Extended Data Fig. 1: Model predictions for V_O/V_C difference between Glacial and Holocene [CO₂]_atm levels.

The difference between Glacial and Holocene V_O/V_C is predicted to be positive or negligible depending on assumptions regarding growing season photosynthesis temperature, the extent of glacial cooling, and the response of c_i/c_a to a change in pCO₂. Orange, blue, and purple lines are model predictions assuming photosynthesis temperatures in the Last Glacial period were 3, 6, or 9 °C colder than in the Holocene. 3–9 °C spans the range of mean annual surface temperature differences observed in recent modeling efforts and proxy reconstructions^21,22. Dotted lines use the FvCB model⁹ assuming a single set of Rubisco kinetics¹¹, a temperature-dependent mesophyll conductance term^48,49, and a fixed c_i/c_a of 0.7^13,30,84,85. Upper and lower bounds of envelopes use the same kinetics but assume that c_i/c_a was lower (0.6, upper bound)^32,86 or higher (0.8, lower bound)^33,87,88 during the Last Glacial period compared to the Holocene (0.7). Pentagons use the average output of P-model v.1.0⁵³ for all sample sites (see Methods, Extended Data Fig. 2). Grey line denotes no net change in V_O/V_C between Glacial and Holocene climate states.