We used an ensemble climate-model experiment to explore the timing and nature of an abrupt regional climate change within the 21st century. In response to global warming a North-Atlantic climate transition occurs, which affects climate over Greenland and northwestern Europe. For a high IPCC non-mitigation emission scenario the transition has a high probability to occur before 2100. In a lower IPCC scenario the probability is lower and the transition threshold is approached more gradually. We found that close to the threshold the evolution of the system becomes sensitive to small perturbations. Consequently, natural climate fluctuations limit the predictability of the timing of crossing the transition threshold, and thus of the abrupt climate change, most strongly for the lower IPCC scenario. No transition is projected for a mitigation scenario, in which CO2-equivalent concentrations are stabilized below the IPCC-scenario range.