Skip to content


Repository files navigation

The Community Earth System Model

See the CESM web site for documentation and information:

The CESM Quickstart Guide is available at:

This repository provides tools for managing the external components that make up a CESM tag - alpha, beta and release. CESM tag creation should be coordinated through CSEG at NCAR.

This repository is also connected to slack at

Installing, building and running CESM requires:

  • a Unix-like operating system (Linux, AIX, OS X, etc.)
  • git client version 1.8 or newer
  • subversion client (we have tested with versions 1.6.11 and newer)
  • python3 version 3.6 or newer
  • perl version 5
  • build tools gmake and cmake
  • Fortran and C compilers
  • LAPACK and BLAS libraries
  • a NetCDF library version 4.3 or newer built with the same compiler you will use for CESM
    • a PnetCDF library is optional
  • a functioning MPI environment (unless you plan to run on a single core with the CIME mpi-serial library)

The Fortran compiler must support Fortran 2003 features. However, even among mainstream Fortran compilers that claim to support Fortran 2003, we have found numerous bugs. Thus, many compiler versions do not build or run CESM properly (see for more details on older Fortran compiler versions).

CESM2 is tested on several different systems with newer Fortran compilers: Please see CESM Compiler/Machine Tests for a spreadsheet of the current results.

For more details on porting CESM to a new machine, see

CESM is now released via github. You will need some familiarity with git in order to modify the code and commit these changes. However, to simply checkout and run the code, no git knowledge is required other than what is documented in the following steps.

To obtain the CESM code you need to do the following:

  1. Clone the repository.

    git clone my_cesm_sandbox

    This will create a directory my_cesm_sandbox/ in your current working directory.

  2. Go into the newly created CESM repository and determine what version of CESM you want. To see what cesm tags are available, simply issue the git tag command.

    cd my_cesm_sandbox
    git tag
  3. Do a git checkout of the tag you want. If you want to checkout release-cesm2.1.2, you would issue the following.

    git checkout release-cesm2.1.2

    (It is normal and expected to get a message about being in 'detached HEAD' state. For now you can ignore this, but it becomes important if you want to make changes to your Externals.cfg file and commit those changes to a branch.)

  4. Run the script manage_externals/checkout_externals.


    The checkout_externals script is a package manager that will populate the cesm directory with the relevant versions of each of the components along with the CIME infrastructure code.

At this point you have a working version of CESM.

To see full details of how to set up a case, compile and run, see the CIME documentation at .

The file Externals.cfg in your top-level CESM directory tells checkout_externals which tag/branch of each component should be brought in to generate your sandbox. (This file serves the same purpose as SVN_EXTERNAL_DIRECTORIES when CESM was in a subversion repository.)

NOTE: Just like svn externals, checkout_externals will always attempt to make the working copy exactly match the externals description. For example, if you manually modify an external without updating Externals.cfg, (e.g. switch to a different tag), then rerunning checkout_externals will automatically restore the externals described in Externals.cfg. See below documentation Customizing your CESM sandbox for more details.

You need to rerun checkout_externals whenever Externals.cfg has changed (unless you have already manually updated the relevant external(s) to have the correct branch/tag checked out). Common times when this is needed are:

  • After checking out a new CESM branch/tag
  • After merging some other CESM branch/tag into your currently checked-out branch

checkout_externals must be run from the root of the source tree. For example, if you cloned CESM with:

git clone my_cesm_sandbox

then you must run checkout_externals from /path/to/my_cesm_sandbox.

To see more details of checkout_externals, issue

./manage_externals/checkout_externals --help

There are several use cases to consider when you want to customize or modify your CESM sandbox.

If you have already checked out a tag and HAVE NOT MADE ANY MODIFICATIONS it is simple to change your sandbox. Say that you checked out release-cesm2.1.2 but really wanted to have release-cesm2.1.3; you would simply do the following:

git checkout release-cesm2.1.3

You should not use this method if you have made any source code changes, or if you have any ongoing CESM cases that were created from this sandbox. In these cases, it is often easiest to do a second git clone.

Each entry in Externals.cfg has the following form (we use CAM as an example below):

tag = trunk_tags/cam5_4_143/components/cam
protocol = svn
repo_url =
local_path = components/cam
required = True

Each entry specifies either a tag or a branch. To point to a new tag:

  1. Modify the relevant entry/entries in Externals.cfg (e.g., changing cam5_4_143 to cam5_4_144 above)

  2. Checkout the new component(s):


Keep in mind that changing individual components from a tag may result in an invalid model (won't compile, won't run, not scientifically meaningful) and is unsupported.

After making this change, it's a good idea to commit the change in your local CESM git repository. First create a CESM branch in your local repository, then commit it. (Unlike with subversion, branches are stored locally unless you explicitly push them up to github. Feel free to create whatever local branches you'd like.) For example:

git checkout -b my_cesm_branch
git add Externals.cfg
git commit -m "Update CAM to cam5_4_144"

If you'd like to modify a component via a branch and point to that branch in your CESM sandbox, use the following procedure (again, using CAM as an example):

  1. Create a CAM branch. Since CAM originates from a subversion repository, you will first need to create a branch in that repository. Let's assume you have created this branch and called it my_branch.

  2. Update Externals.cfg to point to your branch. You can replace the tag entry with a branch entry, as follows:

    branch = branches/my_branch/components/cam
    protocol = svn
    repo_url =
    local_path = components/cam
    required = True
  3. Checkout your branch:


It's a good idea to commit your Externals.cfg file changes. See the above documentation, Committing your change to Externals.cfg.

Developers who have not already done so should follow the recommended one-time setup directions for git. Developers may also want to set up ssh keys and switch to using the form of the github URLs.