Attachment A

National Research Council (NRC), 1998a. Decade-to-Century-Scale Climate Variability and Change: A Science Strategy (pre-publication). National Academy Press, Washington, DC. pp. 204.

A1. “Also, we need a better understanding of the cloud/water-vapor/radiation processes and feedbacks that will strongly influence the climatic response to the increased radiative forcing associated with increased greenhouse gas concentrations.” (pg. xviii)

A2. “It [the Dec-Cen program] also must 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 components of the International Geosphere-Biosphere Programme, such as the PAGES (Past Global Changes) program.” (pg. xx)

A3. “Freshwater is the very basis of terrestrial life, and is arguably its most precious natural resource. Water influences nearly every aspect of society and day-to-day life... Any changes or disruptions in the freshwater cycle as we have come to know and rely on it can thus have widespread consequences, with implications for all levels of society and every individual in it.” (pg. 7)

A4. “...how do the impacts of aerosols on the Earth's radiation budget vary by region? Although aerosols appear to constitute a critical radiative-forcing factor that is active on dec-cen time scales, the mechanisms and processes involved remain poorly characterized. Many of them appear to have short--time-scale influences with long-time-scale implications. (This is true for both tropospheric and stratospheric aerosols.) For example, indirect effects of aerosols on cloud properties and formation, as well as on solar and thermal radiation, remain a major uncertainty--but may represent the primary impact of aerosols on climate.” (pg. 63)

A5. “A number of processes that operate predominantly on short time scales need to be better understood and parameterized in order to properly evaluate their role in dec-cen climate variability and change. For example, how do the composition and properties of aerosols determine both their direct and indirect radiative effects? How do tropospheric aerosols contribute to climate change on long time scales? How do aerosols contribute to cloud formation, precipitation, and radiative interaction? Cloud processes in general and their relationship to atmospheric water vapor and the radiation balance, although they occur on time scales far shorter than decadal, remain a major uncertainty in the prediction of future radiation balances; parameterizations need to be improved for cloud formation and distribution as a function of water-vapor distribution, surface boundary conditions, and rate of the hydrologic cycle. These parameterizations must also include the associated radiative impacts.” (pg. 65)

A6. “Here we highlight the issues that especially important for atmospheric circulation.

...

What are the mechanisms of interaction between the atmosphere and land-surface processes on dec-cen time scales? Land-surface characteristics, such as snow and ice cover and soil moisture content, are known to affect short-term (interannual) climate variability. Longer-term



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GEWEX—CLIVAR: Coordination of U.S. Activities Attachment A National Research Council (NRC), 1998a. Decade-to-Century-Scale Climate Variability and Change: A Science Strategy (pre-publication). National Academy Press, Washington, DC. pp. 204. A1. “Also, we need a better understanding of the cloud/water-vapor/radiation processes and feedbacks that will strongly influence the climatic response to the increased radiative forcing associated with increased greenhouse gas concentrations.” (pg. xviii) A2. “It [the Dec-Cen program] also must 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 components of the International Geosphere-Biosphere Programme, such as the PAGES (Past Global Changes) program.” (pg. xx) A3. “Freshwater is the very basis of terrestrial life, and is arguably its most precious natural resource. Water influences nearly every aspect of society and day-to-day life... Any changes or disruptions in the freshwater cycle as we have come to know and rely on it can thus have widespread consequences, with implications for all levels of society and every individual in it.” (pg. 7) A4. “...how do the impacts of aerosols on the Earth's radiation budget vary by region? Although aerosols appear to constitute a critical radiative-forcing factor that is active on dec-cen time scales, the mechanisms and processes involved remain poorly characterized. Many of them appear to have short--time-scale influences with long-time-scale implications. (This is true for both tropospheric and stratospheric aerosols.) For example, indirect effects of aerosols on cloud properties and formation, as well as on solar and thermal radiation, remain a major uncertainty--but may represent the primary impact of aerosols on climate.” (pg. 63) A5. “A number of processes that operate predominantly on short time scales need to be better understood and parameterized in order to properly evaluate their role in dec-cen climate variability and change. For example, how do the composition and properties of aerosols determine both their direct and indirect radiative effects? How do tropospheric aerosols contribute to climate change on long time scales? How do aerosols contribute to cloud formation, precipitation, and radiative interaction? Cloud processes in general and their relationship to atmospheric water vapor and the radiation balance, although they occur on time scales far shorter than decadal, remain a major uncertainty in the prediction of future radiation balances; parameterizations need to be improved for cloud formation and distribution as a function of water-vapor distribution, surface boundary conditions, and rate of the hydrologic cycle. These parameterizations must also include the associated radiative impacts.” (pg. 65) A6. “Here we highlight the issues that especially important for atmospheric circulation. ... What are the mechanisms of interaction between the atmosphere and land-surface processes on dec-cen time scales? Land-surface characteristics, such as snow and ice cover and soil moisture content, are known to affect short-term (interannual) climate variability. Longer-term

