Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis

Abstract

A decrease in the ¹³C/¹²C ratio of atmospheric CO₂ has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the ¹³C-Suess effect, is driven primarily by the input of fossil fuel-derived CO₂ but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO₂ from fossil fuel, land, and oceans can explain the observed ¹³C-Suess effect unless an increase has occurred in the ¹³C/¹²C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO₂ levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C₃ plants at times of altered atmospheric CO₂, but increasing discrimination has not previously been included in studies of long-term atmospheric ¹³C/¹²C measurements. We further show that the inferred discrimination increase of 0.014 ± 0.007‰ ppm^-1 is largely explained by photorespiratory and mesophyll effects. This result implies that, at the global scale, land plants have regulated their stomatal conductance so as to allow the CO₂ partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO₂ concentration.

Keywords: carbon cycle; carbon-13 Suess effect; isotope; photosynthesis; water use efficiency.

Fig. 1.

Measured and modeled trends in δ¹³C of atmospheric CO₂. Measurements are based on average monthly values at Mauna Loa and South Pole stations, after removing seasonal cycles. (A) Measurements vs. standard model run. (B) Measurements and standard model run as residuals from spline fit. The linear fit through the model residual with slope of −0.055‰ decade⁻¹ quantifies the failure of the standard model to account for the observed δ¹³C trend. This slope error is independent of the overall offset of the model from the data, which depends on the assumed preindustrial model value of −6.4‰. (C and D) Measurements vs. optimized model run, where the optimization is based on radiocarbon and other constraints, also allowing for ocean warming and wind shift effects (SI Appendix). Gray shading reflects 5% and 95% confidence limits on optimized run. Also shown are runs for three scenarios with variable land discrimination Δ, each of which was optimized to also match the rate of change in δ¹³C since 1978. All model runs initiated at a value of δ¹³C = −6.4‰ in year 1765.

Fig. 2.

(A) Variation in the isotopic discrimination of land photosynthesis for three variable Δ scenarios, each of which was optimized to match the rate of change in δ¹³C since 1978. (B) Measured vs. modeled δ¹³C. Ice core data sources: WAIS divide (4), Law Dome (53), and Siple Station (54). All runs initiated at a value of δ¹³C = −6.4‰ in year 1765. In the model–data comparison, trends are significant but not absolute differences.