GEOS-Chem is a global 3-D model of atmospheric composition driven by assimilated meteorological observations from the Goddard Earth Observing System (GEOS) of the NASA Global Modeling Assimilation Office (GMAO) .  It is developed and used by research groups worldwide as a versatile tool for application to a wide range of atmospheric composition problems.

The GEOS data are available from the NASA Global Modeling Assimilation Office as a continuous archive from 1979 to present.  The operational GEOS-Forward Processing product (GEOS-FP) product has a horizontal resolution of 0.25° latitude x 0.3125° longitude, with 72 levels in the vertical. The consistent MERRA-2 reanalysis product for 1979-present has a horizontal resolution of 0.5° latitude x 0.625° longitude, again with 72 levels in the vertical. Both products are generated by the GEOS-5 Data Assimilation System (DAS) from a cubed-sphere simulation with the GEOS-5 general circulation model (GCM). The high-performance GEOS-Chem (GCHP) can use the cubed-sphere output. GEOS-Chem simulations can be conducted at the native resolution, at lower resolution, or in a nested mode with native resolution over selected regions. GEOS-Chem is grid-independent and compatible with the Earth Science Modeling Framework (ESMF), so that it can be used as a stand-alone chemical module in Earth System Models (ESMs). The ESMF framework also enables GEOS-Chem simulations to be conducted using GCHP on massively parallel computing architectures with Message Passing Interface (MPI) protocol.

All current applications of the GEOS-Chem model use the same standard code maintained by the GEOS-Chem Support Team. Updated versions of this standard code are released regularly. Detailed GEOS-Chem on-line documentation is available including a User's Guide.  The code is fully modular and highly parallelized. It is presently being used on Sun, IBM, Linux, Ubuntu, Fedora, and CentOS cluster platforms. It supports Intel Fortran and the open-source GNU Fortran compilers.

An adjoint of the GEOS-Chem model is maintained by the GEOS-Chem community under the direction of Adjoint Model Scientist Daven Henze for inverse modeling, data assimilation, and sensitivity studies. See the GEOS-Chem Adjoint web site for more details.

Several software tools facilitate the visualization of GEOS-Chem model outputs. The IDL-based GAMAP package—which is developed and maintained by the GEOS-Chem Support Team—allows for easy generation of a wide variety of plots and animations. Other open-source software visualization tools have been developed by the GEOS-Chem community to process and display model output using Matlab, NCL, R, and Python.

GEOS-Chem is a freely accessible community model. It is owned and supported by its user community under the purview of Working Groups and an international Steering Committee. User participation, responsibility, and feedback are essential. See our welcome page for new users. The model is maintained as a robust state-of-the-science facility by combining a nimble grass-roots approach to code development with strong version control. All development is done by users in support of their own research needs and then shared with the community. General management of GEOS-Chem is supported by the US NASA Earth Science Division and the Canadian National Science and Engineering Research Council.