The ever-increasing atmospheric CO₂ concentration is a key driver of modern global warming. However, the low heat capacity of atmosphere and strong convection processes in the troposphere both limit heat retention. Given the higher heat capacity and CO₂ concentration in soil compared to the atmosphere, the direct contributions of soil to the greenhouse effect may be significant. By experimentally manipulating CO₂ concentrations both in the soil and the atmosphere, we demonstrated that the soil-retained heat and the slower soil heat transmission decrease the amount of heat energy leaking from the earth. Furthermore, the soil air temperature was affected by soil CO₂ concentration, with the highest value recorded at 7500 ppm CO₂. This study indicates that soil and soil CO₂, together with atmospheric CO₂, play a crucial role in the greenhouse effect. The spatial and temporal heterogeneity of soils and soil CO₂ should be further investigated, given their potentially significant influence on global climate change.