken advantage of through short-term adaptive
measures, such as altering the crops planted, adjusting
water-resource management strategies, or reallocating energy
resources.
On longer time scales, climate variations can lead to prolonged
droughts, or can alter the frequency and distribution of severe
weather events for many years. They also can influence the nature
of short-term events, such as the frequency with which El
Niño occurs, or their duration or severity. Such long-term
changes have the potential for greatly surpassing shorter-scale
variations in their societal, economic, and political impacts.
Areas of the economy that could be significantly affected include
agriculture, energy production and utilization, fisheries,
forestry, insurance, recreation, and transportation. Water
resources and quality, air quality, human health, and natural
ecosystems also could suffer or improve. Consequently, responses to
climate changes on decade-to-century time scales may involve
investments in infrastructure and changes in policy.
For example, if it becomes clear that midwestern floods like
those of 1993 and 1997 will occur more often in the next few
decades, or that El Niño events will continue to be as
frequent as in the 1990s, or that sea level will keep rising, then
efforts to mitigate such effects will certainly involve both policy
(e.g., modifications of building codes) and infrastructure changes
(e.g., construction of protective and adaptive structures such as
dikes and irrigation systems). Such predictions also might enable
society to capitalize on opportunities, like improved planning for
water-resource management, expansion or relocation of agricultural
regions, prediction of prime fishing sites and times, or adjustment
of insurance rates to better reflect the true risks over the
lifetime of a policy.
The aforementioned differences, along with the practical need to
subdivide the climate problem into more manageable units, at least
for organizational purposes, have led to this initial division of
climate-prediction efforts on the basis of time scale. This report
represents the first stage in developing a coherent national effort
for addressing decadal to centennial climate variability and
change. It does this in the form of a science strategy that
articulates the fundamental scientific issues that must be
addressed, outlines the concepts underlying an observational and
modeling program, and recommends the development of a formal,
national dec-cen program that would ensure a balanced, effective
approach to the scientific issues and the observational and
modeling needs.
A Dec-Cen Science Strategy
The science strategy proposed in this volume focuses on six of
the attributes of the Earth' s environmental system that are
considered to be most directly relevant to society and that have
displayed variations on dec-cen time scales in the past, and on six
components of the climate system that control these attributes. The
attributes are precipitation and water availability, temperature,
solar radiation, storms, sea level, and ecosystems. The controling
climate components are the atmospheric composition and radiative
forcing, atmospheric circulation, hydrologic cycle, ocean
circulation, land and vegetation, and cryosphere.
This report describes each attribute and component, illustrates
how they have varied in the past over decade-to-century time
scales, explains the mechanisms involved and the interactions among
them, describes their predictabilities, and presents the remaining
issues and questions that surround them. The report also discusses
the current state of knowledge of the climate patterns identified
so far, and the outstanding issues related to such patterns. This
approach maintains a scientifically sound yet socially relevant
focus while naturally leading to an overall science strategy for
future research, since the issues and questions serve to define and
justify the science requirements presented in the plan.
Dec-Cen Issues
The outstanding scientific questions related to dec-cen
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Page 3
climate variability are fairly extensive and disparate. The
panel's selection of principal issues was both guided and
constrained by a few overarching questions:
• What are the patterns in both space and time of dec-cen
variability, and what mechanisms give rise to them?
• What is the relationship between natural dec-cen
variability and observed global warming? For example, what do we
have to know about natural variability in order to detect
anthropogenic change?
• How does variability of forcing (natural and
anthropogenic) affect dec-cen variability?
• What is the role of the interactions among the climate
components in generating and sustaining dec-cen variability?
• To what extent is dec-cen variability predictable, and
what unresolved issues must be addressed to realize that
predictability?
A few of the issues raised in the report are briefly described
below.
Climate Patterns and Long-Term Climate
Change
It has become clear that much of the accelerated global warming
that has occurred since the mid-1970s is associated with a
low-frequency relative phasing of two of the predominant patterns
of the Northern Hemisphere: the Pacific-North American (PNA)
teleconnection and the North Atlantic Oscillation (NAO). This
phasing produces a spatial warming pattern that is similar to one
of the "greenhouse fingerprints" predicted by models. This
similarity raises several questions regarding the interaction
between anthropogenic change and natural variability. For example,
is the accelerated warming the result of natural variability caused
by an unusually persistent coincidence of the NAO and PNA, or the
result of the modification of natural modes (patterns) by
anthropogenic changes in radiative forcing that alter the phasing,
or some combination of both of these?
