Diagnosis of the Contribution of Internal Climate Variability to Global Surface Temperature Projection Under Future Warming

Abstract Internal climate variability (ICV) is a critical source of uncertainty in global surface temperature (GST) projections. We find regional drivers of ICV in GST projections and investigate their evolution under anthropogenic forcing. Among four CMIP6 large ensemble simulations under historical and SSP5‐8.5 scenarios, we focus on MIROC6 and CanESM5, which show contrasting behaviors of ICV related to GST. In MIROC6, ICV, quantified by ensemble variance, increases markedly from the historical to the future period, primarily driven by the tropical variability including El Niño‐Southern Oscillation (ENSO). Conversely, CanESM5 exhibits stable ICV from the historical to the future period. In CanESM5, while both ENSO and Barents‐Kara Sea ice are key drivers in the historical period, ENSO dominates in the future. These contrasts are associated with forced mean‐state changes in tropical Pacific and North Atlantic sea surface temperature.