The concentration of carbon dioxide in the atmosphere was 315 parts per million by volume (ppm) when Charles Keeling started his measurement at the Mauna Loa Observatory, Hawaii, in 1958. It surpassed 400 ppm on May 9, 2013 for the first time in the 55-year continuous record of measurements. The so-called ‘Keeling curve’ that shows the rapidly increasing atmospheric carbon dioxide concentration since 1958 is one of the most famous and important scientific findings of our time – yet a full and detailed understanding of the curve and its variations is still to be achieved. For instance, the year-to-year variability that appears as the ‘wiggles’ on the Keeling curve have long been linked to variations of the natural climate-carbon system. But questions remain about what (ocean versus land), where (tropics versus mid-high latitudes) and how (e.g., temperature versus precipitation) different drivers affect the observed variability. This presentation reviews the scientific literature on these questions and presents a simple yet robust analysis that points toward the most likely answer.