Increasing cotton heat exposure during critical growth stages under projected warming in Xinjiang, China

China contributes approximately 25% to global cotton production, however, over 90% of cotton in China is harvested in Xinjiang, a region increasingly impacted by heat events. Despite this, the extent to which cotton cultivation is exposed to heat events, particularly during critical growth stages, remains uncharacterized. To address this gap, we employed extreme degree days (EDD) and accumulated heat stress days (AHSD) to reflect heat events during the flowering and boll development stages. We analyzed the spatiotemporal patterns of such exposure over the historical period (1961–2020) and projections under two warming scenarios (1.5 °C and 2.0 °C) that derived from the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results revealed a modest upward trend of heat events during critical growth stages of cotton, characterized by considerable interannual variability. Specifically, EDD and AHSD increased at 0.12 d·°C/yr and 0.12 days/yr, respectively. Despite notable spatial heterogeneity, regions such as Hami, Paotai, Yuli, and Mossel were identified as the most vulnerable, with EDD exceeding 25 d·°C and AHSD surpassing 9 days. Future projections suggest a substantial intensification of heat events, with EDD and AHSD values tripling and doubling under the 2.0 °C warming scenario. The findings highlight the critical importance of optimizing growth stage windows to reduce cotton’s exposure to heat stress. Targeted adaptive measures, such as adjusting planting windows and breeding new cultivars, are urgently needed to mitigate the adverse impacts of heat stress and ensure stable cotton yield.