October 4, 2010 | In the early 1960s, NCAR scientists Warren Washington and Akira Kasahara began developing one of the world’s first computer models of atmospheric circulation. They used a CDC 6600 computer in the basement of the Mesa Lab, before today’s computing room was even built. The machine received input via punch cards and seven-channel digital magnetic tape, and generated output via two line printers, a card punch, a photographic plotter, and standard magnetic tape. In 1967, the pair published their first journal paper, “NCAR Global General Circulation Model of the Atmosphere.”
Climate modeling has come a long way since the days of punch cards. In August, NCAR released its latest and most advanced tool: the Community Earth System Model (CESM), a fully coupled, global model that provides state-of-the-art computer simulations of Earth's past, present, and future climates. CESM will be one of the primary models used to conduct simulations in support of the next assessment by the Intergovernmental Panel on Climate Change (IPCC), due in 2013–14.
Building on CCSM
CESM is the successor to the CCSM (Community Climate System Model), whose first version, the Community Climate Model, was created at NCAR in 1983. Scientists steadily improved and added capabilities to the model over the years, renaming it CCSM in 2001. A third version of CCSM was one of the main climate models used for the IPCC’s 2007 assessment report, for which NCAR scientists shared a Nobel Peace Prize. The CCSM’s fourth version, released last April, is now considered a subset of CESM.
CESM builds on CCSM, giving scientists a broader picture of Earth’s climate system by incorporating more influences and feedbacks. With the new model, for instance, researchers can simulate the interaction of marine ecosystems with greenhouse gases; the climatic influence of ozone, dust, and other atmospheric constituents; the cycling of carbon through the atmosphere, oceans, and land surfaces; and the influence of greenhouse gases on the upper atmosphere. In addition, an entirely new representation of atmospheric processes in the CESM will allow researchers to pursue a much wider variety of applications, including studies of air quality and the role of aerosols in climate.
The improved realism of the model should also be helpful for studying and perhaps forecasting the evolution of ocean-atmosphere patterns (such as the El Niño/Southern Oscillation, the North Atlantic Oscillation, and the Pacific Decadal Oscillation), which dominate regional changes in weather and climate on interannual to decadal time scales.
“With CESM, we can pursue scientific questions that we could not address previously,” says Jim Hurrell (NESL/CGD), who is the current chair of the CESM Scientific Steering Committee. “Thanks to its improved physics and expanded capabilities, it gives us a better representation of the real world.”
Some specific questions that the model will be applied to include:
• What impact might warming temperatures have on the massive ice sheets of Greenland and Antarctica?
• How might patterns in the ocean and atmosphere affect regional climate in coming decades?
• How might climate change influence the severity and frequency of tropical cyclones, including hurricanes?
• What are the effects of tiny airborne particles, known as aerosols, on clouds and temperatures?
Preparing for the IPCC
CESM’s advanced capabilities will help scientists study climate change in greater detail, and they’ve already begun using the model for an extremely ambitious set of climate experiments to be featured in the next IPCC assessment. Although the publication of the assessment is still several years off, most of the simulations are scheduled for completion and public release beginning later this year, so that the broader research community can complete its analyses in time for inclusion in the report.
“We are working day and night to get all of the long-term climate change runs completed,” Jim says. “It takes the commitment of nearly everyone in the project, as well as strong collaboration with CISL.”
“It’s a big investment of time and takes a lot of effort from a lot of people,” concurs CGD/ACD scientist Jean-François Lamarque, who is working on the atmospheric chemistry component of the IPCC runs. One of the advantages of CESM for Jean-François is its ability to simulate interactive chemistry. “By having interactive chemistry, you can better capture feedbacks between chemistry and climate,” he says. “We can do this in CESM, but couldn't in CCSM4."
CESM, which is supported by NSF and the Department of Energy, is freely available to researchers worldwide.
Simulating a century or more of climate is nothing new for NCAR’s community climate models. But this year the CESM is also zeroing in on shorter decadal time-scale predictions for the next IPCC assessment. One of the major advances in the new report will be attempts by modeling groups to incorporate the recent, observed history of slow-moving ocean currents and other factors that determine climate’s path over a decade or more. “Potential forecasting skill has been going untapped because we haven’t been assimilating the present state of the oceans,” says Jim Hurrell (NESL/CGD).
With this new approach, NCAR and other research centers are producing the first “decadal predictions.” These will reveal how well the models reproduce past climate and what they have to say about the near future. For each of the decadal predictions, the CESM will produce an ensemble of simulations, each with slightly different starting conditions to capture uncertainty in the observed atmosphere and ocean.
The time frames to be simulated (see graphic) include
• decade-long spans starting every five years from 1960 to 2005;
• 30-year spans starting in 1960, 1985, and 2005; and
• as in past IPCC reports, a variety of periods out to 2100 and beyond (not shown in graphic)—but with more sophisticated models and a wider variety of conditions and scenarios.
Beyond these core experiments, each center will carry out additional runs suited to that lab’s particular model and the interests of its scientists.
