Abstract
It is well known that terrestrial photosynthesis and13C 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 CO2and photosynthesis in C3 vegetation have the potential to influence the13C 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 time‐varying discrimination for the period of 1981–1994. If the net land carbon sink and plant13C 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 δ13C and CO2as compared with previous studies that assumed discrimination was invariant.
Highlights
[2] El Nino-induced changes in ocean circulation and global climate affect atmospheric CO2 concentrations through several different mechanisms
net primary production (NPP) in Carnegie-Ames-Stanford Approach (CASA) is derived from satellite-derived estimates of absorbed photosynthetically active radiation (PAR) [Bishop and Rossow, 1991; Los et al, 1994] and a light use efficiency term that has a partial dependence on local environmental conditions
Decreases in PPT during the 1997 – 1998 El Nino event were substantial over many regions when compared with PPT during the 1999 to 2000 La Nina period (Figure 2d)
Summary
[2] El Nino-induced changes in ocean circulation and global climate affect atmospheric CO2 concentrations through several different mechanisms. Previous applications of the double deconvolution approach have assumed that plant discrimination against 13C during photosynthesis remains constant from year-to-year. Studies of environmental and biological controls on 13C discrimination during photosynthesis have revealed a number of factors that cause discrimination to vary [Farquhar et al, 1989]. These include atmospheric humidity, solar radiation, drought stress, and plant type, all of which may be expected to respond to interannual climate variability. Allowing discrimination to covary with GPP, in a double deconvolution inversion (presented below) suggests that land/ocean interannual flux variability may be substantially smaller than that predicted in previous analyses. [7] We constructed the NPP-weighted time series of climate variables (PPT, temperature, or VPD; denoted generically by W ) using the following equation: lPand 12 mPonths Pðx; iÞ Á Cðx; i; tÞ
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