Likewise, there appears to have been a distinct change in the
character (frequency and severity) of El Niño and La
Niña events during this period of accelerated warming. Is
this a consequence of the influence of anthropogenic change on the
dominant natural modes of climate variability, or is it a natural,
low-frequency (dec-cen) modulation of a high-frequency
(interannual) mode? In addition to answering these specific
questions, it is clear that we must determine the longevity of the
current patterns, as well as their spatio-temporal variability;
identify the best ways of characterizing the patterns; identify new
patterns, especially in data-poor regions such as the Southern
Hemisphere; determine which ones are statistical products of our
analysis tools, rather than true dynamic modes; and determine the
mechanisms and underlying physics that control the patterns, their
sensitivities, and their evolution in space and time, including
their interaction with anthropogenic and natural changes in
radiative forcing, and their interaction amongst themselves.
Radiative Forcing
The redistribution of greenhouse gases between the ocean,
atmosphere, and biosphere, and the manner in which these fluxes
influence tropospheric greenhouse-gas concentrations on dec-cen
time scales, are of central importance to dec-cen climate change.
Accurate predictions of changes in green-house-gas concentrations
and their effects on radiative forcing will require an improved
understanding of the complex interactions between the physics
(including radiation), biology, and chemistry (including
photochemistry) of the climate system that are driving the fluxes,
redistributions, and changes. For some of these gases, such as
methane, N2O, and extratropical
ozone, it is not even clear what has caused the recent changes.
Better understanding of the cloud/water-vapor/radiation processes
and feedbacks is needed as well, because they will strongly
influence climatic response to the increased radiative forcing
associated with higher green-house-gas concentrations.
Understanding of the radiation, chemistry, and dynamic coupling
in the upper troposphere and lower stratosphere must be improved in
order to more accurately predict how stratospheric ozone will
recover from depletion by anthropogenic halocarbons, and to
determine how aerosols influence dec-cen climate variability. A
number of issues must be addressed regarding the potential role of
changes in solar radiation on climate. For instance, how
representative are proxies for solar activity (e.g., sunspots) of
actual total solar irradiance on dec-cen time scales? To what
extent are dec-cen climate changes related to changes in the sun's
output?
Other Issues
A variety of discipline-specific issues require attention in
order to better understand dec-cen climate variability. Many of
these issues require inter- and multi-disciplinary perspectives and
efforts. A better understanding needs to be obtained of the nature
of dec-cen changes and variations that have occurred in the
portions of the climate system that are especially sensitive to
climate changes. These sensitive components include cloud cover,
sea-ice fields, terrestrial water, and the subsurface ocean.
Improved understanding is also needed of how the various components
of the climate system interact to produce dec-cen climate
variability, as distinguished from the variability that is a
product of variations internal to a single component. The
atmosphere works on very fast time scales, so coherent
decadal-to-centennial variability within the atmosphere is expected
to be attributable, at least in part, to interactions with the
other, slower components of the climate system, or else to be a
response to changes in the radiative forcing. (Nonlinear internal
feedbacks are a distinct possibility, though). On the other hand,
modeling studies suggest that the ocean is susceptible to cycles of
variability on dec-cen and longer scales that are caused by
mechanisms internal to the ocean alone. Are these
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or other mechanisms responsible for driving dec-cen climate
variability and its spatial patterns? A related set of questions
is: How do processes and changes within one component influence
other aspects of the climate system, how are changes in one region
transmitted to other regions, and what components and time scales
are involved in such telecommunication? For example, how do
large-scale dec-cen changes in the atmospheric circulation
influence the seasonal-to-interannual variability of severe storms?
How do the details of the planetary boundary-layer physics and
bio-geochemistry, and the Earth's surface characteristics,
influence the propagation of climatic variability or the transfer
of greenhouse gases between various components of the climate
system?
