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7
Conclusions and Recommendation
Conclusions
Climate is constantly changing and will continue to do so; we
cannot assume a stable climate system even in the absence of
anthropogenic influences. Such variability manifests itself over a
continuum of time scales, from seasonal and interannual to decadal
and centennial, or longer.
This report documents decadal-to-centennial variations of those
climatic attributes—freshwater, temperature, sea level, solar
radiation, storms, and ecosystems—that directly affect
societies and economies. It also suggests how this variability is
governed by the interacting components of the climate system:
atmospheric composition and radiative forcing, atmospheric
circulation, the hydrologic cycle, ocean circulation, the
cryosphere, and land and vegetation.
Because of the duration of dec-cen-scale climate changes, and
the potentially large magnitude of their effects, mitigation and/or
adaptation measures are likely to involve investments in
infrastructure and changes in policy. Unfortunately, the subtlety
of slow change over long time scales, relative to the obvious
diurnal, seasonal, and interannual variations, can disguise its
potential long-term severity. Society's willingness to address
problems of climate variability in advance is limited both by the
inconspicuousness of the changes and by uncertainty in our ability
to forecast them. This underscores the importance of improving our
understanding of dec-cen climate change, the rate and range of its
variability, the likelihood and distribution of its occurrence, and
the sensitivity of climate to changes in the forcings, both natural
and anthropogenic.
A firm understanding of these characteristics will constitute
the foundation on which future policy decisions and infrastructure
management can be rationally based. Development of this
understanding will call for the ability to answer questions that
are both fundamental and overarching. Specifically, we need to
know:
• What are the spatio-temporal patterns of dec-cen
variability, and what mechanisms give rise to them?
• What is the relationship between natural dec-cen
variability and observed global warming? What do we have to know
about the natural variability in order to detect anthropogenic
change?
• How does variability in the forcings, both natural and
anthropogenic, affect dec-cen variability?
• What is the role of interaction among the climate
components in generating and sustaining dec-cen variability?
• To what extent is dec-cen variability predictable?
The resolution of these questions will depend on progress in
addressing a number of more specific scientific issues, which are
presented at the conclusion of Chapter 3, near the end of each of
the climate-component discussions of Chapter 5, and in Chapter 6.
The panel has selected those issues they consider most urgent for
effectively advancing our understanding of dec-cen variability and
change. In general, these issues revolve around basic concerns such
as: how the various climate components have changed in space and
time, and what mechanisms drive the changes; the interactions and
feedbacks among the various atmospheric constituents, radiative
forcing, and surface boundary conditions, and how they influence
dec-cen climate variability and change; how the sources and sinks
of greenhouse trace gases and the partitioning of carbon between
reservoirs vary on dec-cen time scales, and what mechanisms drive
their variations; and identifying the externally forced, internally
forced, and coupled modes of variability and change in the Earth's
climate system, and the processes and mechanisms driving them.
Recommendation
The Dec-Cen panel recommends that the United States initiate
a Dec-Cen Program designed to increase understanding of climate
variability on decade-to-century time scales, and determine its
predictability. The initial design of this program would
address those issues currently identified in Chapters 3, 5, and 6.
Flexibility and adaptability will
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have to be maintained, so that new directions and opportunities
can be pursued as our understanding is improved and research
directions are refined. In order to achieve its goal, the U.S.
Dec-Cen Program must represent a balance between the following
elements:
1. A long-term, stable observing system to constantly
monitor, with sufficient accuracy and resolution, a subset of
crucial earth-system variables (i.e., key state variables and
primary forcings) on dec-cen time scales. A modeling
activity should be an integral part of this system, to add
value to the observations by assimilating them into suitable
models. By more clearly delineating the actual nature of decadal
variability in the climate system, the capability for detecting
anthropogenic climate change will be improved, thereby making an
important contribution to the IPCC's goals. The observing system
can be built incrementally, but must be initiated immediately,
because a long, sustained climate record is required to document
the wide range of climate variability and change over dec-cen time
scales. (Specific details are outlined in Chapter 6.)
2. A hierarchical program of modeling studies that has
the ultimate goal of simulating and predicting the entire Earth
system on dec-cen time scales. This element requires an
infrastructure that makes powerful computational and communications
resources readily available to the full modeling community. (Again,
specific details are outlined in Chapter 6.)
3. Process studies that address those geophysical,
chemical, and biological processes which are so poorly understood
that they hinder our ability to define, understand, and predict
dec-cen variability. These studies should include analytic,
model-based, and observational ones, as well as combinations of the
three. (Fundamental processes requiring particular attention are
listed following the issues in each subsection of Chapter 5.)
4. Producing and disseminating long-term proxy and
instrumental datasets for use in the study of dec-cen
variability. These datasets provide the only means for immediately
securing a reliable, albeit preliminary, assessment of past dec-cen
variability, and for establishing the pre-anthropogenic-effect
baseline of natural variability. Because the sources of some of the
most well-established proxy records are at risk of permanent
destruction in the very near future (e.g., alpine glaciers and
long-lived stable tropical corals), it is imperative that these
records be secured while it is still possible to do so.
The recommended U.S. Dec-Cen Program would be part of the U.S.
contribution to the DecCen and ACC (Anthropogenic Climate Change)
components of the Climate Variability and Predictability (CLIVAR)
Programme of the WCRP. It would also contribute to the
International Geosphere-Bio-sphere Programme (IGBP). The scope of
the U.S. program would be broader than that of either the WCRP or
IGBP alone; the research objectives would deal in an integrated
manner with the biological, terrestrial, oceanic, and atmospheric
aspects of climate variability on dec-cen time scales. It would
thus draw on the contributions already being made by such programs
and projects as the IGBP's PAGES, the WCRP's GEWEX, SPARC, ACSYS,
and WOCE, and GCOS, GOOS, and GTOS.
The U.S. Dec-Cen Program must also be well coordinated with the
U.S. and WCRP seasonal-to-interannual program, the Global
Ocean-Atmosphere-Land System (GOALS) Program, because the dec-cen
influence on short-term variability and change may aggravate the
difficulties of short-term climate prediction. Indeed, several
components of the proposed Dec-Cen Program cannot be separated from
those of the GOALS Program (e.g., determining the influence of
dec-cen variability on the frequency, intensity, and duration of E1
Niño events).
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 reliably determined. If it can be
shown that they are predictable, the ultimate practical aim of the
Dec-Cen Program would be to design and implement a system that
predicts the various aspects of climate at the temporal range
permitted by their predictability. This system would build on the
emerging seasonal-to-interannual prediction systems now being
constructed, predict future decadal-to-centennial variations to the
extent possible, and learn to use these predictions for the benefit
of all.
The dec-cen prediction system would have to investigate the
future responses to whatever external-forcing scenarios are
imposed, both naturally and anthropogenically, upon the Earth's
climate system. It would provide a basis for distinguishing natural
variability from anthropogenic change, to the extent theoretically
and practically possible. The understanding of this distinction
will yield a strategy for the detection and attribution of any
anthropogenic climate change that is superimposed on the background
of natural variability.
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 model-aided 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:
natural variability