"Certain uncertainty": The role of internal climate variability in projections of regional climate change and risk management

Internal climate variability co-exists with anthropogenic climate change and places limits on the accuracy of regional climate projections due to its inherent unpredictability. This "certain" uncertainty in regional projections introduced by internal variability contrasts with uncertainty resulting from structural differences amongst climate models, which is potentially reducible as climate models improve. Initial-condition "Large Ensembles" of simulations with individual climate models provide a new perspective on the expected range of future climate change outcomes. Their value for climate risk assessment, adaptation management, and decision-making has yet to be fully realized.