Revisiting Trends in Wetness and Dryness in the Presence of Internal Climate Variability

Changes in the hydrological cycle have profound societal impacts. Previous literature has proposed a theoretical basis for intensification of the hydrological cycle under global warming, i.e., Wet regions get Wetter and Dry regions get Drier (WWDD). Recent studies have documented uncertainties in the WWDD theory, calling it ‘potentially misleading' over land. We employ the ‘perfect model framework' in four climate models from the Coupled Model Intercomparison Project phase 5 to re-examine historical and projected trends in terrestrial wetness and dryness. We found that two main sources of uncertainties over land are: (1) unforced climate variability which strongly affects local trends, and (2) dry-land areas because of its limitations to evaporation. The global warming signal that supports the WWDD theory emerges over oceans in late 20th century and all-through 21st century. We also found that the WW signal predominates over water sufficient land areas.