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In response to the President's CCRI directive, the following research strategy options are recommended for consideration. The summary in the remainder of this document follows the outline of the three categories of NAS recommendations, identified above as A, B and C.

We recommend high priority studies to address the most important unknowns in climate forcing including studies of the chemical and radiative properties of aerosols and in climate feedbacks, including studies of the role of clouds, water vapor, precipitation and evaporation, and aerosols in feedback processes. Global climate models used for prediction of future climate are also sensitive to emissions scenarios. We recommend the development of new emissions scenarios that recognize the importance of aerosols and greenhouse gases other than carbon dioxide and methane. In order to discern climate change due to human activity from natural variability, we recommend studies that will extend and improve predictions of natural variability. This will also substantially improve inference from climate models.

Aerosols and tropospheric ozone play unique, but poorly quantified, roles in the atmospheric radiation budget. This occurs, in part, because both aerosols and tropospheric ozone are short-lived and have spatially and temporally heterogeneous distributions. The role of aerosol particle radiative forcing is complex and current understanding of the problem is fragmented. There are two distinct, but related effects of aerosols. The first is the direct effect of particles on the radiation budget of the planet; the second is the effect of changing particle concentrations on cloud properties (the indirect effect) and the subsequent effect of those clouds on the radiation budget.

Research Objectives

• Develop global aerosol climatology with improved representation of regional distribution of aerosols by major type and radiative properties. Establish the climatic importance of radiative-forcing from aerosols that absorb solar radiation, such as carbonaceous and mineral dust aerosols.

• Determine the linkages between ozone and aerosol sources and sinks to global distributions, and therefore to radiative forcing, in order to understand regional impacts of current and projected concentrations.

Proposed Strategy

• Establish new and augment existing in-situ monitoring sites, including aircraft sampling, in and downwind of major population areas (e.g. Asia, Eastern North America and South America) to establish temporal and spatial distributions, trends, and aerosol chemical and radiative properties.

• Integrate the available remote sensing (satellite and ground-based) and in situ data into a single product of known and uniform data quality.

• Develop integrated chemistry, aerosol and climate models that combine basic ozone and aerosol chemistry and physics with transport and removal. Integrated models can be used to study regional patterns, evaluate our understanding of source and sink processes, and project future distributions

• Conduct focused field campaigns and/or appropriate laboratory experimentation to experimentally assess the importance of aerosol concentration and composition on cloud microphysical and radiation properties and on precipitation.

• Develop cloud models with comprehensive aerosol-cloud microphysics to determine areas of uncertainty and those processes that are most critical to the alteration of cloud properties.

Ongoing plans and activities

The specific observations and research proposed here will complement the more comprehensive atmospheric chemistry studies proposed as part of the ongoing USGCRP work plan. For example, the USGCRP plan emphasizes the importance of characterizing the distribution of all major aerosol species and their variability through time, the separate contribution of aerosols from various human activities, and the processes by which the separate contributions are linked to global distribution.

In the past few years, several major field experiments in the Indian Ocean, Asia, and Africa have been carried out primarily to address the direct effect aerosols play through the scattering of radiation. These experiments will continue to contribute to our understanding of aerosol processes in the atmosphere and aerosol forcing of the climate. Similarly, the MODIS, MISR and OMI instruments on NASA satellites are providing a vastly improved assessment of the global aerosol. The new AQUA satellite will also have a MODIS instrument. At present, NPOESS instrument and algorithm development includes observing the refractive index and size distribution of aerosols, and through those, inferred measurement of the chemical composition of aerosols. These measurements can be acquired as early as the NPP mission, and together with vertical profile aerosol distributions data from EESP-3 CENA will constitute an unprecedented observational record, capable of addressing the indirect effect of aerosols. Work funded under the GACP is synthesizing observations made in several programs and also linking models and observations.

Deliverables

• Identification and evaluation of role of aerosols that absorb solar radiation such as black carbon and mineral dust

• Evaluation of the regional impact of current and projected concentrations of ozone and aerosols on climate

b. Understand future emissions of radiatively active gases and aerosols

The discussion of climate forcing and feedback in the preceding paragraphs illustrates that the range of human system activities that are relevant for understanding future climate is much wider than fossil fuel use and methane emissions. In addition to the emissions of the usual greenhouse gases it is important to understand direct emissions of particulates. These may include black carbon/soot emissions from biomass burning, aerosol precursors such as SO₂, and various forms of nitrogen emissions that may affect biological productivity. Moreover, deforestation affects transpiration, surface roughness, and runoff.

