Projections of the shifting envelope of Water cycle variability

The time of departure of water cycle variables from historical climatic conditions is quantified from multi-model suites of climate change simulations. Two thresholds are examined: the historical (1860–2005) mean and the range of variability during that same period. The metric is the year when the future range of variability in RCP4.5 and RCP8.5 moves permanently above or below the historical mean for seasonal average values of precipitation, runoff and surface soil moisture. There is a large range of inter-model variability, so both multi-model means of the year of departure and the fraction of models exceeding thresholds by the end of the 21st century are calculated. There is a strong tendency for transition to wetter winter conditions at high northern latitudes for precipitation, but a predominance of dry departures in JJA, with a slight tendency for positive departures in precipitation when considered globally across the year. For runoff there are approximately equal areas of permanently wetter and drier conditions, but runoff is the most sensitive term with the largest areas of departure early in the 21st century, and the greatest propensity to move outside the historical range of variability. Soil moisture trends are predominantly negative with large regions shifting to permanently drier conditions. For all three variables there is a clear increase over time in the total area becoming permanently wetter or drier than the historical mean. The termination of many models’ simulations at 2100, often before permanent transitions are reached, affects the calculations, leading to earlier transition dates than if simulations are extended.