Biases and spread in the estimates of each term of the global carbon budget challenge the robust detection of a trend in their central estimates, and moreover inhibit the attribution of a trend in atmospheric CO2 to anthropogenic emissions. We outline the key sources of bias and spread in each term of the global carbon budget, highlight examples of progress made in recent years and opportunities for further progress in the coming decades. Overall, we suggest that the capacity to verify changes in atmospheric CO2 on sub-decadal timescales will require concerted effort to incrementally address biases and uncertainties across all components of the budget
