A Rumsfeldian analysis of uncertainty in air-sea gas exchange

The title of this paper alludes to the oft-repeated statement by ex-US Defense Secretary Donald Rumsfeld, considering 'known knowns' (KKs), 'known unknowns' (KUs) and 'unknown unknowns' (UUs); a qualitative analysis which we apply to classify the degree of knowledge about uncertainties in air-sea gas exchange. We have tried to be comprehensive in our analysis, so this paper is broad in scope rather than deep in detail. We present a table summarizing the uncertainties we can identify and their 'Rumsfeldian classification' and go on to explain each source of uncertainty listed. We interpret the classifications as follows: a 'known known' uncertainty is one which is relatively well constrained, with some process-based understanding of causes and likely magnitude; a 'known unknown' is a source of uncertainty whose magnitude can be estimated at least for typical conditions/gases, but where only limited empirical evidence is available to validate magnitude and underlying processes; an 'unknown unknown' is an uncertainty for which we have little or no means to quantify the importance or likely magnitude of a process or phenomenon, with limited theoretical and/or little or no empirical evidence. We hope that this paper will provide a useful 'checklist' for researchers quantifying gas exchange and that it will aid in prioritizing future research on air-sea gas fluxes.