Community Earth System Model



Figure 1: Correlation of monthly mean Nino3 SST anomalies with global SST anomalies for (top) observations; (b) CCSM4; and (c) CCSM3. Taken from Gent et al. (2010). [high resolution image]

An imperative for NCAR is to develop and continuously improve a comprehensive Earth modeling system that is at the forefront of international efforts to understand and predict the behavior of Earth's climate. For many years the Community Climate System Model (CCSM) met this challenge. Its output, for instance, has been used in hundreds of peer-reviewed studies to better understand the processes and mechanisms responsible for climate variability and climate change. In addition, simulations performed with CCSM have made a significant contribution to both national and international assessments of climate, including those of the Intergovernmental Panel on Climate Change (IPCC) and the U.S. Global Change Research Program (USGCRP). A trademark of the CCSM project is that it is a community collaboration. It is unique in the climate modeling sphere because of its commitment to make the model readily available to, and usable by, the climate research community. Community members are actively engaged in the ongoing process of model development.


Recent Accomplishments



Figure 2: Mean Arctic sea ice concentration for: (top) CCSM4 and (bottom) CCSM3. Simulations were for pre-industrial conditions, and the black lines are the 10% mean concentration values from satellite observations. [high resolution image]

The development of the model is a continuous process. Version 4 of CCSM was released to the community in April 2010, and it features many notable improvements over previous versions of the model (see, http://www.ccsm.ucar.edu/models/ccsm4.0/notable_improvements.html). One example of a significant improvement is the ability of CCSM4.0 to simulate the dominant pattern of interannual weather and climate variability over the globe, namely the El Niño/Southern Oscillation (ENSO) phenomenon. One aspect of this is shown in Fig. 1, which shows the correlation of monthly mean tropical eastern Pacific (Niño3) sea surface temperature (SST) anomalies with global SST anomalies. CCSM4.0 simulates a much more realistic meridional width to the positive correlations in the central and eastern tropical Pacific. It also captures the observed horseshoe pattern of negative correlations in the western tropical Pacific stretching into the middle latitudes of both hemispheres, as well as connections to the other tropical ocean basins. The improved ENSO variability in the CCSM4.0 compared to the CCSM3.0 and earlier versions will allow more plausible assessments of possible future changes in ENSO.


Simulations with CCSM4.0 also reveal improvements relative to CCSM3.0 in Arctic sea ice coverage. The mean sea ice concentration in the Arctic from both models is shown in Fig. 2, where the black lines are the 10% mean concentration values from satellite observations. The figure shows that the sea ice was much too extensive in the Labrador Sea and adjacent North Atlantic in the CCSM3.0, and this is much improved in the CCSM4.0 with the southern Labrador Sea being ice free. This improvement means that deep water formation can occur in the southern Labrador Sea in the CCSM4.0, whereas it was incorrectly located further east in the North Atlantic in earlier simulations. Credible simulations of Arctic sea ice are important, as there is a great deal of interest in what will happen to Arctic sea ice in the near future

Development of CESM1.0

To address a wider range of pressing scientific questions, additional capabilities have been added to CCSM4.0. These include, for instance, an interactive carbon cycle in the land component and an ecosystem-biogeochemical module in the ocean component. There is also an updated atmospheric chemistry component, a global dynamic vegetation component, and land use changes due to human activity in the land component. A new version of the atmospheric component model now allows scientists to study both the direct and indirect effects of aerosols. The model can be run using the Whole Atmosphere Community Climate Model (WACCM), in order to better understand the role of the upper atmosphere in climate variability and change. There is also an early version of a land-ice component that can be used to simulate changes to the Greenland ice sheet and its role in future climate change (e.g., the rate at which land ice will melt under future emission scenarios and the contribution of the melt to future sea level rise). Since the most widely used description of a model with these capabilities is an "Earth System Model", the project and supported model is now called the "Community Earth System Model", or CESM. The release of Version 1 of CESM occurred in June 2010.


Both CCSM4.0 and CESM1.0 are being used to conduct an extremely ambitious set of simulations in support of the upcoming Fifth Assessment Report (AR5) of IPCC. The simulations are part of the so-called CMIP-5 experimental design, which is a set of coordinated climate model experiments on two time scales: (1) near-term, initialized decadal prediction simulations; and (2) long-term simulations from 1850 through the end of the current century and beyond. The CESM project completed a large number of the simulations in 2010 so that they can be fully utilized by the community in time to be assessed in the AR5.

CESM Tutorial

The first, annual, CESM tutorial occurred in July 2010 and, based on participant feedback, was extremely successful. More than 180 applications were received, and 72 participants were selected. This included 40 'full' participants and 32 'auditors'. The auditors attended all lectures and were able to work on the practical sessions independently. The 40 full participants were almost exclusively graduate students, and the auditors comprised a mix of advanced graduate students, postdoctoral scientists and early career faculty. No single institution had more than 2 'full' participants in the tutorial. The represented institutions included 35 U.S. and Canadian (UCAR member) universities, six foreign universities, two NOAA facilities and four DOE National Laboratories.

2011 and Beyond

A major goal moving forward is to complete the CMIP-5 simulations using both CCSM4.0 and CESM1.0. Although CMIP-5 is a 5-year experimental protocol designed to address outstanding scientific questions that arose from the Fourth Assessment Report (AR4) of IPCC, the CESM project completed many integrations in 2010 and will strive to complete others in in early 2011. Among the many CMIP-5 simulations, a few points to note in particular include:

  • To complete the decadal prediction experiments, CESM will fully integrate recent NOAA- and NSF-funded developments within the NCAR Data Assimilation Research Testbed (DART), permitting the capability of ensemble data assimilation for both the atmosphere and the ocean. The assimilation capability will also address the desire for ensemble predictions, which is necessary to optimally assess forecast uncertainty. Further work involves simulation of tropospheric chemistry at high-resolution.
  • CESM will make a unique contribution to AR5 through the completion of CMIP-5 experiments with more than one configuration of the atmospheric model. Different atmospheric components of CESM, for instance, will allow community scientists to estimate the indirect aerosol effect, determine the role of upper-atmospheric processes in climate, and analyze transient chemistry-climate simulations.
  • Completion of simulations with the Community Ice Sheet Model (CISM) will place CESM among the very few international modeling groups to directly estimate the response of the Greenland and Antarctic ice sheets to future changes in climate.
  • Simulations with CESM will allow scientists to understand and quantify anticipated changes to the carbon cycle and their feedback to climate.
  • Work will be completed on ozone, deposition (nitrogen and aerosols) and aerosol concentration fields for both historical and future emission scenario simulations using CESM. Output will also be used as part of the IGBP Atmospheric Chemistry and Climate (AC&C) activity, which is focused on the understanding of the various radiative forcing agents and their changes from 1850 to 2100.