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GEWEX—CLIVAR: Coordination of U.S. Activities fluctuations may also be related to variations in land-surface characteristics. Changes in the lower boundary conditions may initiate regime changes in the atmosphere, which may emerge as dec-cen-scale climate variations. The land surface's ability to extend the memory of the climate system, even though smaller than the ocean's, can be important in such regime shifts. ... How do dec-cen-scale changes in atmospheric trace gases, aerosols, and cloud cover affect radiative balance and thus atmospheric circulation, and vice versa? The delicate feedback mechanisms associated with the interaction between radiation and dynamics are not well understood. Sub-grid-scale parameterizations are involved in modeling these feedback mechanisms, and need to be put on a more robust physical basis. There has been considerable controversy recently about the sufficiencies of model simulations of upper-tropospheric moisture transport and phase changes, about the role of anthropogenic aerosols in climate change, and about the role of clouds in stabilizing the global climate. These important issues need to be resolved if we are to simulate and predict dec-cen variability.” (pg. 77) A7. “Observations should focus on describing both the state variables--wind, pressure, temperature, humidity, and rainfall--and the forcings or related variables--solar radiation, clouds, aerosols and chemical composition.” (pgs. 77-78) A8. “In particular, the processes controlling the feedbacks and interactions between water vapor, cloud-formation processes, and atmospheric circulation must be better understood and parameterized... Also, the processes controlling the boundary-layer physics, including all interactions (e.g., exchange of heat, moisture, and momentum) and feedbacks, need to be better understood. Much of the dec-cen variability in the atmospheric circulation arises through interactions and coupling with the boundaries, yet this complex interface region is poorly understood, and poorly resolved in models.” (pg. 78) A9. “To understand the hydrologic cycle better, we need to improve both our knowledge and the model representations of the processes controlling the rates, pathways, storage, and redistribution of water in all its forms in the hydrologic cycle. One of the dimensions of the WCRP 's Global Energy and Water Cycle Experiment (GEWEX) program is an investigation of the detailed land-surface hydrology in major drainage basins. (The Mississippi basin is the primary U.S. focus, but concurrent land experiments are proposed for other parts of the world.) The basic issue is the parameterization of aspects of multi-scale land-surface properties and processes that are reliable across a wide range of climatic conditions and are appropriate for use in relatively coarse-scale models (e.g., NRC, 1998b). Among the most urgent questions are the following: What are the patterns of and mechanisms causing prolonged drought on dec-cen time scales?... The hydrologic aspects of an analysis to assess the initiation and persistence of droughts would include a better accounting of the global water balance, of changes in land-surface conditions, and the human use and natural drawdown of the near-surface and deeper reservoirs. A thorough examination of the patterns of past hydrologic variability, and the testing of plausible mechanisms with focused observational and simulation strategies, should lead to a better predictive understanding of this critical climate feature... How do the distribution of water vapor, precipitation, and clouds respond to and interact with surface boundary conditions and changes in forcings on dec-cen time scales? The hydrologic cycle plays a poorly defined role in producing and responding to the patterns of climate variability we have identified in Chapter 3. These patterns and large-scale

