The climate variability, its implication in coastal processses and the uncertainty that it therefore introduces in the morphological evolution of the coast are addressed. Historical evidences in the Iberian Peninsula allow relating the occurrence of significant variations in sea level position during the Holocene and its effect on the morphology, to natural climate changes. It is still unknown the way that long term climate variability will affect sea level position and the severity of other meteorological agents, which is a source of uncertainty that adds to the stochastic nature of coastal long term proccesses. In a decadal scale, under the assumption that sea level and other parameters that describe the climatic forcing remain stationary, the methodology by Baquerizo and Losada (2008) is used to predict the impact of the construction of a reservoir in the delta of the river Guadalfeo (Spain) and to illustrate how to deal with the uncertainties of the prediction for management purposes. Among other results, it is found that the probability that the shore retreats more than 120 m at any location is about 0.95, which allow to conclude that the construction of the dam will have a severe impact.