You can have global variables by creating Global Blackboards through the “Tools/ParadoxNotion/NodeCanvas/Create/Global Scene Blackboard” menu command. Global Blackboards are added in the scene and as such you can have scene object references assigned in their variables. Global Blackboards are persistent between loading scenes (if DontDestroyOnLoad is enabled in its inspector) and you can have as many as you wish in the scene. For each GlobalBlackboard in your scene though, you must specify a completely unique Identifier name in its inspector.
Later on, whenever you encounter a BBParameter for variable selection, you will see the global variables categorized underneath the GlobalBlackboard names as illustrated below.