A disaster is an event that causes such serious harm to people, buildings, economies, or the environment that the affected community cannot manage without external assistance or relief. The United Nations Office for Disaster Risk Reduction defines a disaster as "a serious disruption of the functioning of a community or a society at any scale due to hazardous events interacting with conditions of exposure, vulnerability and capacity, leading to one or more of the following: human, material, economic and environmental losses and impacts". Natural disasters like avalanches, floods, earthquakes, and wildfires are caused by natural hazards. Human-made disasters like oil spills, terrorist attacks and power outages are caused by people. It may be difficult to separate natural and human-made disasters because human actions can make natural disasters worse. Climate change also affects how often disasters due to extreme weather hazards happen.