Long‐term change of CO₂ latitudinal distribution in the upper troposphere

We analyzed temporal variations in the annual mean latitudinal distribution of upper tropospheric CO 2 using the aircraft measurements taken between Japan and Australia over the period 1993–2013, plus earlier data from 1984 and 1985. The observed CO 2 latitudinal gradient between 30°N and 30°S showed large interannual variations that are clearly associated with El Niño–Southern Oscillation events. We also found long‐term increasing trends of the CO 2 gradients in the most northern latitudes that are proportionally associated with increasing fossil fuel emissions, while decreasing trends were found around the tropical regions. Extrapolation of the changes in the CO 2 gradient back to zero fossil fuel emissions showed a negative north‐south gradient with lower CO 2 in the Northern Hemisphere than in the Southern Hemisphere, as well as a regional CO 2 elevation in the tropical regions. These features provide a useful constraint on model estimates of CO 2 fluxes from the ocean and the land biosphere.