Fig. 2.

Changes with atmospheric CO₂ concentration of the global and latitudinal ratio (R) of the β factors calculated with CLM4.5 between including and lacking explicit consideration of mesophyll conductance (g_m). R fluctuates erratically for the first half of the 20th century when the atmospheric CO₂ is close to the baseline reference of 296 ppm in 1901, causing the denominator of R to vary around zero. Therefore, values of R when the atmospheric CO₂ is less than 320 ppm are not shown. The dashed line at R = 1 separates the underestimation (R > 1) from the overestimation (R < 1) of the CO₂ fertilization effect on gross primary production by CLM4.5 lacking g_m compared with CLM4.5 including g_m.