The Azimuth Project
Airborne fraction



Wikipedia defines:

The Airborne fraction (AF) is a scaling factor defined as the ratio of the annual increase in atmospheric carbon dioxide (CO 2CO_2)to the CO 2CO_2 emissions from anthropogenic - human activities - sources. It represents the proportion of human emitted CO 2CO_2 that remains in the atmosphere. The fraction averages about 45%, meaning that approximately half the human-emitted CO 2CO_2 is absorbed by ocean and land surfaces. There is some evidence for a recent increase in airborne fraction, which would imply a faster increase in atmospheric CO 2CO_2 for a given rate of human fossil-fuel burning. However, other sources suggest that the “fraction of carbon dioxide has not increased either during the past 150 years or during the most recent five decades”. Changes in carbon sinks can affect the airborne fraction.


M. Gloor, J. L. Sarmiento, and N. Gruber in their paper What can be learned about carbon cycle climate feedbacks from CO 2CO_2 airborne fraction? states:

the notion that the sinks have already begun to deviate from a linear response to the atmospheric CO 2CO_2 perturbation is a source of substantial concern. While there remains discussion about whether this trend in the AF is actually statistically significant (Knorr, 2009), we focus our discussion here on whether the inferred conclusion is possible, i.e. whether an increasing trend in the AF implies a decreasing efficiency of the carbon sinks.


  • Canadell 2007, Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks

Abstract: The growth rate of atmospheric carbon dioxide (CO2), the largest human contributor to human-induced climate change, is increasing rapidly. Three processes contribute to this rapid increase. Two of these processes concern emissions. Recent growth of the world economy combined with an increase in its carbon intensity have led to rapid growth in fossil fuel CO2 emissions since 2000: comparing the 1990s with 2000–2006, the emissions growth rate increased from 1.3% to 3.3% y −1. The third process is indicated by increasing evidence (P = 0.89) for a long-term (50-year) increase in the airborne fraction (AF) of CO2 emissions, implying a decline in the efficiency of CO2 sinks on land and oceans in absorbing anthropogenic emissions.

Since 2000, the contributions of these three factors to the increase in the atmospheric CO2 growth rate have been ≈65 ± 16% from increasing global economic activity, 17 ± 6% from the increasing carbon intensity of the global economy, and 18 ± 15% from the increase in AF. An increasing AF is consistent with results of climate–carbon cycle models, but the magnitude of the observed signal appears larger than that estimated by models. All of these changes characterize a carbon cycle that is generating stronger-than-expected and sooner-than-expected climate forcing.

Creative Commons

Abstract: The ratio of CO 2CO_2 accumulating in the atmosphere to the CO 2CO_2 flux into the atmosphere due to human activity, the airborne fraction (AF), is central to predict changes in earth’s surface temperature due to greenhouse gas induced warming. This ratio has remained remarkably constant in the past five decades, but recent studies have reported an apparent increasing trend and interpreted it as an indication for a decrease in the efficiency of the combined sinks by the ocean and terrestrial biosphere. We investigate here whether this interpretation is correct by analyzing the processes that control longterm trends and decadal-scale variations in AF.

To this end, we use simplified linear models for describing the time evolution of an atmospheric CO 2CO_2 perturbation. We find firstly that the spin-up time of the system for the AF to converge to a constant value is on the order of 200–300 years and differs depending on whether exponentially increasing fossil fuel emissions only or the sum of fossil fuel and land use emissions are used. We find secondly that the primary control on the decadal time-scale variations of the AF is variations in the relative growth rate of the total anthropogenic CO 2CO_2 emissions. Changes in sink efficiencies tend to leave a smaller imprint. Before interpreting trends in the AF as indication of weakening carbon sink efficiency, it is therefore necessary to account for these trends and variations, which can be achieved based on a predictive equation for the AF implied by the simple models.

Using atmospheric CO 2CO_2 data and emission estimates for the period 1959 through 2006 we find that those controls on the AF, omissions in land use emissions and extrinsic forcing events can explain the observed trend, so that claims for a decreasing trend in the carbon sink efficiency over the last few decades are unsupported by atmospheric CO2 data and anthropogenic emissions estimates.

category: climate