Global Transpiration Changes and Its Drivers in the Future

Abstract

Plant transpiration (Et), the dominant element of evapotranspiration, plays an important role in hydrological, ecological, and climate processes. However, attributing projected future Et change to elevated atmospheric carbon dioxide concentration ([CO2]), air temperature (Ta), and leaf area index (LAI) remains challenging because these drivers exert opposing and interacting influences. Here, we develop a quantitative first‐order attribution framework that combines grid‐cell‐level sensitivities diagnosed from CMIP6 single‐forcing experiments with projected driver changes under Shared Socioeconomic Pathways (SSPs). Our results show that simulated future global Et shifts from +10.42 ± 11.84 mm yr−1 (+3.9%) under SSP1‐2.6 to −14.33 ± 29.91 mm yr−1 (−5.4%) under SSP5‐8.5, largely because the diagnosed negative physiological contribution of elevated [CO2] increasingly exceeds the positive contributions associated with increasing Ta and LAI under stronger forcing. Spatially, the [CO2]‐related reduction is widespread, whereas the Ta‐related contribution is more pronounced in humid regions and the LAI‐related contribution is larger in arid and semi‐arid regions. Inter‐model spread in the attributed contributions arises substantially from differences in diagnosed sensitivities of ΔEt to changes in [CO2], Ta, and LAI, with the LAI‐related term showing the largest spread among the three first‐order terms considered. These results provide a consistent quantitative basis for interpreting how elevated [CO2], warming, and vegetation dynamics jointly shape projected Et and highlight the need to better represent plant physiological responses, vegetation structural dynamics, and land‐atmosphere coupling in Earth System Models.