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CHAPTER FOURTEEN A National Strategy for Advancing Climate Modeling Over the next few decades, climate models and observed trends in both greenhouse gas emissions and diverse climate indicators suggest that global warming and its myr- iad consequences will further unfold and may accelerate. The Arctic Ocean will be a new frontier of shipping and undersea exploration as perennial sea ice disappears. The Greenland and Antarctic ice sheets may respond in surprising ways with surprising speed. Regional droughts in desert margins such as the southwestern United States and the Mediterranean may become more frequent, as may intense flooding events. Large-scale ecosystem changes, associated with pests and disease, may become increasingly hard to ignore, and national and international planning for changes in water resources and agricultural strategy may become essential, challenging the capa- bility of some semi-arid countries to adapt. Pressure for climate engineering “solutions” to delay the consequences of warming will come from diverse quarters. To plan for how to mitigate these changes and to adapt to those that are not forestalled, citizens and policy makers across the United States and around the world will increasingly demand the most accurate global and regional-scale climate projections possible. Over the next two decades, the U.S. climate modeling enterprise will have to evolve substantially to meet national needs and stay internationally competitive. As de- scribed throughout the report, a primary driver for this evolution will be the need to work effectively and increasingly closely with a diverse user community, from design of simulations to choice of outputs, tools for their analysis and distribution, and com- municating uncertainty. Another important driver will be the changing design of su- percomputers. Over the next decade and beyond, individual computer processors or cores are not expected to speed up. Instead, computers will be developed with 107-109 cores, requiring a level of coding parallelism far larger than at present. Past experience suggests more computing power will lead to better and more useful climate simula- tions. However, making high-end climate modeling codes work well in this architec- ture is one Grand Challenge problem, and managing the vast data sets they produce is a second. The lessons learned from previous reports on how to improve the U.S. climate mod- eling enterprise (Chapter 2) emphasize the usefulness of practical recommenda- tions. The large number of specific recommendations that the committee has made 239
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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G throughout this report (Box 14.1) represent stepping stones to a larger strategy, one that emphasizes an evolutionary change in U.S. climate modeling institutions away from developing multiple completely independent models toward a collaborative ap- proach in which different groups pursue different niches or methodologies where sci- entifically justified. The recommendations in this box are not prioritized or weighted. This chapter attempts to summarize these recommendations into a larger strategy, then gives an outlook of the national capability for climate modeling is 10-20 years if this strategy is followed. ELEMENTS OF A NATIONAL STRATEGY FOR ADVANCING CLIMATE MODELING The two principles underlying the committee’s vision for U.S. climate modeling a de- cade hence are that • U.S. climate modeling groups need to work together more closely, while fully engaging the user, academic, and international communities; and • taking full advantage of exascale computing will be critical to progress on both longstanding and new climate science frontiers. As a critical step toward more useful climate models, the committee envisions an evo- lutionary change in U.S. climate modeling institutions away from developing multiple completely independent models toward a collaborative approach. A collaborative approach does not mean only one center of modeling; rather it means that different groups pursue different niches or methodologies where scientifically justified, but within a single common modeling framework. An overarching thread of the commit- tee’s vision is to promote unification of the decentralized U.S. climate modeling en- terprise—across modeling efforts, across a hierarchy of model types, across modeling communities focused on different space and time scales, and across model developers and model output users. The committee recommends a national strategy for advancing the climate modeling enterprise in the next two decades, consisting of four main new components and five supporting elements that, while less novel, are equally important (Figure 14.1). The nation should 1. Evolve to a common national software infrastructure that supports a diverse hierarchy of different models for different purposes, and which supports a vigorous research program aimed at improving the performance of climate models on extreme-scale computing architectures (Recommendations 10.1, 10.2, and 3.2); 240
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A National Strategy for Advancing Climate Modeling 2. Convene an annual climate modeling forum that promotes tighter coordina- tion and more consistent evaluation of U.S. regional and global models, and helps knit together model development and user communities (Recommen- dations 13.1 and 13.2); 3. Nurture a unified weather-climate modeling effort that better exploits the synergies between weather forecasting, data assimilation, and climate model- ing (Recommendation 11.1); and 4. Develop training, accreditation, and continuing education for “climate inter- preters” who will act as a two-way interface between modeling advances and diverse user needs (Recommendation 12.1). The nation should increase efforts to 5. Sustain the availability of state-of-the-art computing systems for climate mod- eling (Recommendation 13.3); 6. Continue to contribute to a strong international climate observing system ca- pable of comprehensively characterizing long-term climate trends and climate variability (Recommendation 5.1); 7. Develop a training and reward system that entices the most talented com- puter and climate scientists into climate model development (Recommenda- tions 