Warming is expected to increase methane (CH₄) emissions from lakes and reservoirs, yet how warming affects CH₄ cycling in oxygenated surface waters－the layer directly coupled to the atmosphere－remains unresolved. Here we show that CH₄ accumulation in a stratified reservoir increases nonlinearly with temperature, coinciding with a microbial shift towards production. Using methylphosphonate (MPn) as a model substrate, we find that oxic CH₄ production is substantially more temperature sensitive than aerobic CH₄ oxidation. This kinetic asymmetry enhances CH₄ accumulation under experimental warming, particularly when MPn is available. Our results reveal a warming-driven feedback in which oxic CH₄ production outpaces its oxidation, potentially increasing CH₄ emissions from stratified waters beyond current projections.