Synergistic interactions between warming and elevated atmospheric CO₂ increase soil organic carbon sequestration

Warming and elevated CO₂ (eCO₂) are two potentially opposing climate-carbon (C) feedback mechanisms that modulate the magnitude of the land C sink, with warming decreasing and eCO₂ increasing C sequestration. However, their net effect on soil organic C (SOC)—the largest terrestrial C stock—remains uncertain. Here, we quantify how warming, eCO₂, and their interactions influence SOC by using 5558 paired observations from 1392 global studies across ecosystem types. Our study shows that warming reduces SOC by 8.0%, primarily by suppressing aboveground C inputs (−1.0%) and decreasing microbial C use efficiency (−9.9%). Concurrent warming and eCO₂ increase SOC by 7.6%—a synergistic effect larger than eCO₂ alone (+4.8%), primarily contributed by croplands. This outcome may result from increases in plant C inputs and soil nitrogen availability under eCO₂, which facilitate microbial C assimilation and necromass formation (+3.9%). These processes promote the accumulation of mineral-associated C (+9.9%) and offset the negative effects of warming. The combined effect of warming and eCO₂ is projected to increase SOC by 27.4 Pg C by 2100. Our findings highlight that synergistic interaction between warming and eCO₂ increases SOC sequestration and enhances SOC stability under future climate change.