Proposed Strategy

• Extend the control strategy projections applied to SO₂ emissions in the recent IPCC scenarios to all relevant local environmental pollutants.

• Improve the modeling of agricultural activities, including long-term growth in productivity and its impact on land use and conversion, and the ability to provide sufficient nutrition to developing countries.

• Improve population forecasts, including the important determinants of total completed fertility, death rates, and migration. These determinants must reflect an integration of the demographics models and socio-economic models.

• Revisit with care the determinants of long-term demands for food and energy, the two economic activities most responsible for greenhouse gas emissions.

• Develop tools to provide more disaggregated results for relevant emissions and levels of economic activity.

Ongoing plans and activities

The current set of IPCC scenarios includes population forecasts that need updating, confusing combinations of business as usual and policy intervention scenarios, and perhaps unwarranted optimism about economic growth in much of the developing world. Using improved, integrated and more comprehensive assessment models to develop a revised set of emissions scenarios would be a very helpful step.

Deliverables

Research objectives for carbon cycle science in the next decade include combinations of modeling, inventory, observations, process research, and assessment, integrated according to topic areas that represent some of the field's greatest areas of uncertainty. Scientific understanding of the carbon cycle has now advanced to the point where a small number of targeted investments can yield major returns in five years. The proposed investments in carbon cycle research will provide decision-makers, resource managers and the public with solid, quantitative information on the role of the U.S. as both a source and a sink for carbon. Policy makers and resource managers will have useful assessments of the potential of U.S. forests, soils, and coastal systems for carbon sequestration and the first reliable estimates of the time scales over which these managed sinks could be maintained. Decision support tools will be available to explore impacts of energy policies, land use policies, and climate change policies on management options. Resource managers will have more efficient and reliable methods for inventorying forests, rangelands, and croplands and assessing the impact of various management practices on crop yields, timber volume, and soil fertility.

Error budgets in the global carbon balance will be significantly reduced, and policy makers will have a better understanding of where the global hot spots of carbon uptake and release are located. It is unlikely that this information will be resolved at a national scale -- except for very large nations -- but it will be useful for international negotiations and identifying regions where mitigation activities are most needed or would have the most impact. Similarly, projections of climate change and the scenarios used to inform assessments will be improved, and additional insight into the societal risks of climate change and human efforts to mitigate climate change will be derived.

Research Objectives

• Quantify the North American region's carbon sources and sinks, describe the natural and human system processes controlling changes in them, and document North America's contribution to the Northern Hemisphere carbon sink.

• Reduce uncertainties in regional patterns of carbon sources and sinks on a global scale.

• Quantify the main mechanisms resulting in sub-decadal variability in natural fluxes of carbon between ocean and atmosphere, and land and atmosphere.

• Quantify the role of land management and land use practices on storage of carbon.

• Assess options for managing carbon in the environment, their effectiveness, and their impacts on the environment and human activities for purposes of mitigating climate change.

• Provide more realistic model projections of future atmospheric carbon dioxide concentrations and scenarios of future climate change.

Proposed Strategy

• Integrated North American Carbon Study -- An intensive focus on North American land and adjacent ocean basin carbon sources and sinks, to improve monitoring techniques, reconcile approaches for documenting carbon storage, and elucidate key controlling processes and land management regimes regulating carbon fluxes from the land and ocean.

• Carbon Observations in Under-Sampled Areas -- A focus on augmenting observations of globally relevant parameters in key under-sampled oceanic and continental regions around the globe, selected to reduce high uncertainty in current flux estimates (such as the Southern Ocean, continental tropics and boreal regions). Required to achieve deliverable: Multiple Constraint Modeling and Data Assimilation to develop and support framework to utilize carbon-related measurements in the most efficient and systematic way

• Process Studies and Manipulative Experiments -- Terrestrial and oceanic process studies with changing environmental factors such as elevated CO₂, enhanced nutrients, water stress, temperature, manipulated to discover managed and unmanaged ecosystem sensitivity to potential global changes.

