Detection significances of CH₄, CO₂, and DMS from POSEIDON retrievals of K2-18 b’s JWST NIRISS SOSS + NIRSpec G395H transmission spectrum. Each grid cell represents a Bayesian model comparison between two retrievals, one with and one without a molecule, for a specific NIRISS SOSS and NIRSpec G395H data variant pair. The data variants are ordered with increasing spectral resolution to the right (for NIRSpec) and down (for NIRISS). The cells are colored and annotated according to the detection significance equivalent to the calculated Bayes factor (e.g., R. Trotta 2008; B. Benneke & S. Seager 2013). Bayes factors <3 (equivalent to <2.1σ), corresponding to no evidence on the Jeffreys’ scale, are donated by “—.” Our retrievals demonstrate strong evidence for CH₄, but find no reliable statistical evidence for CO₂ or DMS in K2-18 b’s atmosphere.