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

A decrease in the C/C ratio of atmospheric CO has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the C-Suess effect, is driven primarily by the input of fossil fuel-derived CO 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 from fossil fuel, land, and oceans can explain the observed C-Suess effect unless an increase has occurred in the C/C isotopic discrimination of land photosynthesis. A trend toward greater discrimination under higher CO levels is broadly consistent with tree ring studies over the past century, with field and chamber experiments, and with geological records of C plants at times of altered atmospheric CO, but increasing discrimination has not previously been included in studies of long-term atmospheric C/C measurements. We further show that the inferred discrimination increase of 0.014 ± 0.007‰ ppm 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 partial pressure within stomatal cavities and their intrinsic water use efficiency to increase in nearly constant proportion to the rise in atmospheric CO concentration.