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Photorespiration can limit gross primary productivity in terrestrial plants. The rate of photorespiration relative to carbon fixation increases with temperature and decreases with atmospheric [CO₂]. 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–CH₃) 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 isotopologue¹³CH₂D correlates with temperature (18–28 °C) and atmospheric [CO₂] (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-[CO₂] world. This work enables reconstructing photorespiration rates in the geologic past using fossil wood. 

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO₂record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO₂thresholds in biological and cryosphere evolution. 

Abstract Throughout the Phanerozoic, estimated CO₂levels from CO₂proxies generally correlate well with independent estimates of temperature. However, some proxy estimates of atmospheric CO₂during the Late Cretaceous and early Paleocene are low (<400 ppm), seemingly at odds with elevated sea surface temperature. Here we evaluate early Paleocene CO₂by applying a leaf gas‐exchange model toPlatanitesleaves of four early Paleocene localities from the San Juan Basin, New Mexico (65.66–64.59 Ma). We first calibrate the model on two modernPlatanusspecies,Platanus occidentalisandP. × acerifolia, where we find the leaf gas‐exchange model accurately predicts present‐day CO₂, with a mean error rate between 5% and 14%. Applying the model to the early Paleocene, we find CO₂varies between ∼660 and 1,140 ppm. These estimates are consistent with more recent CO₂estimates from boron, leaf gas‐exchange, liverwort, and paleosol proxies that all suggest moderate to elevated levels of CO₂during the Late Cretaceous and early Paleocene. These levels of atmospheric CO₂are more in keeping with the elevated temperature during this period.