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 CO2 introduces 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 CO2 product for process studies and for accurate long‐term average maps of ocean carbon uptake. Plain Language Summary Closing the global carbon budget is key to keeping tabs on society's progress toward a stabilized climate. Therefore, oceanographers go to great lengths to reduce uncertainty in the quantification of ocean carbon uptake. While there has been much attention on improving almost every aspect of the calculation of air‐sea exchange of carbon dioxide, one aspect has been seldom examined: How atmospheric carbon dioxide (CO2) concentrations vary across the globe. For instance, westerly winds draw elevated CO2 from Asia and North America over the neighboring oceans. This promotes higher ocean CO2 uptake than would be estimated if we neglect that spatial variation. Luckily, the errors introduced by ignoring spatial variability average to a very small number over large enough scales (though this was not a foregone conclusion, given that the elevated atmospheric concentrations are found over very windy, high ocean uptake regions). However, in the “tailpipe” of the industrialized continents (i.e., the western North Pacific and North Atlantic), neglecting the elevated atmospheric CO2 concentrations would lead to a low bias in ocean carbon uptake estimates. Overall, the work suggests that local ocean carbon uptake studies should measure atmospheric CO2 locally or make use of atmospheric CO2 estimates that resolve spatial variability. Key Points Atmospheric CO2 is elevated downwind of highly industrialized continents, the “tailpipe regions,” east of Asia and North America Atmospheric CO2 anomalies swept over the neighboring ocean enhance ocean uptake above estimates using zonal mean atmospheric CO2 Errors average out over hemispheric‐scales, but introduce important biases on local scales