• Carbon Management Strategies -- Empirical, process and modeling studies to elucidate the potential decision pathways and develop systems for managing carbon sources, such as fossil fuel consumption, and carbon sinks, such as agricultural and forest management.

• Dynamic carbon-climate projections --Improvement in model parameterizations, incorporation of human decision pathways, and coupling of dynamic carbon cycle models to global climate models to more realistically project future CO₂ and related greenhouses gases.

Ongoing plans and activities

Significant investments over the past decade have resulted in an unprecedented opportunity to study the carbon cycle over a scale not previously attempted-- the continental/basin scale. Observational resources such as the USDA forest and soil inventory, Ameriflux, CMDL tall towers, Atlantic and Pacific ocean time series and ships of opportunity, vegetation and ocean color remote sensing have all contributed to a better understanding of the components of the carbon cycle over North America and adjacent basins. Building on our existing observational and research activities, we recommend enhancement of sensors on existing networks, augmentation of sites in networks in some cases, new observations to connect the scales of activity, and innovative new diagnostic modeling frameworks to ensure that data are being used in the most efficient manner to constrain regional patterns of sources and sinks.

To achieve adequate resolution of global carbon sources and sinks on a regional scale in the next five years, we will build on our pilot capabilities to observe carbon storage in land and ocean and exchanges with the atmosphere. New observational locations will be deployed as determined by optimal sampling schemes and requirements for model improvement. A number of pioneering efforts have successfully been launched in the past decade to study the impact of rising carbon dioxide on the growth and flows of carbon in a variety of ecosystems, including large forest areas and savanna ecosystems. In addition, great strides have been made in understanding key processes limiting phytoplankton growth in the ocean, which is a factor in uptake of carbon dioxide by the ocean. Building on this work in the next five years we will quantify ecosystem respiration on land, and study the responses of ecosystems to multiple stresses.

Carbon management is a relatively new topic but an increasingly relevant one as nations consider options for responding to climate change. The next critical step in understanding managed ecosystems is understanding the role that agricultural and forestry management practices play in carbon storage, and how multiple changes in environmental factors may affect carbon storage, pest distributions, and crop production. The recommended priorities target steps for continued improvement on the investments of the past few years.

Research in the past decade has focused on improving dynamic models of the components of the carbon cycle. A quantum leap forward is possible by coupling dynamic component models to existing and improved general circulation models.

Efforts to improve the measurement and quantification of carbon storage and fluxes will be aided by new and emerging technologies. Routine quantification of changes in carbon sources and sinks around the world will require new remote sensing capabilities for measuring atmospheric carbon dioxide and estimating biomass.

• An analysis and quantification of regional carbon sources and sinks and prospects for carbon management in U.S. managed systems for the 1/3 of the Northern Hemisphere centered on North America -- land, ocean, atmosphere, and human system;

• Estimates of carbon flux strength in currently highly uncertain regions, thereby reducing uncertainty both for regional budgets of carbon flux as well as global estimates of uptake, with the ability to reveal key gaps in our process understanding and observational system necessary for improving dynamic prediction models.

• Identification of critical potential feedbacks in the regulation of carbon storage and fluxes by the land and ocean ecosystem, including sensitivities to climate, species distribution shifts, and circulation pattern changes.

• Analysis of options available to decision-makers for management of the carbon system, assessment of on-going management practices, and development of management systems for deployment by landowners.

• Improved, more realistic climate change scenarios from prognostic models projecting future atmospheric greenhouse gas concentrations and carbon-climate interactions and feedbacks.

Water plays a key role in the radiative balance of the atmosphere: in the vapor phase, it is the most important of the so-called "greenhouse gases"; in condensed phase (both liquid and ice clouds) it affects both vertical heating profiles and geographic heating patterns. Results from climate models suggest there will be an increase in water vapor as the climate warms. Water vapor is a natural greenhouse gas and its increase produces a positive feedback that approximately doubles the warming due to increased concentrations of anthropogenic greenhouse gases. Predictions of global warming vary in large part because of differences in the way that the various feedback processes are represented in the models. It is ironic that among the uncertainties regarding feedbacks in the climate system, those associated with the representation of water vapor, the most important greenhouse gas, and cloud processes are the greatest. For example, it is not known how the amount and distribution of clouds will change, both vertically and horizontally, as the water vapor in the atmosphere increases. More importantly, we do not know the impact on climate of the associated changes in radiative forcing and precipitation. The feedbacks could be positive or negative. Another related uncertainty is associated with the detrainment of moisture from clouds as a function of height. This is particularly important in the tropical upper troposphere where the radiative impact of water vapor (or ice crystals) could be substantial and significant questions remain regarding the nature of troposphere and stratosphere exchange.

