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

Abstract

We analyzed temporal variations in the annual mean latitudinal distribution of upper tropospheric CO2 using the aircraft measurements taken between Japan and Australia over the period 1993-2013, plus earlier data from 1984 and 1985. The observed CO2 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 CO2 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 CO2 gradient back to zero fossil fuel emissions showed a negative north-south gradient with lower CO2 in the Northern Hemisphere than in the Southern Hemisphere, as well as a regional CO2 elevation in the tropical regions. These features provide a useful constraint on model estimates of CO2 fluxes from the ocean and the land biosphere.