Photorespiration can limit gross primary productivity in terrestrial plants. The rate of photorespiration relative to carbon fixation increases with temperature and decreases with atmospheric [CO ₂ ]. However, the extent to which this rate varies in the environment is unclear. Here, we introduce a proxy for relative photorespiration rate based on the clumped isotopic composition of methoxyl groups ( R –O–CH ₃ ) in wood. Most methoxyl C–H bonds are formed either during photorespiration or the Calvin cycle and thus their isotopic composition may be sensitive to the mixing ratio of these pathways. In water-replete growing conditions, we find that the abundance of the clumped isotopologue ¹³ CH ₂ D correlates with temperature (18–28 °C) and atmospheric [CO ₂ ] (280–1000 ppm), consistent with a common dependence on relative photorespiration rate. When applied to a global dataset of wood, we observe global trends of isotopic clumping with climate and water availability. Clumped isotopic compositions are similar across environments with temperatures below ~18 °C. Above ~18 °C, clumped isotopic compositions in water-limited and water-replete trees increasingly diverge. We propose that trees from hotter climates photorespire substantially more than trees from cooler climates. How increased photorespiration is managed depends on water availability: water-replete trees export more photorespiratory metabolites to lignin whereas water-limited trees either export fewer overall or direct more to other sinks that mitigate water stress. These disparate trends indicate contrasting responses of photorespiration rate (and thus gross primary productivity) to a future high-[CO ₂ ] world. This work enables reconstructing photorespiration rates in the geologic past using fossil wood.