Basic understanding of the processes that control the atmospheric water vapor and clouds must be improved and incorporated into models. Better representation of the distribution of water vapor is critical given its contribution to temperature increases as an active radiative gas as well as its role in cloud formation. Because the physical processes responsible for the transport of water vapor or cloud formation occur at scales that are not resolved by climate models, they must be parameterized. Reducing the uncertainties due to the representation of cloud and water vapor in climate models will require better (three-dimensional) observations, targeted process studies, and model improvements.

Research Objectives

• Improve understanding of water vapor and water condensate concentrations and source and sink processes in the upper troposphere and lower stratosphere, particular in the tropics

• Provide a framework in which cloud parameterizations can be evaluated rigorously against atmospheric data and cloud model sensitivities can be assessed

Proposed Strategy

• Combined in situ and remote sensing and process studies of the injection of water vapor in the upper troposphere by convection and studies of stratospheric-tropospheric exchange in the tropics

• Analysis of detailed, quantitative, three-dimensional data on cloud amount and height, phase and water content, dynamics, cloud radiation and precipitation processes for a variety of synoptic regimes using a combination of ground-based and satellite remote sensing

• Tests of cloud parameterizations in the framework of process-resolving cloud ensemble models

• Tests of cloud parameterizations against observations in the framework of operational regional or global atmospheric circulation models

• Tests of climate model sensitivity to three-dimensional cloud representation employing cloud-resolving models

Ongoing plans and activities

Research on water and clouds will have to be closely linked to investigations of aerosols. A major source of uncertainty related to clouds is the indirect effect of aerosols that serve as the condensation nuclei for cloud droplets. Aerosols can affect the brightness (albedo) of clouds as well as cloud thickness, lifetimes and precipitation characteristics.

While the studies that we describe here will substantially improve our understanding of feedbacks, other studies proposed as part of "A Plan for a New Initiative on the Global Water Cycle" and the USGCRP Strategic Plan will be critical to predicting the impact of climate change on precipitation and water availability, for example, determining long term trends in the global water cycle including the character of hydrologic events and their causes; developing the ability to bridge climate and weather modeling; and determining the relationship between the water cycle and biogeochemical/ecological processes.

Deliverables

New, observationally-tested cloud parameterizations for global climate models

It has been argued that Polar Regions, especially the Arctic, are the most sensitive areas for detecting global change and have great potential for causing abrupt climate change. Modeling studies indicate that under a representative global warming scenario, temperature increases will be amplified in the Arctic due to feedbacks involving the snow and ice cover that, in turn, feedback to global climate.

First, the Arctic Ocean's stratification and ice cover provide a control on the surface heat and mass budgets of the north polar region, and therefore on the global heat sink. If the distribution of Arctic sea ice were substantially different from the present, the altered surface fluxes would affect both the atmosphere and the ocean and would likely have significant consequences for regional and global climate.

Second, the export of low-salinity waters out of the Arctic Ocean, whether in the form of liquid or desalinated sea ice, has the potential to influence the overturning cell of the global ocean through control of convection in the subpolar gyres. Third, Arctic soils serve as significant reservoirs of carbon dioxide and methane and warming of the region could result in increased atmospheric emissions of these greenhouse gases.

For the past decade or more, climate-driven environmental changes in the Arctic have been severe, and there is a strong possibility that these changes will continue into the future and cause consequences throughout the Northern Hemisphere. Regardless of the causes for these changes, the decade-long warming of the Arctic also has the potential for resulting in abrupt climate change associated with rapid melting of much of the Arctic sea ice or extensive thawing of permafrost and release of greenhouse gases. Recent measurements of sea ice thickness in some parts of the Arctic show that it is only 40% of the thickness 50 years ago. Continued thinning could lead to rapid melting and breakup of the ice. Longer and deeper permafrost thaw could release large quantities of CO₂ and methane. The greatest uncertainties in our ability to judge the likelihood of these two scenarios are, respectively, the restricted area of available sea ice thickness measurements and the poor coverage of permafrost stations.

