English: Time series of an observable for each ensemble member are represented by thin light-gray lines. The dark-gray line represents a single member. The thick black line represents the ensemble mean. The gray dot represents a perturbed initial state. Ensemble members are initially close to each other and all evolve in phase. This would correspond to an accurate weather forecast. After some time, however, members start to spread and only the low-frequency part of the signal is visible in the ensemble mean (there is still some memory of the initial state). This could, for instance, correspond to the loss of the predictability of synoptic weather systems after a few days/weeks, but with atmospheric regimes or ENSO still providing some sub-seasonal to seasonal predictability. Eventually, even the low-frequency variability is lost and only the long-term trend associated with forcing is visible in the ensemble mean. This could be the signal associated with the anthropogenic forcing in climate projections.