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 most recent GEOS–5 product has a horizontal resolution of 0.5° latitude x 0.667° longitude, with 72 levels in the vertical. GEOS–Chem simulations can be conducted at the native resolution, at lower resolution, or in a nested mode with native resolution over selected regions. We have also adapted GEOS–Chem to run with climate data from the NASA/GISS general circulation model for simulations of atmospheric composition in past and future climates.

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.

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.

An IDL-based Global Atmospheric Modeling (output) Analysis Package (GAMAP) facilitates the visualization of GEOS–Chem model outputs.  GAMAP takes in 5-D model output fields (concentrations in 3-D space, time, tracer #) and through an user-friendly interface allows easy generation of a wide variety of plots and animations.  GAMAP was originally written at Harvard and is being maintained and further developed by the GEOS–Chem Support Team.

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.