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Abstract A simplifying assumption in many studies of ocean carbon uptake is that the atmosphere is well‐mixed, such that zonal variations in its carbon dioxide (CO₂) content can be neglected in the calculation of air‐sea fluxes. Here, we examine this assumption at various scales to quantify the errors it introduces. For global annual averages, we find that positive and negative errors effectively cancel, so the use of atmospheric zonal‐average CO₂introduces reassuringly small errors in fluxes. However, for millions of square kilometers of the North Pacific and Atlantic that are downwind of the highly industrialized northern hemisphere continents, these biases average to over 6% of the annual ocean uptake and can cause errors of up to 30% on a given day. This work highlights the need to use a high quality, spatially‐resolved atmospheric CO₂product for process studies and for accurate long‐term average maps of ocean carbon uptake.