CarbonTracker is a data assimilation system that combines observed carbon dioxide concentrations from 81 sites around the world with model predictions of what concentrations would be based on a preliminary set of assumptions (“the first guess”) about sources and sinks for carbon dioxide. CarbonTracker compares the model predictions with reality, and then systematically tweaks and evaluates the preliminary assumptions until it finds the combination that best matches the real world data.

The CarbonTracker has modules for atmospheric transport of carbon dioxide via weather systems, for photosynthesis and respiration, for air-sea exchange, and for fossil fuel combustion and fires. Users can download the source code, the measurements of carbon dioxide that feed the assimilation, and the surface carbon flux estimates that CarbonTracker produces to conduct their own analyses or to contribute to the further improvement of the software. Launched in 2007, the CarbonTracker is updated at least once annually with software improvements, bug fixes, and new carbon dioxide observations.