In the Antarctic, the West Antarctic ice sheet (WAIS) is the only remaining marine ice sheet from the last glacial period. The increasing number of major tabular icebergs that calve from the ice sheet has led to concern that the ice sheet may be susceptible to runaway "grounding line retreat", leading to rapid disintegration. This would result in rapid sea level rise. Were the WAIS to completely melt, the water released is sufficient to raise global sea level by 5-6 meters. The likelihood of such a scenario, and the ice sheet's sensitivity to climate forcing are topics of ongoing research. The grounding line position and net balance are key parameters for observing ice sheet changes. Those data, when combined with accurate basal environment information, are critical for supporting model-driven hypotheses concerning the response of the WAIS to climate change and sea level change.

The Amundsen Sea Embayment (Pine Island Bay/Thwaites Glacier area) is the only major West Antarctic drainage not buttressed by a large ice shelf and thus is the drainage most likely to participate in a collapse. Recent observations using interferometric SAR and repeat satellite altimetry show a speed-up, thinning, and grounding-line retreat of ice flowing into Pine Island Bay. Model studies focused on this drainage are needed to assess the possibility that the ongoing thinning will lead to retreat from a prominent bedrock sill, which in turn might trigger major changes in the ice sheet, contributing to sea level. However, available radar data do not provide sufficient geometric detail or information on internal layers and basal conditions to allow confident modeling.

Research Objectives

• Measure sea ice thickness in the Arctic ice margin environment for five years to determine whether thinning observed in the central Arctic is present across the entire basin;

• Measure permafrost temperature and thaw patterns in sufficient detail for five years to establish regional thaw patterns;

• Establish the mass balance and ice dynamic regime of the Pine Island/Thwaites drainage system of the West Antarctic ice sheet; and

• Quantify the boundary conditions of the Pine Island/Thwaites drainage system in a fashion suitable for 3-D ice sheet modeling and develop atmosphere/ocean/ice models to assess the likely stability of this part of the ice sheet.

Proposed Strategy

• Expand the network of ice-ocean buoys, especially in the Arctic ice margin, and surface observations to track changes in ocean/ice/lower-atmosphere temperature, ocean salinity and circulation, and sea ice thickness;

• Expand the arctic-wide network of permafrost stations and boreholes to sufficient numbers to determine regional thaw patterns that reflect the pace of climate change;

• Expand the network of automatic weather stations over the West Antarctic ice sheet to include the Pine Island/Thwaites ice drainage system and establish a program of mass balance studies (accumulation stakes, shallow snow pit and ice core studies, short-pulse ground penetrating radar measurements to identify near surface layering) and ice dynamics measurements (e.g. ice velocity and surface elevation- using both satellite data and ground based measurements); and

• Collect geophysical data (surface and bed elevation, ice thickness, bed roughness, internal layering, gravity anomalies, magnetic anomalies) to provide a better picture of the subglacial topography, using airborne remote sensing techniques. These data will provide input for a new generation of coupled ice sheet- ocean-atmosphere models.

Ongoing plans and activities

• Enhanced meteorological observations are also a key to reducing uncertainties in the response of the Polar Regions to climate change and are addressed in the Climate Observing System section of the Climate Change Research Initiative. Proposed integrated Arctic science studies, such as SEARCH, will be critical to putting the measurements proposed here into a larger context.

Deliverables

• An assessment of the likelihood of large-scale polar sea ice thinning or of an ice-free Arctic;

• An assessment of the likelihood of large-scale release of greenhouse gases presently sequestered in permafrost; and

• An assessment of the likelihood of West Antarctic Ice Sheet collapse.

Incorporating improved understanding of mechanisms producing different patterns of climate variability into climate models provides the potential to extend and improve predictions of climate variations and their regional impacts. ENSO is known to induce major impacts on weather and climate extremes such as hurricanes, floods, and droughts in many regions of the globe. One of the major accomplishments of recent climate research has been the successful application of ENSO predictions several seasons in advance; however, while the models have demonstrated some skill in predicting ENSO patterns in the tropical Pacific, they cannot predict the remote, non-local, impacts.

