Two unintended experiments provide insight into the role of anthropogenic emissions in the global carbon cycle. One was the temporary reduction in emissions during the COVID-19 lock-down. The other was the emission of a radioactive isotope of carbon during the testing of nuclear weapons in the 1950s and the abrupt cessation of these tests in 1963. Together they imply the existence of two distinct reservoirs which exchange carbon with the atmosphere, viz.: the mixed layer and the deep ocean. Exchanges with the former are noisy because they are influenced by sea surface temperature which, in turn, depends on local weather conditions. They completely mask any variations caused by the COVID-19 lock-down. Exchanges with the latter are steady, long-term and uninfluenced by the weather. They result in half the carbon content of the atmosphere and mixed layer being replaced every eleven years.