The Community Climate Model (CCM) was created by NCAR in 1983 as a freely available global atmosphere model for use by the wider climate research community. The formulation of the CCM has steadily improved over the past two decades, computers powerful enough to run the model have become relatively inexpensive and widely available, and usage of the model has become widespread in the university community, and at some national laboratories.

A limitation of the original CCM was that it did not include models of the global ocean and sea ice. Accordingly, in 1994, NCAR scientists submitted a plan to National Science Foundation (NSF) to develop and use a Climate System Model (CSM) that was to include models of the atmosphere, land surface, ocean, and sea ice. These components were to be coupled without resorting to any "flux adjustments." The plan was to focus initially on the physical aspects of the climate system, and then in a subsequent version to improve biogeochemistry and coupling to the upper atmosphere. The first phase of this project was the model development by the NCAR staff, after which the model and associated data sets were to be made available to the scientific community. In addition, a new governance structure was promised, in which the interested scientific community would be given a fair opportunity to participate in all aspects of the CSM. NSF approved the plan, and model development began immediately.

In May 1996, the first CSM Workshop was held in Breckenridge, Colorado. At this workshop the CSM components and the results of an early equilibrium climate simulation were presented. Working groups began to form, and the nature of future CSM governance was discussed. At the final plenary session of the workshop, the proposed management structure was discussed, modified, and adopted. At that point, the second phase of CSM, including full participation of the scientific community, had begun.

The period since this 1996 workshop has been a time of substantial organizational progress. A Scientific Steering Committee (SSC) has been formed to lead the CSM activity, working groups have been producing useful output, and the previously existing Climate Modeling, Analysis and Prediction (CMAP) Advisory Council has been reorganized as the CSM Advisory Board (CAB). In addition to support from NSF, interest in the CSM from other agencies, notably the Department of Energy (DOE) and NASA, has developed. While working toward the second version of CSM, we believed that it was also time to recognize the community of users and sponsors by changing the name of the model to the Community Climate System Model (CCSM).

The period since May 1996 has also seen substantial scientific progress. A 300-year run has been performed using the CSM, and results from this experiment have appeared in a special issue of the Journal of Climate, 11, June, 1998. A 125-year experiment has been carried out in which carbon dioxide was prescribed to increase at 1% per year from its present concentration to approximately three times its present concentration. More recently, the Climate of the 20th Century experiment was run, with carbon dioxide and other greenhouse gases and sulfate aerosols prescribed to evolve according to our best knowledge from 1870 to the present. Three scenarios for the 21st century were developed: a "business as usual" experiment, in which greenhouse gases are assumed to increase with no economic constraints; an experiment using the Intergovernmental Panel on Climate Change (IPCC) Scenario A1; and a "policy-limited" experiment, in which emissions are assumed to be constrained, so that the concentration of carbon dioxide levels off at 550 parts per million by volume (ppmv) shortly after 2100.

Changes in climate, whether anthropogenic or natural, involve a complex interplay of physical, chemical, and biological processes of the atmosphere, ocean, and land surface. As climate system research seeks to explain the behavior of climate time scales of years to millennia, the focus necessarily turns to the interactions among the physical, chemical, and biogeochemical subsystems. The paleoclimate record reveals large correlated changes in atmospheric and oceanic circulation and biogeochemistry. The challenges of modeling the roles of anthropogenic emissions of carbon dioxide, reactive trace gases, and of changing land use in the earth system require a coupled-climate-system approach. While an appreciation that land-ocean-atmosphere interactions influence climate is not new, the emergence of coupled-climate-system questions as central scientific concerns of geophysics constitutes a major change in the research agendas of atmospheric science, oceanography, ecology, and hydrology.

Development of a comprehensive CESM that accurately represents the principal components of the climate system and their couplings requires both wide intellectual participation and computing capabilities beyond those available to most U.S. institutions. The CESM, therefore, must include an improved framework for coupling existing and future component models developed at multiple institutions, to permit rapid exploration of alternate formulations. This framework must be amenable to components of varying complexity and at varying resolutions, in accordance with a balance of scientific needs and resource demands. In particular, the CESM must accommodate an active program of simulations and evaluations, using an evolving model to address scientific issues and problems of national and international policy interest.

The CESM project will address important areas of climate system research. In particular, it is aimed at understanding and predicting the climate system. The long-term goals of the CESM project are simple but ambitious. They are:

  • To develop and to work continually to improve a comprehensive CESM that is at the forefront of international efforts in modeling the climate system, including the best possible component models coupled together in a balanced, harmonious modeling framework;
  • To make the model readily available to, and usable by, the climate research community, and to actively engage the community in the ongoing process of model development;
  • To use the CESM to address important scientific questions about the climate system, including global change and interdecadal and interannual variability; and
  • To use appropriate versions of the CESM for calculations in support of national and international policy decisions.

Complementary efforts using simplified models are also important and will be undertaken by many individuals, including some CESM participants. However, the CESM project will remain focused on comprehensive climate modeling.

We anticipate many important changes in the climate modeling enterprise over the next five years, including:

  • Increasing computer power, both in the U.S. and abroad, that can support more elaborate and more sophisticated models and modeling studies, using increased spatial resolution and covering longer intervals of simulated time;
  • Improved understanding of many of the component processes represented in the CESM, including cloud physics; radiative transfer; atmospheric chemistry, including aerosol chemistry, boundary-layer processes, polar processes, and biogeochemical processes; and the interactions of gravity waves with the large-scale circulation of the atmosphere;
  • Improved understanding of how these component processes interact;
  • Improved numerical methods for the simulation of geophysical fluid dynamics; and
  • Improved observations of the atmosphere, including major advances in satellite observations.