Scientists have also identified other important patterns of natural climate variability such as the North Atlantic Oscillation (NAO)/ Arctic Oscillation (AO), and the Pacific Decadal Oscillation (PDO). These modes involve both internal and coupled atmospheric and ocean dynamics as critical components, and models differ in their ability to simulate them. For example, it is still an open question whether NAO is an uncoupled atmospheric process or a process coupled to the ocean. All of the modes influence weather systems substantially. We do not yet know to what extent these natural modes of climate variability are predictable, nor how a changing climate will affect them, or the impacts such changes might have on regional climate, extreme weather events, or the potential for abrupt global climate change. One of the major challenges for global climate models is the accurate characterization of the multiple processes of the ocean and atmosphere that are important for predicting changes in climate.

Observations of current and past climates will play an important role in improving the characterization of physical processes in the ocean, atmosphere, land surface and cryosphere, and in validation of climate models. The need for refining, extending (both backwards and forwards), and analyzing long-term observational records to better discriminate natural climate variability from global change is self-evident. Field programs and modeling experiments will be used in identifying key physical processes and regions that need to be better observed and monitored, which will help in designing the future climate observing system.

Changes in ocean circulations, such as a suppression of the large scale thermohaline "conveyer belt" that transports heat, would greatly impact the natural climate of the U.S. Gulf states, the North Atlantic, and western Europe. Paleoclimate data suggests the thermohaline circulation of the North Atlantic can shut off over a matter of decades resulting in significant shifts in the distribution of surface temperature, rainfall and storms. The response of the thermohaline circulation under a changing climate is a major issue with vast potential consequences, including the possibility of abrupt climate transitions, such as those observed in the historical and geologic past. Modeling studies indicate that a potential response to changes in global temperature and the melting of high-latitude ice is a rapid reorganization of the three-dimensional ocean circulation. A first step is to develop methods to determine the likelihood of abrupt climate change and to identify critical indicators/evidence needed to assess whether we are currently experiencing an abrupt change.

Research Objectives

• Simulate with climate models the weather and climate extremes observed as a result of the ENSO cycle.

• Determine the mechanisms controlling variability of the high air-sea heat flux regions adjacent to subtropical western boundary currents by a field experiment coordinated with high-resolution ocean modeling

• Characterize the temporal variability and spatial structure of the Indonesian Through-flow, the primary link between the western Pacific and eastern Indian Oceans, a poorly understood component of the meridional overturning circulation, and a potential influence on the Asian monsoon.

• Test competing hypotheses forcing the NAO/AO, e.g., tropical and/or midlatitude SST, stratospheric dynamics, sea ice, by means of modeling and empirical studies.

• Develop a modeling strategy for resolving weather phenomena in climate change projections by combining high-resolution global or regional fine-mesh models with coarser representations suitable for long-term integration.

• Develop methods for determining the likelihood of abrupt climate change, such as the collapse of the ocean thermohaline circulation or the loss of the polar ice cap, and the expected global and regional manifestations of such changes.

Proposed Strategy

• Analyze and clarify connections between observed ENSO variations and the geographic distribution, frequency, and intensity of weather systems (intense mesoscale systems) through diagnostic studies of global and regional data.

• Develop a modeling strategy for incorporating interactive simulations of ENSO and weather systems dynamics, at the required spatial resolution, by alternatively switching from relatively coarse resolution to mesoscale-resolving spatial grids, or the use of embedded regional fine-mesh models.

• Deploy Lagrangian and Eulerian instrumentation in high flux regions of the ocean, together with ship-based surveys, acoustic thermometry and a buoy-based meteorological observing array. Exemplars include, the regions near the Kuroshio Extension in the N. Pacific, the Gulf Stream Extension and North Atlantic Current in the N. Atlantic and the Brazil/Malvinas Confluence in the S. Atlantic.

• Conduct simultaneous measurements of flow through the northern Indonesian passages, flow through southern Indonesian passages, and the hydrography of the Banda Sea and analysis with contemporaneous data about seasonal atmospheric and oceanic patterns in the western Indian Ocean and in the western Pacific. Ideally this should be an international effort and include scientists from the U.S. and Indonesia.

