Risk are categorized by possible impact and influence as:
- Existential (our whole Earth or all civilization can be destroyed)
- Global risks (impact is global and international cooperation to manage it is required)
- Local severe risks (impact is local or regional, but induced impacts are on the global level)

High

Existential risks, definition:
http://www.nickbostrom.com/existential/risks.html 

Overview of the global risks:
http://reports.weforum.org/global-risks-2017/