Because of the high density and effects like the urban heat island affect, weather changes due to climate change are likely to greatly effect cities,[8] exacerbating existing problems, such as air pollution, water scarcity,[9] and heat illness in the metropolitan areas. Moreover, because most cities have been built on rivers or coastal areas, cities are frequently vulnerable to the subsequent effects of sea level rise, which cause coastal flooding[8]: SPM-33 and erosion, and those effects are deeply connected with other urban environmental problems, like subsidence and aquifer depletion.