• Conduct a focused field experiment, using combined Eulerian and Lagrangian instrumentation, to understand processes that modulate the air-sea heat flux in the NAO region.

• Conduct a hierarchy of sensitivity experiments to determine the relative role of tropical, mid-latitude SST and sea-ice in climate variability due to NAO/AO.

• Couple a high resolution climate model with a stratospheric model of high enough resolution and complete enough physics to represent the upper atmospheric dynamic processes believed to drive the NAO/AO.

• Conduct a series of model experiments designed to improve the coupled model physics, leading to improved simulation of the NAO/AO.

• Apply models to predict future climates, with and without human-induced forcing, to examine the impacts of the forcing on the characteristics and behavior of the NAO/AO.

• Conduct an intercomparison / evaluation of the various techniques for "downscaling" information from coarse resolution century integrations to the local and regional levels. Use the ENSO cycle as the "large-scale" climate forcing.

• Based on the formalized evaluation, apply the best technique(s) for the next round of assessments of how climatic extremes are likely to change over the next century and the potential impact on the United States in terms of hurricanes, floods and droughts.

• Organize paleoclimate, contemporary climate, and related data and information about past abrupt climate changes

• Identify critical indicators/evidence needed to assess whether we are currently experiencing a climate transition.

Ongoing plans and activities

The research foci discussed above address the most important questions about natural climate variations and their possible modification by climate change because of their significance as sources of climate prediction uncertainty. All are research efforts for which targeted resources will produce scientific results over the next five-years that will impact significantly the reduction of those uncertainties, although for several, additional research will be required.

The scientific strategy to attain these objectives requires long-term base-funded observation and research activities, including: theoretical and modeling studies using established dynamical constraints to project the behavior of the climate system; basic research tools, such as a hierarchy of process models, diagnostic analysis of climate information, long time series of in situ and space-based observations of weather and climate variables, clouds, precipitation, and ocean circulation; and regional studies of current climate change impacts. Current base-funded support for these activities comprises elements of a number of ongoing and planned interagency activities such as: Global Water Cycle/GEWEX, CLIVAR (Climate Variability), GLOBEC (global change and marine ecosystems), ESH (Earth System History), SEARCH, etc. This work will also be enhanced by analysis and synthesis of existing data sets from large field experiments completed during the 1990s and existing satellite data sets.

Deliverables

• Predictions with significantly reduced uncertainty of (i) ENSO cycle, including associated extreme weather events within the US and around the world; and (ii) climate variability associated with major modes of atmospheric/oceanic oscillations, such as the Pacific Decadal Oscillation and the Arctic Oscillation/North Atlantic Oscillation, and the changes that may be expected under climate change.

• Capability for downscaling climate information to regional spatial scales, in order to produce probabilistic estimates of change in the distribution, frequency and intensity of extreme weather events that may result from natural variability and human influences on climate.

• Assessment of the scope for possible abrupt climate transition or change that could result from the non-linear (chaotic?) interplay between ocean, atmosphere, land surface, and ice.

Computer simulation is one of the most important components of a comprehensive climate research program. Because the Earth system cannot be isolated and studied in a physical laboratory, models are an essential tool for synthesizing observations and theory to investigate how the system works and how it is affected by human activities. The continued development and refinement of computational models that can simulate the past and future conditions of the Earth system is crucial for developing capabilities to provide more accurate projections of future change. Comprehensive climate models represent the major components of the climate system (atmosphere, oceans, land surface, cryosphere) and the transfer of water and energy among them. Current research has demonstrated that more accurate simulation and prediction that reduces uncertainty about projected future change requires the inclusion of more process, particularly the biological and chemical processes that influence the atmospheric concentrations of carbon. Accordingly, future projections must be made with Earth system models that simulate more than the climate alone.

The four principal US agencies that support climate model development and application commissioned the National Research Council (NRC) to analyze US modeling efforts as well as to suggest ways that the agencies could further develop the US program so that the need for state-of-the-art model products can be satisfied. These reports, The Capacity of US Climate Modeling to Support Climate Change Assessment Activities (1998) and Improving the Effectiveness of U.S. Climate Modeling (2001), provide valuable guidance on how to improve US climate modeling efforts. Also, an Ad Hoc Working Group on Climate Modeling charged by the Subcommittee on Global Change Research (SGCR) prepared the report, High-End Climate Science: Development of Modeling and Related Computing Capabilities in late 2000.

