A step in the annual average CO₂ difference between Mauna Loa and the Southern Hemisphere of 0.8 ppm in 2009–2010 has no precedence in 50 years. Variations in interhemispheric exchange coincide with the step, the subsequent 5-year north-south CO₂ stability, and historic differences. This behaviour provides a critical test for studies that employ atmospheric transport models to interpret global carbon budgets and inform management of anthropogenic emissions.