Terrestrial carbon sink in the Northern Hemisphere estimated from the atmospheric CO₂ difference between Mauna Loa and the South Pole since 1959

The difference between Mauna Loa and South Pole atmospheric CO₂ concentrations from 1959 to the present scales linearly with CO₂ emissions from fossil fuel burning and cement production (together called fossil CO₂). An extrapolation to zero fossil CO₂ emission has been used to suggest that the atmospheric CO₂ concentration at Mauna Loa was 0.8 ppm less than that at the South Pole before the industrial revolution, associated with a northward atmospheric transport of about 1 Gt C yr^-1 (Keeling et al., 1989a). Mass conservation requires an equal southward transport in the ocean. However, our ocean general circulation and biogeochemistry model predicts a much smaller pre-industrial carbon transport. Here, we present a new analysis of the Mauna Loa and South Pole CO₂ data, using a general circulation model and a 2-box model of the atmosphere. It is suggested that the present CO₂ difference between Mauna Loa and the South Pole is caused by, in addition to fossil CO₂ sources and sinks, a pre-industrial interhemispheric flux of 0.5-0.7 Gt C yr^-1, and a terrestrial sink of 0.8-1.2 Gt C yr^-1 in the mid-latitude Northern Hemisphere, balanced by a tropical deforestation source that has been operating continuously in the period from 1959 to the present.