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Atmos. Chem. Phys. Discuss., 10, 9045–9075, 2010 Atmospheric www.atmos-chem-phys-discuss.net/10/9045/2010/ Chemistry ACPD © Author(s) 2010. This work is distributed under and Physics 10, 9045–9075, 2010 the Creative Commons Attribution 3.0 License. Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry Airborne fraction and Physics (ACP). Please refer to the corresponding final paper in ACP if available. trends and carbon sink efficiency M. Gloor et al. What can be learned about carbon cycle Title Page climate feedbacks from CO2 airborne Abstract Introduction fraction? Conclusions References M. Gloor1, J. L. Sarmiento2, and N. Gruber3 Tables Figures 1The School of Geography, University of Leeds, Leeds, LS2 9JT, UK J I 2Atmospheric and Oceanic Sciences Department, Princeton University, 300 Forrestal Road, Sayre Hall, Princeton, NJ 08544, USA J I 3Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich,¨ Universitatsstr.¨ 16, 8092 Zurich,¨ Switzerland Back Close Received: 22 March 2010 – Accepted: 29 March 2010 – Published: 8 April 2010 Full Screen / Esc Correspondence to: M. Gloor ([email protected]) Printer-friendly Version Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion 9045 Abstract ACPD 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 10, 9045–9075, 2010 surface temperature due to greenhouse gas induced warming. This ratio has remained 5 remarkably constant in the past five decades, but recent studies have reported an ap- Airborne fraction parent increasing trend and interpreted it as an indication for a decrease in the effi- trends and carbon ciency of the combined sinks by the ocean and terrestrial biosphere. We investigate sink efficiency here whether this interpretation is correct by analyzing the processes that control long- term trends and decadal-scale variations in AF. To this end, we use simplified linear M. Gloor et al. 10 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 increas- Title Page ing fossil fuel emissions only or the sum of fossil fuel and land use emissions are used. Abstract Introduction We find secondly that the primary control on the decadal time-scale variations of the Conclusions References 15 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 Tables Figures 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 J I equation for the AF implied by the simple models. Using atmospheric CO2 data and 20 emission estimates for the period 1959 through 2006 we find that those controls on J I 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 Back Close the last few decades are unsupported by atmospheric CO2 data and anthropogenic Full Screen / Esc emissions estimates. Printer-friendly Version Interactive Discussion 9046 1 Introduction ACPD Central for predicting future temperatures of the Earth’s surface is how much and for how long carbon dioxide from fossil fuel emissions and land use change stays in the 10, 9045–9075, 2010 atmosphere, and how much gets removed by the carbon sinks on land and in the 5 ocean (e.g., Solomon et al., 2009). A straightforward measure of this redistribution Airborne fraction is the ratio between the increase rate in atmospheric CO2 and the CO2 emitted to trends and carbon the atmosphere by human activity (fossil fuel burning and land use change). Keeling sink efficiency (1973) termed this quantity the “airborne fraction” (AF) and it was investigated in many subsequent studies (e.g. Bacastow and Keeling, 1979; Oeschger and Heimann, 1983; M. Gloor et al. 10 Enting, 1986). Because of the large uncertainties in land fluxes, these early studies could estimate the value of AF only to within a wide range from 0.38 to 0.78 (Oeschger and Heimann, 1983). Recently several studies have extended the estimation of AF Title Page over the last two decades, with a suggestion of a positive trend in AF (Canadell et al., Abstract Introduction 2007; Raupach et al., 2008; LeQuere et al., 2009). Moreover, this positive trend has 15 been interpreted as evidence for a decreasing trend in the efficiency of the ocean and Conclusions References land carbon sinks. Given the model-based projection of a substantial reduction in the Tables Figures sink strength of the ocean and land in the future (e.g. by a large-scale dieback of the Amazon old-growth forest, Cox et al., 2000), the notion that the sinks have already J I begun to deviate