The first Joint Workshop on Representation Uncertainty in the Earth Sciences was held in March 2021. This brought together the Earth observation, data assimilation and forecast verification and post-processing communities, alongside metrologists to discuss the definition and quantification of representation uncertainty within the Earth Sciences. The aim of the workshop was to facilitate cross-disciplinary discussion, establishing where existing methodologies could be shared and to foster future collaboration. A key outcome of the workshop was a working definition of representation uncertainty applicable across all the Earth Science communities, which is presented in this white paper. The cross-disciplinary discussions at the workshop highlighted the need for scientists to work with metrologists to establish a common vocabulary for uncertainties, accessible to Earth Science applications. Further recommendations included regular workshops to discuss progress in defining and quantifying representation uncertainty and awareness of cross-disciplinary funding opportunities to further address representation uncertainty issues.