to maintain. However, access to these external systems can be unreliable, and they often have operating protocols that are not suited to the very long simulations often needed for climate models. In addition, the external centers often have very different priorities for allocating resources to particular proposed models and simulations than just their importance for furthering climate science. Despite its obvious drawbacks, this is a “resource of opportunity” that the climate modeling community should continue to exploit for extreme-scale computing challenges.

To effectively use both forthcoming climate-dedicated computers and more experimental systems of opportunity, the climate community needs to aggressively invest in research into how to design models that achieve maximum performance from such systems (HECRTF, 2004). This problem is not unique to climate science, but the complexity of climate model codes exacerbates this issue considerably, as noted in Chapter 7. The design challenge is complicated by the diverse landscape of possible architectures, but the basic issue is architecture independent—achieving much higher concurrency in climate model codes than is now realizable through code refactoring, compiler tools, new algorithms, etc. This investment leverages off the proposed national software infrastructure, which would facilitate the transfer of software tools and methodologies developed using one model across to other climate models, allowing the community as a whole to navigate hardware transitions more nimbly.

Should the United States Invest in a National Climate Computing Facility?

The committee debated whether the current combination of institution-specific computing and use of external computer resources of opportunity was the best national strategy for climate computing. In particular, we envisioned a national facility dedicated to climate supercomputing (which we will refer to as the National Climate Computing Facility or NCCF) to enable Grand Challenge calculations that have the potential to provide breakthrough scientific results through simulations at spatial resolutions and/or with representations of processes not previously possible. An NCCF is not intended as a new U.S. climate modeling center; rather, it is envisioned to be a central cutting-edge climate computational resource for pioneering calculations that benefit the entire U.S. climate modeling community and explore the next generation of climate modeling capabilities. In this section, we list some advantages and disadvantages of this approach.

Achieving a large positive impact on climate modeling would require a substantial additional national investment in climate computing of $100 million per year or more, in addition to the resources needed to follow the committee’s other recommendations on software infrastructure and research into optimization of climate codes for ex-



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