An in-depth analysis is performed on the record of atmospheric 14CO2, an isotopic tracer of CO2 that was perturbed by nuclear testing. In addition to long-term behavior, we examine short-term changes that have been largely ignored. It pays to look closely. Those changes reveal the underlying mechanisms responsible for the observed decline of atmospheric 14CO2 and, thereby, for removal of overall CO2. They represent effective absorption that is considerably faster than appears in the average decline of 14CO2 , initially and then later in its long-term decline. The average decline of 14CO2 is slowed initially by periodic re-enrichment from the stratosphere, which offsets direct absorption. Eventually, however, its decline is slowed by re-emission of absorbed 14CO2 from the Earth’s surface, which likewise offsets direct absorption.
With CO2 absorption revealed by the record of nuclear-perturbed 14C, fundamental principles are then shown to reproduce the observed evolution of 14CO2. Applying the same considerations to anthropogenic emission of CO2 recovers effective absorption that is an order of magnitude faster than operates on 14CO2 . The difference follows from interaction between emission, absorption, and reemission of CO2 from the Earth’s surface. Supported by fundamental principles, the observed behavior of 14CO2 provides an upper bound on the anthropogenic perturbation of atmospheric CO2 . It represents only a few percent of the observed increase.
Carbon in the atmosphere is represented almost entirely by CO2 , with methane being a distant second. Its observed evolution, inclusive of its annual cycle, has recently been reproduced in numerical simulations (Harde and Salby, 2021). In both, the abundance of atmospheric CO2 is controlled by a competition between two opposing influences: Introduction of CO2, through emission at the Earth’s surface, and removal of CO2 , through absorption at the Earth’s surface. This competition governs time-mean CO2, which is determined by mean emission and absorption. It also governs changes of CO2 , which follow from perturbations to mean emission and absorption.
In each of these components of CO2 behavior, absorption figures centrally. By opposing emission, absorption determines if and how fast CO2 grows, as well as the magnitude of its perturbation, for example, by anthropogenic emission. Yet, actual observations of CO2 absorption are scarce. Observations of global absorption, the property which regulates the overall abundance of CO2, are nonexistent. The impact of global absorption on atmospheric CO2 , however, is represented in carbon 14, an isotope of atmospheric carbon that has been observed in the troposphere since the 1950s (CDIAC, 2017).