7.1 and 7.2); 8. Enhance the national IT infrastructure that supports climate modeling data sharing and distribution (Recommendations 5.3, 10.3, and 10.4); and 9. Pursue advances in climate science and uncertainty research (Recommenda- tions 4.1, 4.2, 4.3, 4.4, and 6.1). If adopted, this strategy provides a path for the United States to move forward into the next generation of climate models to provide the best possible climate information for the nation. VISION FOR U.S. CLIMATE MODEL CAPABILITIES IN 10-20 YEARS Our national strategy positions the U.S. climate community to fully exploit likely ad- vances in computing, allowing our global climate models to be routinely run at 5-10 km resolution in 10 years and 1-5 km resolution within 20 years. Key processes that are currently parameterized (e.g., ocean eddies and atmospheric cumulus cloud systems, including hurricanes) will be explicitly simulated. Mountain ranges and coastlines will be much better represented. The higher grid resolution will allow improved fidelity of all aspects of climate simulation—clouds, precipitation, upper-ocean structure, ex- treme weather events, etc. 241
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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 14.1 SPECIFIC RECOMMENDATIONS FROM THIS REPORT Recommendation 3.1: To address the increasing breadth of issues in climate science, the climate modeling community should vigorously pursue a full spectrum of models and evaluation ap- proaches, including further systematic comparisons of the value added by various downscaling approaches as the resolution of climate model increases. Recommendation 3.2: To support a national linked hierarchy of models, the United States should nurture a common modeling infrastructure and a shared model development process, allowing modeling groups to efficiently share advances while preserving scientific freedom and creativity by fostering model diversity where needed. Recommendation 4.1: As a general guideline, priority should be given to climate modeling activities that have a strong focus on problems that intersect the space where (i) addressing societal needs requires guidance from climate models and (ii) progress is likely, given adequate resources. This does not preclude climate modeling activity focused on basic research questions or “hard problems,” where progress may be difficult (e.g., decadal forecasts) but is intended to allocate efforts strategically. Recommendation 4.2: Within the realm where progress is likely, the climate modeling commu- nity should continue to work intensively on a broad spectrum of climate problems, in particular on longstanding challenges such as climate sensitivity and cloud feedbacks that affect most aspects of climate change (regional hydrologic changes, extremes, sea-level rise, etc.) and require continued or intensified support. Progress can be expected as resolution, physical parameteriza- tions, observational constraints, and modeling strategies improve. Recommendation 4.3: More effort should be put toward coordinated global and regional cli- mate modeling activities to allow good representation of land-surface hydrology and terrestrial vegetation dynamics and to enable improved modeling of the hydrologic cycle and regional water resources, agriculture, and drought forecasts. This will require better integration of the various national climate modeling activities, including groups that focus on models of surface hydrology and vegetation dynamics. The annual climate modeling forum discussed in Chapter 13 might provide a good vehicle for a working group with this focus. Recommendation 4.4: At least one national modeling effort in the next decade should aim to simulate historical and future climate change (i.e., the period 1900-2100) at a resolution of less than 5 km, to enable eddy-resolving models of ocean dynamics and more realistic representation of cumulus convection and land-surface exchanges with the atmosphere. Parallel efforts need to aim for century-scale global atmospheric simulations at 1-2 km, to enable cloud-resolving physics. These national efforts would be facilitated by advances in climate model software infra- structure and computing capability discussed in Chapter 10. Recommendation 5.1: The committee reiterates the statements of previous reports that call on the United States to continue and to augment the support for Earth observations and to address the potential for serious gaps in the space-based observation system. A particular priority should 242
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A National Strategy for Advancing Climate Modeling BOX 14-1 CONTINUED be maintaining fundamental climate-quality observational data sets that have been gathered for 20 years or longer. Recommendation 5.2: To better synthesize the diversity of climate-relevant observations, the United States should establish a national Earth system data assimilation effort that builds from existing efforts and merges weather observations, satellite radiances or retrievals for precipita- tion and various trace constituents, ocean measurements, and land and other observations into the same Earth system model simultaneously. Recommendation 5.3: Building from existing efforts, the United States should develop a na- tional IT infrastructure for Earth system data, so as to facilitate and accelerate data display, visu- alization, and analysis. Recommendation 6.1: Uncertainty is a significant aspect of climate modeling and should be properly addressed by the climate modeling community. To facilitate this, the United States should more vigorously support research on uncertainty, including • nderstanding and quantifying uncertainty in the projection of future climate change, u including how best to use the current observational record across all time scales; • ncorporating uncertainty characterization and quantification more fully in the climate i modeling process; • ommunicating uncertainty to both users of climate model output and decision makers; c and • eveloping deeper understanding on the relationship between uncertainty and decision d making so that climate modeling efforts and characterization of uncertainty are better brought in line with the true needs for decision making. Recommendation 7.1: The United States should attempt to entrain top students into choosing