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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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Page 124 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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Page 125 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.