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 first phase of TransCom 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
The second phase of TransCom (TransCom 2) examined the transport of sulfur hexafluoride (SF6) emissions using 11 tracer transport models. The goal was to utilize a known source function to diagnose aspects of transport differences that were evident in the results coming out of the first phase of the TransCom experiment (TransCom 1)
The Atmospheric Carbon Cycle Inversion Intercomparison (TransCom 3) 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. This provided an opportunity to assess both the model-data mismatch and the spread of results achieved in individual model inversion efforts. All aspects of the inverse estimate were controlled for except for simulated transport.
Three new Transcom experiments have been initiated. The first involves a comparison of upper-air model output. The second is a simulation of hourly CO2, radon and SF6 for 2002-2003 using prescribed fluxes. The third is a forward simulation comparison of CH4 concentrations. There is also a new intercomparison (led by Philippe Peylin) in which the latest inversion model efforts are intercompared. These new experiments we classify as "TransCom 4".