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