The Azimuth Project Airborne fraction (Rev #4, changes)

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Contents

Idea

Wikipedia states: defines:

The airborne Airborne fraction (AF) is a scaling factor defined as the ratio of the annual increase in atmospheric CO2 to the CO2 emissions from anthropogenic sources. It represents the proportion of human emitted CO2 that remains in the atmosphere. The fraction averages about 45%, meaning that approximately half the human-emitted CO2 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 CO2 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”.$CO_2$)to the $CO_2$ emissions from anthropogenic sources. It represents the proportion of human emitted $CO_2$ that remains in the atmosphere. The fraction averages about 45%, meaning that approximately half the human-emitted $CO_2$ 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 $CO_2$ 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.

Changes in carbon sinks can affect the airborne fraction.

Details

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

the notion that the sinks have already begun to deviate from a linear response to the atmospheric $CO_2$ 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.

References

Creative Commons

Abstract: The ratio of CO2 accumulating in the atmosphere to the CO2 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.$CO_2$ accumulating in the atmosphere to the $CO_2$ 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 CO2 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 CO2 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.$CO_2$ 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 $CO_2$ 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 CO2 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.$CO_2$ 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