Here we show that land-surface models improved their ability to simulate the increase in the amplitude of seasonal CO₂-cycle exchange (SCA_NBP) by ecosystems compared to estimates by two atmospheric inversions. We find a dominant role of vegetation growth over boreal Eurasia to the observed increase in SCA_NBP, strongly driven by CO₂ fertilization, and an overall negative effect of temperature on SCA_NBP. Biases can be explained by the sensitivity of simulated microbial respiration to temperature.