climate model development as a career by providing more graduate and postgraduate training opportunities, enhanced professional recognition and career advancement for participation in climate model development projects, and adequate incentives to attract software engineers who could also choose private-sector careers. Recommendation 7.2: In order to assess future needs on the climate model development workforce, the United States should obtain quantitative information about the workforce needs and required expertise base to support climate modeling. Recommendation 8.1: To advance in the next 10-20 years, U.S. climate modeling efforts should continue to strive for a suitable balance among and support for • he application of current generation models to support climate research activities, as t well as national and international projects such as CMIP/IPCC; • ear-term development activities that lead to incremental but meaningful improvements n in models and their predictions; and • he investment of resources to conduct and capitalize on long-lead-time research that t 243
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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G BOX 14-1 CONTINUED offers the potential for more fundamental and transformational advances in climate modeling. Recommendation 8.2: The United States should continue to support the participation of U.S. scientists and institutions in international activities, such as model intercomparisons, including support for systems to archive model output, because such activities have proven effective in robustly addressing user needs for climate information and for advancing U.S. climate models. Recommendation 8.3: To enhance their robustness, national and regional climate change/ad- aptation assessments should incorporate projections from leading international climate models as well as those developed in the United States. Recommendation 9.1: To better address user needs for short-range climate predictions, the U.S. and international modeling communities should continue to push toward a stronger operational component for prediction of seasonal climate and regular experimental simulation of climate change and variability on decadal time scales. Recommendation 10.1: To promote collaboration and adapt to a rapidly evolving computa- tional environment, the U.S. climate modeling community should work together to establish a common software infrastructure designed to facilitate componentwise interoperability and data exchange across the full hierarchy of global and regional models and model types in the United States. Recommendation 10.2: In order to address the climate data needs of decision makers and other users, the United States should invest in more research aimed at improving the performance of climate models on the highly concurrent computer architectures expected in the next 10- 20 years, and should sustain the availability of state-of-the-art computing systems for climate modeling. Recommendation 10.3: The United States should support transformational research to bring analysis to data rather than the other way around in order to make the projected data volumes useful. Recommendation 10.4: The data-sharing infrastructure for supporting international and na- tional model intercomparisons and other simulations of broad interest—including archiving and distributing model outputs to the research and user communities—is essential for the U.S. 244
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A National Strategy for Advancing Climate Modeling BOX 14-1 CONTINUED climate modeling enterprise and should be supported as an operational backbone for climate research and serving the user community. Recommendation 11.1: To fully exploit a multiscale approach to model advancement, the Unit- ed States should nurture a unified weather-climate prediction system capable of state-of-the-art forecasts from days to decades, climate-quality data assimilation, and Earth system reanalysis. Recommendation 11.2: To reduce sources of uncertainty in climate simulations, the United States should pursue a coordinated research effort to use weather and/or seasonal/interannual hindcast simulations to systematically constrain uncertain parameters and to improve param- eterizations in its major climate models. Recommendation 12.1: To promote the effective application of climate models, the United States should develop climate interpretation certification and continuing education programs to train a cadre of climate interpreters who can facilitate the interpretation of climate model output into usable information for a variety of decision makers and communicate user needs to climate modelers. Recommendation 13.1: To promote communication and collaboration across the climate modeling enterprise, annual U.S. climate modeling forums should be organized to bring together scientists from the global and regional modeling efforts across the United States, scientists from other institutions that are involved in model development and analysis, and model users. Recommendation 13.2: Model intercomparison activities are key to advancing climate models and one activity at the climate modeling forum should be discussion and planning of carefully designed suites of simulations to compare the behavior of U.S. climate models with each other and with observational benchmarks. Regional climate models are a particularly pressing focus for this activity. Such simulations could take advantage of a shared software infrastructure to facilitate comparisons, including on a component basis. Recommendation 13.3: In order to advance climate modeling in the United States in the next 10-20 years, the United States should invest in initiatives that enable the climate modeling com- munity to exploit extreme-scale computing capabilities through the development of new and common software architectures that can be shared across modeling centers and thus spur a national effort to push the computational frontiers of climate science. 245
