Modeling the Atmospheric Airborne Fraction in a Simple Carbon Cycle Model

The fraction of anthropogenic CO2 emissions remaining in the atmosphere, known as the airborne fraction (AF), has remained roughly constant over the last 50 years, averaging about 50-60%. Since AF is widely considered a fundamental property of the carbon cycle, its historical constancy has been taken as evidence that anthropogenic emissions have not as yet triggered a “climate-carbon-cycle feedback”. Projections of future atmospheric CO2 levels and hence climate are often justified on this basis. On the other hand, recent measurements show a slight increase in AF, leading some authors to suggest that climate change feedbacks are now becoming significant. However, increases in AF are also predicted by complex carbon cycle models, even in the absence of such feedbacks. Thus, it remains unclear how one should interpret the observed historical constancy of AF, or its recent trends. Given the importance attributed to AF, particularly in formulating climate policy, it is therefore imperative to understand the reason for its long-term constancy, and why it has stabilized around a particular value. A related question is whether a future increase in AF necessarily imply the operation of climate-carbon feedbacks. In order to address these questions, we have developed and applied a simple box model of the anthropogenically-perturbed global carbon cycle. We show that our model output is in good agreement with both historical observations and the predictions of far more sophisticated carbon cycle models. We then carry out a series of idealized experiments and find that, rather than being a fundamental property of the carbon cycle, AF depends sensitively on both the shape of the emission history and its growth rate. Specifically, our results suggest that both the quasi-constancy of AF over the past half-century, and its particular numerical value of ≈50%, are essentially a consequence of exponentially growing emissions with a nearly-constant growth rate of ≈(40 y)−1. Furthermore, our results suggest that although we cannot rule out climate-carbon feedbacks, fluctuations in the emission growth rate and/or changes in the absorbing capacities of the ocean and land biosphere, offer plausible, and perhaps simpler, explanations for the recent observed trends in AF.