Global inverse modeling of CH4 sources and sinks: an overview of methods

S. Houweling, P. Bergamaschi, F. Chevallier, M. Heimann, T. Kaminski, M. Krol, A.M. Michalak and P. Patra

Anthropogenic emissions of methane are second only to those of carbon dioxide in terms of their impact on global warming since the start of the industrial period. The two primary ways of quantifying methane emissions are (1) to build inventories that catalog source types and their contributions, and (2) to use concentrations of methane in the atmosphere to infer emissions in upwind regions. This literature review surveys the history and state of the science for the second approach, also known as atmospheric inverse modeling, with a special emphasis on the opportunities and challenges associated with the use of satellite-based observations of atmospheric methane.

Figure: Conceptual diagram of ways to extend the use of measurements in CH4 flux inversions. Square boxes represent models, ovals represent measurements, and the rounded box represents the target variable of the CH4 inversion. Call outs provide examples of the kind of measurements that are meant by the ovals (without attempting to be complete). Black arrows: coupled and assimilated into inversions already; dashed arrows: not (yet) coupled or assimilated.


The aim of this paper is to present an overview of inverse modeling methods that have been developed over the years for estimating the global sources and sinks of CH4. It provides insight into how techniques and estimates have evolved over time and what the remaining shortcomings are. As such, it serves a didactical purpose of introducing appren- tices to the field, but it also takes stock of developments so far and reflects on promising new directions. The main focus is on methodological aspects that are particularly relevant for CH4, such as its atmospheric oxidation, the use of methane isotopologues, and specific challenges in atmospheric transport modeling of CH4. The use of satellite retrievals receives special attention as it is an active field of methodological development, with special requirements on the sampling of the model and the treatment of data uncertainty. Regional scale flux estimation and attribution is still a grand challenge, which calls for new methods capable of combining information from multiple data streams of different measured parameters. A process model representation of sources and sinksin atmospheric transport inversion schemes allows the integrated use of such data. These new developments are needed not only to improve our understanding of the main processes driving the observed global trend but also to support international efforts to reduce greenhouse gas emissions.

Houweling, S., P. Bergamaschi, F. Chevallier, M. Heimann, T. Kaminski, M. Krol, A.M. Michalak, P. Patra (2017) "Global inverse modeling of CH4 sources and sinks: an overview of methods", Atmospheric Chemistry and Physics, 17 (1), 235-256, doi:10.5194/acp-17-235-2017.