NCAR Imperative 3 | Community Access: CESM Ensembles

To explore the possible impacts of uncertainties in recent past variations and future projections of natural and anthropogenic forcings in the framework of internal variability of the coupled climate system, NCAR has provided ensembles of CESM simulations (over 100 so far) for community analysis. CGD scientists in collaboration with colleagues at the University of Colorado Boulder led a project to run the NCAR-based Community Earth System Model (CESM) 40 times from 1920 forward to 2100 – producing the CESM Large Ensemble ( With each simulation, the scientists modified the model's starting conditions ever so slightly by adjusting the global atmospheric temperature by less than one-trillionth of one degree, touching off a unique and chaotic chain of climate events, extending to 2100 with the RCP8.5 scenario. Complementing the Large Ensemble, CGD scientists have provided 15-member CESM ensembles for 2006 forward to 2100 using the RCP4.5 scenario forcings (the Medium Ensemble) and using the same forcing as the RCP 8.5 Large Ensemble except that all aerosol emissions and tropospheric oxidants fixed at 2005 levels.

The CESM Large, Medium, and Fixed Aerosol Ensembles are a display of Earth climates that could have been along with a rich look at future climates that could potentially be.  The datasets generated during the project, which are freely available, have already proven to be a tremendous resource for researchers across the globe who are interested in how natural climate variability and human-caused climate change interact. In a little over a year, about 100 peer-reviewed scientific journal articles have used data from the CESM Large Ensemble. Scientists have so far relied on the CESM Large Ensemble to study everything from oxygen levels in the ocean to potential geoengineering scenarios to possible changes in the frequency of moisture-laden atmospheric rivers making landfall. All three ensembles have been utilized by the project on the Benefits of Reduce Anthropogenic Climate changE (BRACE) to assess avoided impacts on climate extremes, health, tropical cyclones, and agriculture and land use by choosing a path forward of reduced emissions.

Complementing these CESM ensembles for the instrumental period and into the future, a 38-member Last Millennium Ensemble ( from 850 forward to 2005 has been completed by CGD scientists  and is also freely available to the community. The last millennium has a rich archive of annually-dated proxy records – tree rings, ice cores, Arctic lake records, stalagmites, corals – that give a longer perspective on climate variability and change than is available for the instrumental period. The CESM Last Millennium ensemble contains both simulations with all forcings (solar, volcanic, greenhouse gases, orbital, land use, and aerosols) as well as ensembles of simulations with each forcing individually, with the latter providing a longer-term perspective for detection and attribution. Scientists have already used the CESM Last Millennium Ensemble to assess the roles of internal and naturally-forced variability on coastal upwelling to terrestrial aridity to East African hydroclimate.

Running a complex climate model like the CESM many dozens of times takes a vast amount of computing resources, which makes such projects rare and difficult to pull off. With that in mind, NCAR queried scientists from across the community who might make use of the project results — oceanographers, geochemists, atmospheric scientists, biologists, geologists, socioeconomic researchers — about what they really wanted. Scientists have long understood that it makes sense to look at more than one model run. Frequently, however, scientists have done this by comparing simulations from different climate models, collectively called a multi-model ensemble. This method gives a feel for the diversity of possible outcomes, but it doesn't allow researchers to determine why two model simulations might differ: Is it because the models themselves represent the physics of the Earth system differently? Or is it because the models have different representations of the natural variability or different sensitivities to changing natural and anthropogenic forcings? Ensembles with the same model help to resolve this dilemma.

CESM Large Ensemble
Winter temperature trends (in degrees Celsius) for North America between 1963 and 2012 for each of 30 members of the CESM Large Ensemble. The variations in warming and cooling in the 30 members illustrate the far-reaching effects of natural variability superimposed on human-induced climate change. The ensemble mean (EM; bottom, second image from right) averages out the natural variability, leaving only the warming trend attributed to human-caused climate change. The image at bottom right (OBS) shows actual observations from the same time period. By comparing the ensemble mean to the observations, the science team was able to parse how much of the warming over North America was due to natural variability and how much was due to human-caused climate change. Read the full study in the American Meteorological Society's Journal of Climate. (© 2016 AMS.)