Challenges and opportunities to reduce uncertainty in projections of future atmospheric CO₂: a combined marine and terrestrial biosphere perspective

Abstract. Atmospheric CO₂ and climate projections for the next century vary widely across current Earth system models (ESMs), owing to different representations of the interactions between the marine and land carbon cycle on the one hand, and climate change and increasing atmospheric CO₂ on the other hand. Several efforts have been made in the last years to analyse these differences in detail in order to suggest model improvements. Here we review these efforts and analyse their successes, but also the associated uncertainties that hamper the best use of the available observations to constrain and improve the ESMs models. The aim of this paper is to highlight challenges in improving the ESMs that result from: (i) uncertainty about important processes in terrestrial and marine ecosystems and their response to climate change and increasing atmospheric CO₂; (ii) structural and parameter-related uncertainties in current land and marine models; (iii) uncertainties related to observations and the formulations of model performance metrics. We discuss the implications of these uncertainties for reducing the spread in future projections of ESMs and suggest future directions of work to overcome these uncertainties.