Earth system models underestimate carbon fixation by plants in the high latitudes

Most Earth system models agree that land will continue to store carbon due to the physiological effects of rising CO₂ concentration and climatic changes favoring plant growth in temperature-limited regions. But they largely disagree on the amount of carbon uptake. The historical CO₂ increase has resulted in enhanced photosynthetic carbon fixation (Gross Primary Production, GPP), as can be evidenced from atmospheric CO₂ concentration and satellite leaf area index measurements. Here, we use leaf area sensitivity to ambient CO₂ from the past 36 years of satellite measurements to obtain an Emergent Constraint (EC) estimate of GPP enhancement in the northern high latitudes at two-times the pre-industrial CO₂ concentration (3.4 ± 0.2 Pg C yr^-1). We derive three independent comparable estimates from CO₂ measurements and atmospheric inversions. Our EC estimate is 60% larger than the conventionally used multi-model average (44% higher at the global scale). This suggests that most models largely underestimate photosynthetic carbon fixation and therefore likely overestimate future atmospheric CO₂ abundance and ensuing climate change, though not proportionately.