Rising atmospheric carbon dioxide concentrations have a moderate fertilizing effect on agricultural yields by increasing the availability of this essential input for photosynthesis and promoting water use efficiency .
In the equation at right, as the present/preindustrial ratio of atmospheric carbon rises monotonically from unity--doubling by 2100 in the BAU scenario--the magnitude of this effect also grows (up to 4% in BAU). With this (potentially) conservative estimate, the FeliX model reflects the loose consensus that carbon fertilization has or will have a net positive effect of magnitude less than 10%.
 Baldos, U.L.C., Hertel, T.W.: Global food security in 2050: the role of agricultural productivity and climate change. Aust. J. Agr. Resour. Ec. 58, 1–18 (2014)