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

• Published in: Proceedings of the National Academy of Sciences
• Main Authors: Keeling, Ralph F., Graven, Heather D., Welp, Lisa R., Resplandy, Laure, Bi, Jian, Piper, Stephen C., Sun, Ying, Bollenbacher, Alane, Meijer, Harro A. J.
• Language: unknown
• Published: 2023
• Subjects: 54 ENVIRONMENTAL SCIENCES; ice core
• Online Access: http://www.osti.gov/servlets/purl/1535353; https://www.osti.gov/biblio/1535353; https://doi.org/10.1073/pnas.1619240114

First, a decrease in the 13 C/ 12 C ratio of atmospheric CO 2 has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the 13 C-Suess effect, is driven primarily by the input of fossil fuel-derived CO 2 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 CO 2 from fossil fuel, land, and oceans can explain the observed 13 C-Suess effect unless an increase has occurred in the 13 C/ 12 C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO 2 levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C 3 plants at times of altered atmospheric CO 2 , but increasing discrimination has not previously been included in studies of long-term atmospheric 13 C/ 12 C measurements. In our paper, 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 CO 2 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO 2 concentration.