Programmatic Objectives

• Capitalize on the strong US basic research enterprise while simultaneously producing the routine and on-demand products demanded by the impacts, assessment and policy communities. Diverse basic research, performed principally by single investigators or small groups in academia, is the primary source of new basic knowledge about the climate and Earth system. Provide a venue for these researchers to interact with a high-end modeling center that can make use of new information; and

• Develop the capability to produce routine and on-demand high-end climate model simulations and projections required for other communities.

Proposed Strategy

• Establish two complementary high-end modeling centers to address the promise and problems identified above. The first, a high-end research center, would be charged with developing an open and accessible modeling system that integrates basic knowledge from the broad, multi-disciplinary basic research community. The second research center would compliment the first by focusing on model product generation for research, assessment and policy applications as its principal activity;

• Establish and maintain links between the two centers so that the operational needs can be communicated to the research community and the research progress can be transferred into the operational products. A joint Advisory Committee will be established to provide the centers with advice and recommendations. A visiting scientist exchange program will be developed to promote cooperation in all aspects of the center's research such as model framework development, the adoption of standards, the joint evaluation of models and physical parameterization schemes;

• Focus efforts on software engineering with the development of modeling frameworks to improve the compatibility and portability of model codes, thus ensuring that software advances can be more easily shared among centers and laboratories; and

Ongoing plans and activities

In response to the reports, four agencies initiated activities to strengthen the U.S. modeling infrastructure and address several of the high-priority needs that the NRC and Ad Hoc committees identified.

• The Community Climate System Model (CCSM) project is dedicated to the development of a state of the art climate system model for research into climate processes and climate applications. While the CCSM core project teamed is located at NCAR, the CCSM was established through the self-organization of the climate modeling research community with the encouragement of the Federal agencies responsible for modeling. Soon after it was established, CCSM became a magnet for a wide range of both model development and model application research. The model currently includes components that represent the atmosphere, ocean, land and sea-ice physical processes. Current research includes addition of both marine and terrestrial biogeochemical processes to the system. Additional plans are to include atmospheric chemical processes to model changes in greenhouse gases and aerosols. Computational resources for the CCSM activity are available through the Climate Simulation Laboratory at NCAR and additional resources made available through DOE. The development process of the CCSM is governed by a Scientific Steering Committee, which is composed members, from both inside and outside of NCAR and four from the university community. The SSC receives advice from the CCSM Advisory Board, which is composed of members from universities, national laboratories and NCAR.

• The Operational Research Center's core component is NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) at Princeton, New Jersey. GFDL has played a central role in climate research. Much of the pioneering work in climate change, stratospheric modeling, seasonal forecasting, ocean modeling and data assimilation, and hurricane modeling was conducted there. This core research capability will be enhanced to enable product generation and policy related research. Additional research capabilities in carbon cycle, water resources, atmospheric chemistry, paleoclimate, oceans and climate, and integrated assessment modeling will be leveraged through collaborations with GFDL's existing partnerships with Princeton University and Columbia University's Lamont-Doherty Earth Observatory.

• Interagency investigations of the suitability of distributed memory, high-end computers for climate modeling are now underway. (DOE SciDAC, NASA CAN, NSF ITR, NOAA HPCC).

Deliverables

• The venue to provide for interactions within the climate research community is through the proposed research-oriented modeling center; this operates in a way that is suited to the culture of the small-project investigator. Although aligned with the basic research community, the research center will be product-driven in that it will develop, maintain and distribute a state-of-the-art modeling system.

• The operational center will fill this role as the center of a climate modeling service. It will grow out of NOAA's charter to provide the US with prediction products. Although its mission is operational, experience at the ECMWF has shown that high-quality operational centers require a significant in-house research activity that can collaborate and interact with the external research community and transfer knowledge into the center. It will utilize NOAA's delivery mechanisms for climate information that already have been established, e.g. the NWS, the IRI, the RISA program. It will also build new links to the policy community and to the private sector involved in carbon mitigation strategies and long range economic planning.