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May 1997

Science Briefing

Jim Hack
CGD's climate system model (CSM) has completed its first century-scale simulation of global climate for a scenario of increasing carbon dioxide (CO2). In their first look at the results, CSM developers were pleased with the quality of the model's performance.

The simulation was performed in Japan over three weekends through a collaboration between NCAR and the Central Research Institute of the Electric Power Industry (CRIEPI), based in Tokyo, Japan. CRIEPI provided the financial support for computer time as well as technical assistance for the 125-year CSM run. It was performed on a 32-processor SX-4 supercomputer at the NEC Laboratory in Fuchu, with assistance from NEC's U.S. subsidiary, HNSX Supercomputers.

Because of the high volume of output from the CSM, the bulk of the 125-year simulation history was saved as monthly averages of temperature, moisture, and other variables. However, the first and last ten years were sampled every six hours in the model, providing enough temporal detail to track weather features as they grow and decay over a few days. This run marks one of the first times that a climate model as sophisticated and long-range as NCAR's has examined weather systems in such detail.

The monthly-averaged data will be made available to the U.S. research community at a users' conference this summer. Before releasing the six-hour data to the community, CRIEPI will use it with a regional climate model to study the climatology of tropical cyclones and to examine synoptic-scale disturbances over eastern Asia in a climate with increased carbon dioxide. Filippo Giorgi and other NCAR scientists also will use the data sets for regional climate modeling.

For the CRIEPI simulation, carbon dioxide levels in the CSM were specified to increase 1 percent a year, roughly in line with current global trends. The result was a doubling of present-day levels of CO2 after 70 years and a tripling after about 110 years. The global temperature average at that point was roughly 2 degrees C above present values.

The simulation was one of the most exhaustive ever carried out by an NCAR computer model. Each year of model time produced one gigabyte (GB, or one billion bytes) of monthly-averaged data and took about 90 minutes to run on the SX-4. To speed the process of data transmission between Boulder and Japan, the Sony Corporation provided a new high-density tape drive used by CRIEPI to store data and ship them to NCAR. Each tape holds 40 GB; the entire project's initial database is roughly 170 GB.

The CSM output includes data on ocean and ice conditions as well as the atmosphere. Future experiments assessing global climate change will examine more realistic greenhouse gas scenarios and include more sophisticated treatments of atmospheric chemistry (for example, sulfate chemistry). The CSM's key advantage over previous models is that its global climate unfolds naturally from the initially specified conditions without any artificial constraints or corrections being needed.

This graph compares two 125-year computer simulations from NCAR's climate system model (CSM). The top line depicts global average temperatures for a scenario of steadily increasing carbon dioxide (1 percent per year) starting in year 10. The bottom line shows temperatures obtained from a control simulation that holds CO2 levels constant. The horizontal lines above and below the control indicate the range of natural climate variations in the control run. Although the increasing-CO2 graph shows multiyear fluctuations, it also indicates a steady rise over time to a global temperature by year 125 of more than 2 degrees C above current levels. This result is in line with recent global temperature projections from the United Nations Intergovernmental Panel on Climate Change. (Illustration courtesy Lawrence Buja.)

While the most recent simulation is by no means a forecast, it is an important first step in more realistic depictions of future climate. "It's a step toward understanding how regional climates might vary under plausible greenhouse-gas scenarios," says Byron Boville, cochair of the CSM development group. "Dozens of runs of this nature will be needed to ensure the model can simulate past, present, and future climate, and to be able to generate meaningful statistics." By repeating the same climate simulation with slightly varied initial conditions, researchers can generate an ensemble of results and then compare them against each other to see where the model's uncertainty is greatest.

"We hope this is the initial basis for future collaboration between NCAR and CRIEPI," says Jim Hack, who coordinated the simulation. CGD's Lawrence Buja was on site in Japan during the first weekend of the model run, working with CRIEPI scientists and NEC engineers to ensure that all went smoothly.

Overseeing this and other CSM projects is the model's Scientific Steering Committee, which includes scientists from NCAR as well as at least four from beyond the institution. Current members are Byron, Peter Gent, Jeff Kiehl, and Warren Washington (all in CGD); Robert Dickinson (University of Arizona); David Randall (Colorado State University); Jagadish Shukla (Center for Ocean-Land-Atmosphere Studies); and Susan Solomon (NOAA). The commitee's chair is CGD head Maurice Blackmon.

Awards already: The CSM has been selected as one of six finalists in the Environment, Energy, and Agriculture category for the 1997 Computerworld Smithsonian Awards. As part of the honor, the Smithsonian Institution is adding the CSM to its Permanent Research Collection of Information Technology Innovation at the National Museum of American History in Washington, D.C. First-place honors will be announced at an awards dinner in Washington on 9 June, and the entire 1997 collection will be formally presented to the Smithsonian the next day.

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Edited by Bob Henson, bhenson@ucar.edu