A possible global covariance between terrestrial gross primary production and¹³C discrimination: Consequences for the atmospheric¹³C budget and its response to ENSO

Abstract

[1] It is well known that terrestrial photosynthesis and 13 C discrimination vary in response to a number of environmental and biological factors such as atmospheric humidity and genotypic differences in stomatal regulation. Small changes in the global balance between diffusive conductances to CO 2 and photosynthesis in C3 vegetation have the potential to influence the 13 C budget of the atmosphere because these changes scale with the relatively large one-way gross primary production (GPP) flux. Over a period of days to years, this atmospheric isotopic forcing is damped by the return flux consisting mostly of respiration, Fire, and volatile organic carbon losses. Here we explore the magnitude of this class of isotopic disequilibria with an ecophysiological model (SiB2) and a double deconvolution inversion framework that includes timevarying discrimination for the period of [1981][1982][1983][1984][1985][1986][1987][1988][1989][1990][1991][1992][1993][1994]. If the net land carbon sink and plant 13 C discrimination covary on interannual timescales at the global scale, consistent with El Niño-induced drought stress causing a decline in global GPP and C3 discrimination, then less interannual variability in ocean and land net carbon exchange is required to explain atmospheric trends in d 13 C and CO 2 as compared with previous studies that assumed discrimination was invariant.

A joint global carbon inversion system using both CO<sub>2</sub> and <sup>13</sup>CO<sub>2</sub> atmospheric concentration data

“…The impacts of these disequilibrium fluxes on the inverted CO 2 flux determined in case III, case IV and case V are similar to previous results using the double deconvolution technique (Tans et al, 1993;Ciais et al, 1995b;Randerson et al, 2002). However, the influences of these disequilibrium fluxes on the joint inversion could possibly be compromised due to the small number of 13 C observation sites relative to the number of CO 2 observation sites used in the joint inversion.…”

“…The ratio between land and ocean sinks is 1.23, which is close to the value of 1.07 derived from the joint inversion system, indicating that the joint inversion can effectively perform double deconvolution. Our joint inversion system differs from previous double deconvolution systems (Siegenthaler and Oeschger, 1987;Keeling et al, 1989a;Francey et al, 1995;Randerson et al, 2002) in the following ways:…”

“…This information is useful for differentiating between terrestrial and oceanic CO 2 exchanges with the atmosphere because the terrestrial CO 2 flux experiences much greater discrimination against 13 CO 2 than does the oceanic CO 2 flux (Tans et al, 1990;Ciais et al, 1995a;Francey et al, 1995). Observed 13 CO 2 mole fractions can also provide independent information on the net CO 2 exchange over land and ocean because the net carbon flux to the surface discriminates against heavier 13 CO 2 (Fung et al, 1997;Randerson et al, 2002;Suits et al, 2005). The 13 CO 2 observations over the globe, albeit with a limited number of stations, could therefore be used to assist in quantifying the global carbon cycle.…”

A revised 1000 year atmospheric δ13C‐CO2 record from Law Dome and South Pole, Antarctica

“…The KFDD does not take into account climatic and land use change effects on isotopic discrimination by the terrestrial biosphere [Randerson et al, 2002;Scholze et al, 2003Scholze et al, , 2008. We tuned the carbon cycle model to match the latest GCP (Global Carbon Project) net fluxes for recent decades, so this omission is not expected to be a significant problem for the long-term (i.e., preindustrial to modern) change in net fluxes.…”

Compiled records of carbon isotopes in atmospheric CO<sub>2</sub> for historical simulations in CMIP6

“…Observations of δ 13 C in ocean dissolved inorganic carbon have been used to investigate anthropogenic CO 2 uptake (Quay et al, 2003) and to evaluate ocean models that include marine ecosystem dynamics (Tagliabue and Bopp, 2008;Schmittner et al, 2013). With terrestrial biosphere models, simulations of the response of plants and photosynthesis to rising atmospheric CO 2 and changing water availability can be evaluated with δ 13 C observations in atmospheric CO 2 or in leaves or tree rings, because a close relationship exists between processes controlling leaf-level isotopic discrimination and water-use efficiency (Randerson et al, 2002;Scholze et al, 2008;Ballantyne et al, 2011;Keller et al, 2017;Keeling et al, 2017). Additionally, observations of 14 C can be used to constrain models of carbon turnover rates in vegetation and soil carbon at plot-level and global scales (Trumbore, 2000;Naegler and Levin, 2009).…”