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GEWEX—CLIVAR: Coordination of U.S. Activities precipitation seem to co-vary, however, so efforts to use knowledge of climate patterns to predict regional precipitation should be enhanced. There is clearly a need for better documentation and understanding of the nature and sensitivity of these co-varying relationships, and for establishing and refining the mechanisms responsible for driving them. In addition to improving model representations of the hydrologic cycle, we must also better document its temporal variability. The distribution of water vapor in the atmosphere, particularly the vertical distribution, needs to be better understood, because considerable controversy surrounds it. One theory about anthropogenic enhancement of the greenhouse effect suggests that increases in moisture in lower levels of the atmosphere will be offset by decreases in the upper troposphere, reducing surface warming (and its moisture increase) while cooling upper layers. If this offset does indeed occur, it would greatly reduce the net warming that would otherwise be anticipated to result from an increase in total atmospheric water vapor (see, e.g., Lindzen, 1996). As mentioned previously, direct measurements of water vapor in the upper levels of the atmosphere are difficult to make, and the results have been contradictory. Clearly, accurate treatment of upper-level water vapor is essential to realistic modeling of the climate system's radiative response to anthropogenic increases in greenhouse gases (and other external forcing factors), and to reliably estimating the greenhouse-warming response. What combination of remote and in situ observations can be used to measure the large-scale distribution of precipitation and evaporation on dec-cen time scales? Precipitation is vital to nearly all of society's activities, and thus to the global economy. It is also a significant expression of dec-cen variability. While observed co-variations suggest that teleconnections, such as those associated with the NAO and PNA, directly influence land precipitation, measurements that would confirm these relationships are poor, and are being made only sporadically. Existing global observation climatologies significantly disagree among themselves, so that not even a baseline of large-scale evaporation has been established. (A recent GEWEX initiative, the Global Precipitation Climatology Project (GPCP), now provides a baseline for precipitation.) Satellite measurements have now provided a global view of radiative proxies of precipitation, but the calibration needed to translate the proxy fields into precipitation is just starting to become credible, partly because of the GPCP results. In situ measurements are helping to quantify the radiation measurements made by satellites, which will permit absolute calibration of satellite measurements, as well as providing spatial-gradient information. Both the in situ and satellite measurements need to be continued in order to provide optimal estimates of large-scale fields of precipitation. Measuring evaporation is an even more difficult problem, though over the oceans a metric for precipitation-minus-evaporation (P-E) may ultimately be realized through monitoring of surface salinity fields by autonomous platforms. Knowledge of these fields is also critical for determining the ocean thermohaline circulation, as discussed in the next section. What spatial/temporal changes occur in the storage and pathways of land water, including the flux of water to the oceans, over decades and centuries? The buoyancy state of the ocean (defined by temperature and salinity) determines the water-transformation properties of the ocean, the rates of deep- and bottom-water production, and therefore ultimately the transport properties of the ocean and the stability of its internal oscillations (see the next section, “Ocean Circulation ”). Crucial to these processes is the geographic distribution and amount of freshwater inputs--directly, by precipitation over the ocean, and indirectly, by input by runoff, groundwater discharge, and discharge of glaciers and other forms of land snow and

