Solar-driven electrochemical CO2 reduction (eCO2R) is a promising approach for producing value-added chemicals from renewable energy sources. While integrated solar fuel reactors have been primarily studied under fixed conditions, their real-world deployment outdoors introduces significant challenges due to dynamic conditions such as fluctuating irradiance. We investigate the performance of a Cu gas diffusion electrode (GDE) as a model system for eCO2R under simulated diurnal conditions. We find that under varying irradiance conditions, degradation of the Cu GDE can arise from the day-to-night transition, which occurs over extended time periods under ?off? conditions. Operando X-ray tomography reveals that pressure variations during this transition can trigger electrolyte incursion into the microporous layer, leading to increased hydrogen evolution. This work enhances our understanding of phenomena that may occur for CO2R during diurnal operation of a GDE and suggests operational strategies for solar-driven eCO2R devices to improve durability and reliability.