A U.S. Dec-Cen Program
In view of dec-cen climate variability's intrinsic scientific
interest, its direct importance to society, and its involvement
with variability on other time scales, the NRC Dec-Cen panel
recommends the initiation of a national program designed to
increase understanding of this topic. The initial design of this
program would address the issues that are outlined above, while
maintaining flexibility and adaptability so that new directions and
opportunities can be pursued as our understanding is improved and
research directions are refined.
In order to address the key issues, the U.S. Dec-Cen Program
must include:
• a long-term, stable observing system;
• a hierarchical modeling program;
• appropriate process studies; and
• a means for producing and disseminating long-term proxy
and instrumental data sets.
It is essential that all of these elements be present, because
the paradigm developed for the study of climate variability on
seasonal-to-interannual time scales cannot be applied to the study
of dec-cen climate problems. Studies of short-time-scale climate
problems have generally used a process of generating hypotheses and
models that can be quickly evaluated and improved through analysis
of existing historical records or observations of near-term climate
variability. For dec-cen problems, the paleoclimate records are
still too sparse and the historical records too short for this
process to be applied; as for future records, multiple decades of
observations are required before even a nominal comparison to model
predictions can be made. Furthermore, the change in atmospheric
composition as a consequence of anthropogenic emissions represents
a forcing whose future trends can be estimated only with
considerable uncertainty. As a result, making progress in
dec-cen-scale prediction will require heavy reliance on improved
and faster models, an expanded paleoclimate data base, and assumed
scenarios for anthropogenic emission. Without the benefit of
real-time observations for constant model validation and
improvement, a substantial effort will be needed to validate models
through alternate means, to improve understanding of the limits and
implications of the proxy indicators constituting the paleoclimate
records, and to monitor actual rates of emissions and atmospheric
concentrations of radiatively active atmospheric constituents. As
for future observations, we can only now begin collection of the
data that will ultimately aid future generations of scientists in
further understanding dec-cen climate variability and change.
The ultimate research objective of a U.S. Dec-Cen Program would
be to define, understand, and model dec-cen climate variability and
change (natural and anthropogenic), so that the extent to which
they are predictable can be determined. If it can be shown that
they are indeed predictable, the ultimate practical aims of a
Dec-Cen Program would be to design and implement a complete
prediction system, building on the emerging seasonal-to-interannual
prediction systems now being constructed; to predict future
decadal-to-centennial variations to the extent possible; and to
learn to use these predictions for the benefit of all. The program
also would provide a means for detecting climate change, which will
be necessary for national and international policy decisions.
Climate variability and change on dec-cen time scales involve
all of the elements of the U.S. Global Change Research Program:
natural and anthropogenic variability and change; past, present,
and future observational networks and data bases; modeling; and the
physical, chemical, and biological sciences, with their
implications for society. A U.S. Dec-Cen Program should encompass
the dimensions of these elements pertaining to dec-cen climate
variability and change, with particular attention to important
aspects that have not received coordinated programmatic support,
such as the effects of aerosols on climate, and coupled
ocean-atmosphere modeling of long-term climate change, among
others. The U.S. Dec-Cen Program would serve as the primary
contribution of the United States to the DecCen and Anthropogenic
Climate Change (ACC) components of the international Climate
Variability and Predictability (CLIVAR) Programme of the World
Climate Research Programme (WCRP) and would provide invaluable
input to the Intergovernmental Panel on Climate Change (IPCC). It
must also be well coordinated with CLIVAR's seasonal-to-interannual
climate-variability component (GOALS) and other WCRP activities
such as GEWEX, ACSYS, and WOCE, as well as with the components of
the International Geosphere-Biosphere Programme, such as the PAGES
(Past Global Changes) program. A successful Dec-Cen Program
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should lead to a deeper knowledge of the interconnections
between seasonal-to-interannual variability and dec-cen
variability, an improved ability to detect anthropogenic climate
change, and an improved understanding of how natural dec-cen
variability is anthropogenically modified.
Decadal-to-centennial climate variability is a subtle
phenomenon. It proceeds too slowly to be perceptible to our senses,
but its cumulative effects ultimately define the life prospects of
future generations. Informed stewardship of the Earth's resources
for the generations to come must draw on the insights afforded by
model predictions and extrapolation of observations that can give
us a glimpse of the future. A U.S. Dec-Cen Program will be the
first step toward assuming this responsibility.
Representative terms from entire chapter:
radiative forcing