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GEWEX—CLIVAR: Coordination of U.S. Activities ice, either through rivers or through ground discharge. The inputs ' geographic distribution depends on the locations of rivers and their respective flows, and on the amount and distribution of groundwater discharge. River and groundwater flows are determined by the relative amounts of precipitation and evaporation over time in drainage basins. (The detailed paths of water on land, and how to model these pathways on the catchment level, are major themes of the GEWEX program.) When freshwater reaches the ocean, it affects sea level, regional ocean temperature, and chemistry, including salinity and alkalinity. Freshwater and thermal inputs alter the ocean's buoyancy state and water-mass transformation properties, as these inputs are mixed and advected throughout the oceans.” (pgs. 87-89) A10. “In order to develop robust model parameterizations, additional understanding is needed of evapotranspiration, the dynamics of vegetative cover, and the dynamics of soil moisture. Accurate simulation of terrestrial water flow and storage is a difficult modeling problem, particularly because water pathways depend on conditions far more local than climate models can currently resolve or are likely to be able to resolve in the foreseeable future. Better parameterizations of the relationships of water vapor, precipitation, and clouds are required in order to improve their representation in climate models, because their responses and feedbacks are critical to the long-term climate response to changes in forcing and concomitant changes in the fundamental climatic state. Process and larger-scale studies are needed to help us better understand the coupled relationship between the ocean surface's temperature and salinity fields and the overlying P-E fields. Likewise, the interdependencies of soil moisture, soil and vegetative cover characteristics, and P-E must be better understood and represented in models. Improvements in precipitation prediction can likely be made through the use of more comprehensive dynamical coupled models (e.g., that of Stockdale et al., 1998), because precipitation variability and change are so tightly interwoven with the entire climate system... Many hydrological parameters, including the flood potential, reflect an interaction of slow and fast time scales of large-scale climate and regional hydrologic evolution. To advance our knowledge of how regional flood potential may change, a better understanding is needed of how the potential for storms is changed in response to the larger-scale spatial-flow configuration and the local energetics. In addition, a better understanding is required of how the evolving antecedent moisture and baseflow conditions and vegetation on the land surface alter the dynamics of flood generation in a watershed. Long-term hydrological forecasts need to be predicated on the state of the large-scale climate system. This state may be represented through regional or local projections of the phase and amplitude of large-scale space-time oscillatory phenomena. Nonlinear, multivariate time-series modeling approaches accounting for the relationships between atmospheric pressure and precipitation, and between precipitation and flow, may be useful in this regard. However, there is also a question whether purely statistical forecasting approaches will be adequate even if they recognize nonstationary behavior. Forecasts are meaningless without an accompanying understanding of the dynamical processes. Once again, a conceptual framework for the multi-scale evolution of the hydroclimatic state is needed to understand what variables are useful for forecasting and how they should enter into forecasting models, as well as to identify the limits and nature of predictability. Some of the key long-term hydrologic forecasting questions include: Are there regimes that are inherently predictable or unpredictable? How should ensembles of forecasts be constructed? Can it be assumed that hydrologic predictability depends solely on conditions at a given time, not on how they evolved? How should one interpret

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GEWEX—CLIVAR: Coordination of U.S. Activities probabilities--as measures of uncertainty, or as a quantification of the relative local frequency of a climate phenomenon?” (pgs. 90-91) A11. “Related to these oceanic processes and characteristics are the following questions that are central to advancing our understanding of dec-cen climate variability: ... How do freshwater fluxes (which are influenced by evaporation-minus-precipitation, sea ice, and runoff) modulate these processes through the creation of salinity anomalies?” (pgs. 109-110) A12. “Do snow-related changes in surface albedo, surface heat and moisture fluxes, soil moisture, vegetation cover, and cloud formation significantly influence atmospheric patterns or large-scale planetary waves, and thus drive long-term feedbacks in the climate system? For example, do changes in the seasonal or spatial distribution of extensive winter snowfields alter the surface vegetation or soil moisture enough to drive longer-term influences elsewhere in the climate system?” (pg. 125) A13. “How does vegetation influence the transfer of freshwater through the and surface on dec-cen time scales?” (pg. 135) A14. “In order to improve models' abilities to predict dec-cen-scale variability, we need to more realistically parameterize many land-surface processes, such as: interactions between soil and vegetation under various conditions (including frozen soils); surface-atmosphere gas exchange and net uptake (including biogeochemical and physical feedbacks); and the effect of land-surface processes on atmospheric conditions, (including evaporation and precipitation). Clearly our understanding of most of these processes must be improved first.” (pg. 135) A15. “To carry out such a hindcast, long-term observational data for driving the model are essential. The highest priority should be given to: ... (2) Making reliable, long-term observations of a carefully chosen set of basic climatic variables. Important variables include spectra of radiative fluxes at the top of the atmosphere, as well as satellite-observable radiances that are indicators of levels of cloudiness, snow cover, sea ice, vegetation, and possibly soil wetness.” (pg. 154) A16. “Issues related to... common processes and patterns warrant particular attention, and a dec-cen program that is highly coordinated with GOALS and GEWEX would enable them to be studied most effectively. ” (pg. 156)