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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G FIGURE 14.1 Driven by the growing need for climate information, the committee envisions a new gen- eration of climate models that can address a wide spectrum of climate information needs. To achieve this vision and in preparation for the coming transition to radically new computing hardware, the committee recommends a national strategy consisting of four key unifying elements and several other recommendations. In 10-20 years, our global climate models will simulate more ramifications of climate change and variability, such as much more sophisticated modeling of ice sheets and ice margins, and biological responses to climate change in land and ocean. Models of human-climate interaction will be much more sophisticated, better tested, and widely used. A well-documented, nationally organized hierarchy of models will be used for re- search ranging across many space and time scales and turn our diversity of modeling efforts into a more powerful strength. Different modeling groups around the country will specialize in different aspects of the hierarchy or in taking diverse approaches to modeling issues with large scientific uncertainty while sharing both data output standards and, where appropriate, model components. In this collaborative para- 246
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A National Strategy for Advancing Climate Modeling digm, model improvements will rapidly propagate across and between U.S. modeling communities. The grid of some global climate models will nearly reach the local scales at which many users need climate information; interpolation or other simple statistical meth- ods will suffice for many such needs. Climate interpreters using advanced software tools will quickly access and analyze the large, comprehensive, but readily available model data sets to generate needed local-scale information and digest it for end users. Regional climate modeling will still have a place in allowing interactive simulation of additional processes not included in the global model because they require even finer spatial resolution (e.g., ice-sheet calving, estuarine ecosystems) or because they do not feed back substantially on climate (e.g., projection of coastal ecosystems or the climatically viable range of an endangered species or pest). The United States will have an organized process for climate model users and stake- holders to help design new climate model simulations and suggest new directions in climate modeling, centered on a U.S. climate modeling forum. It will also continue to be a strong supporter of a broad-based international effort in climate modeling and the sustained observations that are required both to document climate change and skillfully add new processes into the models. In the United States, research and operational weather, regional climate, and global cli- mate modeling will be done within a common software infrastructure with a set of dy- namical cores and physical parameterizations that work across a broad range of scales. Within a decade, the international climate modeling community will understand whether useful prediction of “decadal” climate variability on time scales of 2-10 years is scientifically viable; if it is, the United States will be a major player in the context of an international collaborative effort. Climate projection uncertainty will remain a big issue. The most important driver of local climate change is global climate change. Uncertainty in projecting local climate change and variability cannot be greatly reduced without reducing uncertainty about the overall rate of global-mean temperature increase. Faster global temperature increase would cause sea ice and ice sheets to melt faster and sea level to rise more and would amplify regional and local precipitation trends due to the changing global hydrologic cycle. Projecting global climate change on multidecadal and longer time scales convolves uncertainties in climate sensitivity and in emissions. The past four decades of climate modeling suggest that both of these uncertainties will remain substantial even 20 years hence. There is hope that climate sensitivity may become somewhat better constrained in the next decade or two by the continuing observa- tional record (if the global climate-observing system is adequately maintained and ad- 247
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A N AT I O N A L S T R AT E G Y F O R A D VA N C I N G C L I M AT E M O D E L I N G vanced), if not by reduced modeling uncertainty. Uncertainty in projection of regional precipitation trends will also remain substantial; we envision gradual progress over the next decade or two as the diverse sources of this uncertainty are all incrementally reduced through model improvements and a longer, higher-quality observational record. A 50 percent reduction in model-related uncertainty in climate sensitivity or precipitation response to a given greenhouse gas change over the next 10-20 years would be an optimistic hope. Climate is complex, multiscale, and multifaceted. Even with the strategic plan we envision, overall improvements in climate models will likely be gradual, not revolution- ary. Nevertheless, they can have huge economic value to the nation, because climate change affects everyone and should be a factor in a myriad of planning decisions around the country. FINAL COMMENTS Climate models are among the most sophisticated simulation tools developed by mankind, and the “what-if” questions we are asking of them involve a mind-boggling number of connected systems. As the scope of climate models has expanded, so has the need to validate and improve them. Enormous progress has been made in the past several decades in improving the utility and robustness of climate models, but more is needed to meet the growing needs of decision makers who are increasingly relying on the information from climate models. The committee believes that the best path forward is a strategy centered around the integration of the decentralized U.S. climate modeling enterprise—across modeling efforts, across a hierarchy of model types, across modeling communities focused on different space and time scales, and between model developers and model output users. A diversity of approaches is necessary for progress in many areas of climate modeling and vital for addressing the breadth of users needs. If adopted, this strategy of increased unification amidst diversity will allow the United States to more effec- tively and efficiently utilize that diversity to meet the climate information needs of the nation in the coming decades and beyond. 248