Measurements of atmospheric CO₂ began in 1957-1958 at a wide range of locations, including at fixed stations, on ice floes, on oceanic expeditions, and on aircraft flights, with logistical and financial support provided by the International Geophysical Year (IGY) program. Although the measurement effort was reduced in scope immediately following the IGY, today, measurements are made at more than 100 locations.  Over this same time interval, emissions of CO₂ from fossil fuel combustion increased from 2.3 thousand million metric tons per year (GtC/yr) in 1958 to 7.1 GtC/yr in 2003 [Marland et al., 2005, and personal communication].  More than 90% of this CO₂ was released into the northern hemisphere where it lingered before mixing fully world-wide.  The atmospheric CO₂ concentration, in response, rose faster in the northern hemisphere than in the southern, the interhemispheric difference increasing from near zero during the IGY to about 3 parts per million (ppm) in 2003. For all northern hemisphere stations where our program has measured CO₂, the gradient changes relative to the South Pole are generally proportional to the rate of fossil fuel CO₂ emissions, disregarding seasonal and short term interannual variability in the CO₂ data.  Here, we use this fact to diagnose how the carbon cycle has evolved over the past half century.