Vegetation Physiological Response to Increasing Atmospheric CO2 Slows the Decreases in the Seasonal Amplitude of Temperature

The seasonal amplitude of temperature (SAT) decreases due to non‐uniform seasonal warming, which exerts critical impacts on ecosystem functions and services. Previous studies attributed the SAT decrease to changed seasonal budget of atmospheric radiative forcing (RAD), however, potential impacts of vegetation physiological response to CO2 (PHY) on SAT are largely underexplored. Here, we examine the role of PHY in modulating SAT using Earth system model simulations. Globally, PHY continuously increases SAT, offsetting about 55% of the RAD‐driven SAT reductions under quadrupled CO2. The PHY‐induced SAT enhancement is particularly pronounced in the northern mid‐to‐high latitudes. Such enhancement is driven directly by stomatal closure‐induced evapotranspiration reduction that amplifies warming, and indirectly by suppressing cloud formation that increases downward shortwave radiation. Our results highlight the critical role of vegetation in mediating temperature seasonality and the necessity for a full understanding of PHY feedback when evaluating the benefits of terrestrial ecosystems in mitigating climate change. Plain Language Summary Increasing atmospheric CO2 warms land faster in colder than warmer seasons, causing decreases in the seasonal amplitude of a temperature (SAT). Vegetation amplifies global warming through its physiological response to rising CO2 (PHY), but to what extent this process affects temperature seasonality is understudied. Using factorial simulations from 11 Earth system models, we find a significant increase in SAT induced by vegetation physiological response to rising CO2 (stomatal closure and leaf area increase), especially in the northern mid‐to‐high latitudes. This enhancement is due to a larger temperature increase in warmer than colder months, primarily caused by stronger stomatal closure‐induced reductions in evapotranspiration and increases in shortwave radiation through decreased cloud cover. Such vegetation‐based enhancement in temperature amplitude alleviates the SAT reductions driven by CO2 radiative forcing. These results highlight the importance of characterizing seasonal dynamics of vegetation biophysical climate feedbacks. Quantification of these feedbacks helps to understand the efficiency of different land‐based climate mitigation strategies. Key Points CO2 physiological forcing (PHY) increases seasonal temperature amplitude, canceling about half of the amplitude decrease by radiative forcing PHY imposes stronger wa