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

Carbon exist in two stable isotopic forms; 99% as ¹²C and 1% as ¹³C. The carbon-fixing enzyme Rubisco preferentially fixes ¹²C, so fossil fuels are enriched for ¹²C, and since the industrial revolution the atmospheric ¹³C / ¹²C ratio has been increasing as the ¹²C-enriched fossil fuels are reconverted to CO₂. Interestingly, measured data do not match the expected change in ¹³C / ¹²C ratio, indicating that some other unaccounted for factor is also affecting it. Keeling et al. looked at various models to identify the source of the unexpected ratio, and determined that it results from an increase in the discrimination between ¹³C / ¹²C that is occurring due to increased atmospheric CO₂. Their findings indicate 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.” Proc. Natl. Acad. Sci. USA  10.1073/pnas.1619240114