The Intergovernmental Panel on Climate Change (IPCC) classifies the human influence on our
climate as extremely likely to be the main reason of global warming over the last decades. Particularly anthropogenic emissions of carbon compounds, with carbon dioxide (CO2) as the main culprit and methane (CH4) as a distant second, are made responsible for the observed temperature changes, while any natural forcings are almost completely excluded.
This post summarizes the results of three studies [1-3] addressing the question, how much human or native emissions can be made responsible for the observed increase of Greenhouse Gases (GHG), in particular the rising mixing ratio of CO2 in the atmosphere.
1. Control of Atmospheric CO2: Relation of Carbon 14 to the Removal of CO2
In a first study we perform an in-depth analysis on the record of atmospheric 14CO2, an isotopic tracer of CO2, to better understand how CO2 is removed from the atmosphere .
The observed CO2 evolution, inclusive of its annual cycle, has recently been reproduced in numerical simulations [4-7]. They show, how the abundance of CO2 in the atmosphere is controlled by a competition between two opposing influences, the feed of CO2 through emission, and its removal through absorption, both at the Earth’s surface. This competition governs time-mean CO2, where absorption figures centrally. It determines if and how fast CO2 grows, as well as the magnitude of its perturbation, e.g., by anthropogenic emission. Yet, actual observations of CO2 absorption are scarce. However, the impact of global absorption on atmospheric CO2 is represented in carbon 14, an isotope of atmospheric carbon that has been observed in the troposphere since the 1950s .
Carbon 14 has a radioactive decay time of 8267 years (e-folding time). On time scales of relevance, the operation of 14CO2 is virtually identical to that of the preponderance of carbon dioxide molecules, 12CO2, comprised chiefly of the stable isotope carbon 12. Dynamical, chemical, and thermodynamic processes acting on those two isotopes of CO2 (including those in the biosphere) are, for practical considerations, indistinguishable.
This feature makes carbon 14 a unique tracer of atmospheric CO2 and provides an unrivalled
means, through which to understand key mechanisms controlling the evolution of atmospheric CO2. Once CO2 is introduced into the atmosphere, whatever influence is experienced by one isotope is experienced by the other. Owing to this property and its artificial enrichment by nuclear testing, 14C is central to estimates of CO2 absorption, which vary widely. Absorption, in turn, is essential to understanding changes of atmospheric CO2.