from a linear response to the atmospheric CO2 perturbation is a 20 source of substantial concern. While there remains discussion about whether this J I 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 Back Close the AF implies a decreasing efficiency of the carbon sinks. Full Screen / Esc Determinants of the AF are the magnitude and time course of the human induced 25 emissions of CO into the atmosphere and the removal of this anthropogenic carbon 2 Printer-friendly Version by the ocean and land biosphere. It has been known since the early 1970’s, pos- sibly earlier, that the AF will eventually asymptote to a constant value if (i) the CO2 Interactive Discussion uptake by the oceans and land ecosystems is linear and (ii) if CO2 emissions to the 9047 atmosphere follow exactly an exponential function (e.g., Bacastow and Keeling, 1979). Thus, given that fossil fuel emissions have risen approximately exponentially over the ACPD last 250 years, and that natural systems tend to respond linearly to small perturbations, 10, 9045–9075, 2010 it is natural to inquire whether time trends in AF may inform us about changes in the 5 linear behavior of carbon uptake by the oceans and land ecosystems (Canadell et al., 2007; Raupach et al., 2008; LeQuere et al., 2009; Rafelski et al., 2009). However, the Airborne fraction 1 dFF relative growth rate RGR ≡ FF dt of fossil fuel emissions FF has varied by more than a trends and carbon factor of two in the last 100 years (e.g., Raupach et al., 2008). In addition, emissions sink efficiency from land use change exhibited an even more varied time course, so that the total emis- M. Gloor et al. 10 sions only very approximately followed a single exponential. Furthermore trends may be an articulation of incomplete spin-up of the system for the AF to reach a stationary value. We examine here the impact of these deviations and controls on the AF, and Title Page what the consequences are for the interpretation of the AF as an indicator for changes in the efficiency of carbon sinks, and in turn the state of the global carbon cycle. Abstract Introduction 15 Before proceeding, it is important to recognize that definitions of the AF in the litera- ture vary. Studies from the 1970s and 1980s defined airborne fraction from cumulative Conclusions References carbon inventory changes as Tables Figures C(tf )−C(ti ) t R f FF(t)dt J I ti J I (Keeling, 1973; Bacastow and Keeling, 1979; Enting, 1986) or alternatively as Back Close C(tf )−C(ti ) 20 t Full Screen / Esc R f FF(t)+LU(t)dt ti (Oeschger and Heimann, 1983) where ti ,tf are the beginning and the end time of the Printer-friendly Version period considered, FF is fossil fuel emissions and LU is the flux to the atmosphere due Interactive Discussion to land use change. The more recent studies define airborne fraction from annual or 9048 monthly inventory changes as either dC (t) dC (t) ACPD AF ≡ dt , or AF ≡ dt FF FF(t) FF+LU FF(t)+LU(t) 10, 9045–9075, 2010 (Canadell et al., 2007; Raupach et al., 2008; LeQuere et al., 2009; Knorr, 2009). We analyze here the time-evolution of the AF as defined by these recent studies. Airborne fraction trends and carbon 5 As our analysis is intricate, reflecting the complicated nature of the problem, we briefly outline the organization of our paper. We start in Sect. 2 with a characterization sink efficiency of the time course of anthropogenic CO2 emissions and carbon sinks, thereby high- M. Gloor et al. lighting that there have been strong variations in the relative growth rate of fossil fuel emissions over the last century. In Sect. 3 we introduce a simple linear model of the 10 evolution of an atmospheric CO2 perturbation, thereby also clarifying the meaning of Title Page “sink efficiency”. In Sect. 4 we explore how the time course of the anthropogenic emis- sions controls variations in the AF, using a predictive equation implied by our simple Abstract Introduction model. We demonstrate that (i) for an atmospheric CO perturbation which is not fol- 2 Conclusions References lowing an exact exponential function, there is an adjustment time for the AF to converge 15 to its constant asymptotic value which is on the order of centuries, and that (ii) varia- Tables Figures tions in the relative growth rate of the anthropogenic emissions are a major control on variations of the AF.

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