Development of the CO2 latitude gradient in recent decades

Because 90% of the CO2 from fossil fuel combustion is emitted in the Northern Hemisphere, annual mean atmospheric CO2 mixing ratios are higher at middle and high northern latitudes than in the Southern Hemisphere. The observed CO2 latitude gradient varies interannually and has generally increased as fossil fuel CO2 emissions have increased. Back extrapolation of the measured CO2 latitude gradient to zero fossil fuel emissions gives a latitude gradient with the Northern Hemisphere lower than the Southern. A linear regression of Mauna Loa minus South Pole annual mean differences versus fossil fuel emissions for 1958 through 1996 gives a slope of 0.5 mu mol mol(-1) (abbreviated as ppm CO2) (Gt C)(-1) (sigma = 0.03) and an intercept (at zero fossil fuel emissions) of -0.8 ppm (sigma = 0.2). Shorter data records yield similar results with larger uncertainties. We argue that this extrapolated gradient does not represent preindustrial conditions but is more correctly viewed as a decadal average gradient due to natural sources and sinks that underlie the anthropogenic perturbation. We interpret the extrapolated gradient as evidence for a contemporary Northern Hemisphere sink that has been proposed on the basis of other measurement and model approaches. The slopes (ppm CO2 per gigaton of C from fossil fuel burning) calculated from sufficiently long records tend to agree with model calculations based on fossil fuel emissions, suggesting that any trend in the Northern Hemisphere sink, during the period of the measurements, has been small relative to the trend in fossil fuel emissions.