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

The diVerence between Mauna Loa and South Pole atmospheric CO₂ concentrations from 1959 to the present scales linearly with CO₂emissions 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 diVerence 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.