The Atmospheric Tracer Transport Model Intercomparison Project (TransCom) was created to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in simulated atmospheric transport. TransCom was conceived at the Fourth International CO₂ Conference in Carqueiranne in 1993.

The TransCom project, consists of four phases:

• TransCom 01, the first phase, compared the model output of two different components of the carbon cycle: the fossil CO₂ source and the seasonal aspect of the biospheric CO₂ source.
• TransCom 02, the second phase, examined the transport of sulfur hexafluoride (SF6) emissions using 11 tracer transport models.
• TransCom 03, the third phase, conducted a series of experiments in which leading chemical tracer transport models from around the world were used to calculate the global carbon budget of the atmosphere.
• TransCom 04, the fourth phase, consists of two experiments, which have just been initiated. The first experiment involves a comparison of upper-air model output. The second experiment is called the continuous experiment, in it the transport models are run for 2002 and 2003 using prescribed fluxes (biosphere, ocean and fossil CO₂, SF6, and radon). Hourly CO₂ concentration is output for comparison with in-situ CO₂ measurements.

The TransCom project was sponsored by a task force of the International Geosphere-Biosphere Programme (IGBP) called Global Analysis, Integration, and Modeling.