SciDAC Review - Science Accomplishments: Climate: Predictions from Improved Models

SCIENCE ACCOMPLISHMENTS

Climate: Predictions from Improved Models

The Community Climate System Model (CCSM) is a coupled climate model, consisting of four components for the Earth's atmosphere, ocean, sea ice, and land surface. Each model component communicates its surface fields and fluxes to the other components through a flux coupler. The CCSM is used for climate change research and to provide periodic Intergovernmental Panel on Climate Change (IPCC) climate assessment reports.

The SciDAC project, "Collaborative Design and Development of the Community Climate System Model (CCSM) for Terascale Computers," was launched to accelerate the development of the CCSM. Over the past five years, six DOE laboratories and researchers from the National Center for Atmospheric Research (NCAR) and the National Oceanic and Atmospheric Administration (NOAA) have worked together on a variety of improvements to the CCSM. These improvements include advancements in software design and engineering, the introduction of new numerical algorithms, and the addition of new physical processes to the CCSM models.

The early stages of this project focused on improving the performance of the CCSM and its portability to a wide variety of vector and scalar computer systems. Flexible data decomposition schemes in the atmosphere and ocean models were introduced to enable the necessary fine-tuning for each platform. The sea ice and land model components were also restructured to improve their performance, particularly on vector computers. Finally, new software was developed to improve the coupling of all four components and produce a fully coupled model. Thanks to these improvements, CCSM scientists were able to produce the world's largest ensemble of climate simulations for the recent IPCC assessment. This ensemble contains over 10,000 years of data simulated at relatively high resolution.

Figure 12. Sources (red) and sinks (green) of atmospheric CO₂ from ocean ecosystems as simulated by a prototype biogeochemistry version of the Community Climate System Model (CCSM). Coupling the carbon cycle with models of global climate dynamics will help determine the safe levels of atmospheric greenhouse gases.

The SciDAC CCSM consortium has also contributed new model algorithms and new scientific capabilities to the community. The consortium participated in the creation of a new finite-volume method for simulating atmospheric dynamics, and also developed a new numerical formulation useful for ocean models.

Until recently, the concentrations of greenhouse gases and atmospheric aerosols have been specified as parameters of the model, whose values are based on the future rate of fossil fuel consumption, the ability of oceans and land to adjust to change, and other assumptions. In recent years the project has primarily focused on adding the carbon and sulfur cycles to the climate system model. By introducing the biological and chemical processes that govern the absorption and emission of greenhouse gases, we can better simulate the response of Earth's climate system to human emissions. A prototype carbon-climate-biogeochemistry model has now been assembled, as a practical demonstration of coupled Earth system simulations at this dramatic new level of complexity. This new model includes a comprehensive formulation of atmospheric chemistry, as well as land and ocean ecosystem models. In this first step towards a truly comprehensive Earth system model, each component exchanges CO2 fluxes with the others. In addition, the ocean component supplies a flux of dimethyl sulfide (DMS), which is used by the atmospheric model to create sulfate aerosols. These aerosols interact with and influence the physical climate system, the oceanic carbon cycle, and terrestrial ecosystems. This prototype model, which was developed under SciDAC, will form a basis for future work towards a comprehensive Earth system model and enable climate researchers to more realistically simulate future climate change and its impact.

A further example of climate research.
The image shows CO₂ gas flux (in kilotonnes/km2/year) between the ocean and atmosphere averaged over December of year 9 of the fully coupled chemistry simulation. Positive values indicate uptake by the ocean; negative values indicate outgassing into the atmosphere. The ocean uptake in the Southern Ocean is due to a seasonal phytoplankton bloom.

Contributor: Dr. Philip Jones, Los Alamos National Laboratories (LANL)