By 2100, under a high emissions scenario, rainfall in wet areas is expected to increase (from small increases to more than 30% increases), while rainfall in dry areas is expected to decrease (from small decreases to decreases of up to 60%). Future precipitation patterns are difficult to project for Hawai'i, due to its steep topography and small-scale variations in climate. Overall precipitation has been trending downward in recent decades, with the sharpest downward trends occurring on western Hawai'i island. Significant decreases have also occurred on the wetter areas of other islands. Seasonal movement of the intertropical convergence zone and ENSO strongly impact precipitation. Patterns of wet and dry years can last for decades, and are produced by various climatic patterns, such as the Pacific Decadal Oscillation. This natural variability increases the region's vulnerability to drought, making water shortages a significant climate-related threat for Hawai'i. Information regarding microclimates within particular islands, such as windward/leeward differences and valley/ridge differences, require more fine-scale models that are currently being developed.