What is CarbonTracker?
CarbonTracker-CH4 is a modeling system that calculates methane emissions at the Earth's surface over time. It estimates the methane exchange from an 'atmospheric point of view'. Since CH4 mole fractions in the atmosphere reflect the sum of all the CH4 exchange at the surface, they form a record of the combined human and natural influence on atmospheric methane levels.
Changes in atmospheric methane burden are due to changes in its emissions, with an approximate ratio of 2.73 Tg CH4 per ppb . Visually, in the figure below, you can think of all the sources on one arm of a balance, having to balance changes in the burden on the other arm. By measuring atmospheric methane frequently, and at many places on Earth, CarbonTracker-CH4 can break down the budget even further, keeping track of regional uptake and release.
Figure 1. Global mass balance of atmospheric methane burden and methane emissions (Credit: Sourish Basu).
What additional information can stable carbon isotopic ratios of methane provide?
Shifting methane between the different source types doesn’t really change this balance in the figure above, since methane from any source looks the same to a methane measurement. Hence the difficulty of deriving source-specific emission changes with atmospheric CH₄ measurements alone. The stable carbon isotopic ratio ¹³C/12C (denoted δ¹³C-CH4) of methane measurements bring an additional constraint on the sources, because methane emitted from microbial sources is isotopically lighter (low δ¹³C-CH4) than fossil and pyrogenic sources (high δ¹³C-CH4). In the figure below, the different methane sources, placed at positions indicative of their source δ¹³C-CH4 signatures, must balance a see-saw whose fulcrum is specified by atmospheric measurements and losses due to atmospheric chemistry. Exchanging methane from one source type to another is no longer trivial, since the new distribution must now balance both the ¹²C and the ¹³C budgets.
Figure 2. Global mass balance of atmospheric methane isotopic burden and flux-weighted stable carbon isotopic ratios of different sources (Credit: Sourish Basu).
How does CarbonTracker work?
CarbonTracker produces model predictions of atmospheric CH4 mole fractions, that are systematically compared with the observed atmospheric CH4 mole fractions and its stable isotopic ratios. The difference between observed and predicted CH4 and its isotopes is attributed to differences in the sources and sinks used to make the prediction (the so-called 'prior') and the sources and sinks affecting the true atmospheric CH4 and isotopes. Using numerical optimization techniques, these differences are used to solve for a set of source emissions (‘posterior’) that most closely matches all of the observed CH4 and δ¹³C-CH4 in the atmosphere. CarbonTracker-CH4 has a representation of atmospheric transport based on a model used for weather forecasting, and representations of emissions and sinks of atmospheric CH4. More details about these different components and why they are needed can be found on our documentation pages.