Simulations of future climate change impacts are highly uncertain, particularly for catchment hydrology, where output from models of complex dynamic systems (global climate) are used as inputs to models of complex dynamic systems (hydrology models). This is problematic where decision-making for adaptation is underpinned by future climate predictions, and where policy-makers have opted to delay adaptation until either uncertainties are reduced, or climate change signals emerge from observations. This paper, using the Boyne catchment in the east of Ireland as a case study, discusses the uncertainties involved in climate change impact assessment for catchment hydrology and highlights why uncertainties are unlikely to be constrained or reduced in the time-scale required for adaptation. In addition, by calculating the time required for climate change signals to emerge from the observational record and the magnitude of change required for detection, it is highlighted that waiting for climate signals to be statistically detectable is not an option for effective adaptation. The paper concludes by considering how a paradigm shift in how we use the output from climate impact assessments can progress the adaptation agenda given the limits to prediction identified.