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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 (jump to TransCom overview). TransCom was conceived at the Fourth International CO2 Conference in Carqueiranne in 1993.
All three phases of TransCom are complete.
A key component in the projection of future global change is the ability to predict future concentrations of atmospheric greenhouse gases such as carbon dioxide (CO2). Unfortunately, the current state of the science cannot completely account for the growth rate and interannual variations of atmospheric CO2 with confidence, so accurate prediction of future concentrations is difficult.

Only about half of the anthropogenic CO2 remains in the atmosphere, and the fate of the other half is not completely understood. Both the ocean and terrestrial biosphere currently act as significant sinks for anthropogenic CO2, but their relative contributions are a matter of intense debate (Schimel et al, 1995). The terrestrial net sink is almost impossible to measure directly, even at a single location, because it results from a small imbalance between large natural uptake and efflux by photosynthesis and ecosystem respiration, neither of which can be accurately measured at large spatial scales. Until the mechanisms involved in the terrestrial uptake are more clearly elucidated, predicting the future behavior of such a sink (and therefore the atmospheric concentration) will be very difficult.

One important approach to understanding and predicting the terrestrial net carbon uptake involves inferring its magnitude and geographic distribution from the spatial and temporal variations in atmospheric CO2 concentration as observed by global flask sampling programs (Conway et al., 1994; Francey et al., 1995; Keeling et al., 1995). The observed concentration field is determined by the surface fluxes of CO2 (which we want to know) and the transport of CO2 in the atmosphere. The atmospheric transport may be simulated by numerical models, and the unknown surface fluxes may then be determined from the observational data by inversion (Enting and Mansbridge, 1989, 1991; Tans et al., 1989, 1990; Keeling et al., 1989; Ciais et al., 1995; Enting et al., 1995; Fan et al., 1998).

The current suite of carbon budget inversion studies produce results which are difficult to reconcile with one another. A recent calculation by Song-Miao Fan and colleagues at Princeton University (Fan et al., 1998) found that the carbon sink in the Northern Hemisphere between 1988 and 1992 was dominated by terrestrial uptake in North America. Their results, if correct, imply that the terrestrial sink approximately compensates for the anthropogenic emissions in this region. Peter Rayner and colleagues at Monash University in Australia performed an inversion using most of the same data but a different CTM and a different mathematical method. They found that the northern terrestrial sink was distributed more evenly across the northern continents, with North America acting as only a weak sink (Rayner et al, 1997). A third study has concluded that the terrestrial sink is dominated by Eurasia (Bousquet et al., 1998)

As high time-resolution global data on additional species become available (d13C and d18O of atmospheric CO2 and atmospheric O2/N2 ratio), the use of synthesis inversion techniques with atmospheric tracer transport models will result in much more reliable estimates of the changing global carbon budget of the atmosphere. Improvements in the quality and quantity of the observational data and in the mathematical formalism associated with the inversion calculation have brought us to the point where one of the biggest sources of uncertainty now lies in the transport models themselves (Law et al, 1996; Denning et al, 1999).

The time has come for a thorough and systematic evaluation of the transport models, comparing model results against one another and against the real world. In addition, we hope to use the results of the experiments described herein to advance the mechanistic understanding of spatial and temporal variations of biogenic trace gas concentrations at the global scale.
Transcom (back to top)
The Atmospheric Tracer Transport Model Intercomparison Project (TransCom) was conceived at the Fourth International CO2 Conference in Carqueiranne in 1993. TransCom is a special project of the International Geosphere-Biosphere Programme (IGBP), Global Analysis, Interpretation, and Modeling (GAIM) Project, the objective of which is to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in simulated atmospheric transport. The project is part of a larger GAIM research program which aims to develop coupled ecosystem-atmosphere models that describe time evolution of trace gases with changing climate and changes in anthropogenic forcing.

Initially coordinated by Peter Rayner at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), TransCom is now being coordinated by Scott Denning and Kevin Gurney at the Department of Earth and Atmospheric Sciences, Purdue University.
transcom 01
Click here for the transcom 01 overview.
transcom 02
Click here for the transcom 02 overview.
transcom 03
Click here for the transcom 03 overview.
Click here for references.
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